Book - Comparative Embryology of the Vertebrates 1-3: Difference between revisions

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# Luteinizing hormone, LH (ICSH), and  
# Luteinizing hormone, LH (ICSH), and  
# Luteotrophin, LTH.  
# Luteotrophin, LTH.  
==The Vertetrate Ovary and Its Relationship to Reproduction==
===A. The Ovary and Its Importance===
One of the editions of the treatise on development, “Exercitationes de
Generatione Animalium,” by William Harvey (1578-1657) contains a picture of Jupiter on a throne opening an egg from which various animals,
including man, are emerging (fig. 25). Upon the egg (ovum) are engraved
the words ovo omnia.'' At the heading of chapter 62 of this work Harvey
placed a caption which explains the phrase ex ovo omnia more explicitly.
This heading reads: “Ovum esse primordium commune omnibus animalibus”
— the egg is the primordium common to all animals. Published in 1651, this
statement still maintains its descriptive force.
Many individual animals arise by asexual reproduction, that is, through
a process of division or separation from a parent organism. In the phylum
Chordata asexual reproduction is found among the Urochordata, where new individuals may arise by budding from a stolon -like base of the parent (fig.
27). This process often is called gemmation, the formation of a new individual by a protrusion of a mass of cells from the parental body followed by
its partial or complete separation. It is a prominent method of reproduction
among the lower Metazoa, particularly the coelenterates and sponges. Nevertheless, all animal species among the Metazoa ultimately utilize an egg as
the primordium from which the new individual arises. Sexual reproduction,
generally associated with the fertilization of an egg by a sperm element, appears
to be a needful biological process.
Fig. 25. Copy of the engraved title appearing in one edition of Harvey’s dissertation
on generation as shown on p. 139 of Early Theories of Sexual Generation by E. J. Cole.
Observe the words “ex ovo omnia” upon the egg which Jupiter is opening. Various animals
are emerging from the egg.
Fig. 26. Copy of Hartsoeker’s figure of human spermatozoan, containing the homonculus or “little man,” published in 1694. This figure represents a marked preformationist
conception of development. However, it is to be noted that Hartsoeker later abandoned
the preformationist concept as a result of his studies on regeneration.
True as the general statement made by Harvey may be, it is not clear what
is meant by the word ovum or egg. We know certain of its characteristics,
but, for the most part, it must be accepted as an accomplished fact enshrouded
in mystery. To Harvey the egg was an indefinite, unorganized association of
substance plus a “primordial generative principle” (see Cole, F. J., ’30, p.
140), Other minds have conceived of other meanings. Nevertheless, descriptive
and experimental embryology has forced the conclusion that the egg, during
its development within the ovary, experiences a profound process of differentiation, resulting in the formation of an invisible organization. Although this organization is invisible, it is imbued with an invincibility which, when
set in motion at the time of fertilization, drives the developmental processes
onward until final fulfillment is achieved in the fully formed body of the
adult organism.
Fig, 27. Forms of asexual reproduction in the subphylum Urochordata 9 #
Chordata. (From MacBride: Textbook of Embryology, Vol. 1, Londo/rt', ^
(A) Budding from "stolon of Perophora listeri, from MacBride after (jR) , (C)
Two stages of budding in an ascidian, from MacBride after Pizon
Beyond the fundamental changes effected in the developing egg while in
the ovary, the latter structure has still other roles to maintain. Through the
mediation of the hormones produced within the confines of the ovarian substance, the female parent is prepared to assume the responsibilities of reproduction. In addition, in many vertebrates the further responsibility of taking
care of the young during the embryonic period stems from the hormones
produced in the ovary. In some vertebrates, the instinct of parental care of
the young after hatching or after birth indirectly is linked to ovarian-pituitary
relationships. Because of these profound and far-reaching influences which
the ovary possesses in producing the new individual, it must be regarded as
the dynamic center of reproduction for most animal species.
===B. Preformationism, Past and Present===
The above statement relative to the importance of ovarian influences and
of the female parent is a position far removed from that held by some in the
past. An ancient belief elevated the male parent and his “seed” or semen.
As Cole, F. J., ’30, p. 38, so aptly places the thinking of certain learned
sources during the 16th century: “The uterus is regarded as the ‘till’d ground
for to sow the seeds on’ — a popular idea, based obviously on the analogy
with plants, which prevailed long before and after this period. The seed of
the male is therefore the chief agent in generation, but cannot produce an
embryo without the cooperation of the female, and whether the result is male
or female depends on which side of the uterus the seed falls, the time of the
year, temperature, and the incidence of menstruation.” Or, in reference to
the Leeuwenhoek’s belief in an intangible preformationism, Cole, F. J., ’30,
p. 57, states: “He asserts that every spermatic animalcule of the ram contains
a lamb, but it does not assume the external appearance of a lamb until it has
been nourished and grown in the uterus of the female.” This statement of
A. van Leeuwenhoek (1632-1723) was made as a criticism of N. Hartsoeker
(1656-1725) whose extreme adherence to a seminal preformationism led
him to picture the preformed body of the human individual, the homonculus,
encased within the head of the spermatozoon (fig. 26). Hartsoeker, however,
later abandoned this idea.
In fairness it should be observed that the egg during these years did not
lack champions who extolled its importance. While the Animalculists considered the sperm cell as the vital element in reproduction, the Ovists, such as
Swammerdam (1637-80), Haller (1708-77), Bonnet (1720-93) and Spallanzani (1729-99) believed that the pre-existing parts of the new individual
were contained or preformed within the egg.
An extreme form of preformationism was advocated by certain thinkers
during this period. For example, Bonnet championed the idea of encasement
or “emboitement.” To quote from Bonnet:
The term “emboitement” suggests an idea which is not altogether correct. The
germs are not enclosed like boxes or cases one within the other, but a germ forms
part of another germ as a seed is a part of the plant on which it develops. This
seed encloses a small plant which also has its seeds, in each of which is found a
plantule of corresponding smallness. This plantule itself has its seeds and the latter
bears plantules incomparably smaller, and so on, and the whole of this ever
diminishing series of organized beings formed a part of the first plant, and thus
arose its first growths. (Cole, ’30, p. 99.)
On the other hand, there were those who maintained that for some animals,
neither the sperm nor the egg were important as “many animals are bred
without seed and arise from filth and corruption, such as mice, rats, snails,
shell fish, caterpillars, moths, weevils, frogs, and eels” (Cole, ’30, p. 38).
This concept was a part of the theory of spontaneous generation of living
organisms -a theory ably disproved by the experimental contributions of
three men: Redi (1626-97); Spallanzani; and Louis Pasteur (1822-95).
Modern embryology embraces a kind of preformationism, a preformationism which does not see the formed parts of the new individual within
the egg or sperm but wi.ich does see within the egg a vital, profound, and
highly complex physiochemical organization capable of producing a new individual by a gradual process of development. This organization, this selfdetermining mechanism, is resident in the nucleus with its genes and the
organized cytoplasm of the fully developed oocyte or egg. However, as shown
later, this organization is dependent upon a series of activating agencies or
substances for its ultimate realization. Some of these activating substances
come from without, but many of them are produced within the developing
organism itself.
===C. General Structure of the Reproductive System of the Vertebrate Female===
====1. General Structure of the Ovary====
Morphologically, the ovary presents a series of contrasts in the different
vertebrate classes. In teleost fishes the size of the ovary is enormous compared
to the body of the female (fig. 28), while in the human (fig. 29), cow, sow,
etc., it is a small structure in comparison to the adult body. Again, it may
contain millions of mature eggs in the ling, cod and conger, during each breeding season, whereas only a single egg commonly is matured at a time in the
cow, elephant, or human. During the reproductive season the ovary may
assume a condition of striking colored effects as in the bird, reptile, shark,
and frog, only to recede into an appearance drab, shrunken, and disheveled
in the non-breeding season.
Fig. 28. Dissection of female specimen of the common flounder, Limanda ferruginea.
It particularly shows the ovary with its laterally placed ovarian sinus. Observe that the
ovary, during the breeding season, is an elongated structure which extends backward into
the tail. There are two ovaries, one on either side of the hemal processes of the caudal
vertebrae.
Its shape, also, is most variable in different species. In mammals it is a
flattened ovoid structure in the resting condition, but during the reproductive
phase it may assume a rounded appearance, containing mound-like protrusions.
In birds and reptiles it has the general form of a bunch of grapes. In the
amphibia it may be composed of a series of lobes, each of which is a mass
of eggs during the breeding season, and in teleost and ganoid fishes it is
an elongated structure extending over a considerable area of the body.
Regardless of their many shapes and sizes, the ovaries of vertebrates may
be divided morphologically into two main types, namely, compact and saccular
forms. The compact type of ovary is found in teleost, elasmobranch, cyclostome, ganoid, and dipnoan fishes, as well as in reptiles, birds and mammals.
It has the following regions (figs. 30, 31):
( 1 ) the medulla, an inner zone containing relatively large blood and lymph
vessels;
(2) the cortex, an area outside of and surrounding the medulla (except
at the hilus), containing many ova in various stages of development;
(3) a tunica albuginea or connective-tissue layer surrounding the cortex;
and
(4) the germinal epithelium or the covering epithelium of the ovary.
The germinal epithelium is continuous with the mesovarium, the peritoneal
support of the ovary, and the particular area where the mesovarium attaches
to the ovary is known as the hilus. Within the mesovarium and passing
through the hilus are to be found the blood and lymph vessels which supply
the ovary (fig. 30).
The ovary of the teleost fish is a specialized, compact type of ovary adapted
to the ovulation of many thousands, and in pelagic species, millions of eggs
at one time. It has an elongate hilar aspect which permits blood vessels to
enter the ovarian tissue along one surface of the ovary, whereas the opposite
side is the ovulating area. In many teleosts the ovulating surface possesses
a special sinus-like space or lumen (fig. 28) which continues posteriad to
join the very short oviduct. At the time of ovulation the eggs are discharged
into this space and move caudally as the ovarian tissue contracts. In other
teleosts this ovulatory space is not a permanent structure but is formed only
at the time of ovulation. In Tilapia macrocephala, for example, the ovulatory
lumen is formed on the side of the ovary opposite the area where the blood
vessels enter. The formation of this space at the time of ovulation is described
by Aronson and Holz-Tucker (’49) as a rupture of the elastic follicles during
ovulation whereupon the follicle walls shrink toward the ovarian midline.
Fig. 29. Diagrammatic representation of a midsagittal section of the reproductive
organs of the human female. (Slightly modified from Morris: Human Anatomy, Philadelphia, Blakiston.)
Fig. 30. Schematic three-dimensional representation of the cyclic changes which occur in the mammalian ovary.
carrying the interstitial tissue and immature ova. This shrinking away of the
tissues of the ovary leaves a space between these tissues and the outside
ovarian wall. A lumen thus is formed along the lateral aspect of the ovary
which is continuous with the oviduct. Many teleosts have two ovaries (e.g.,
flounder); in others there is but one (e.g., perch).
The amphibia possess a true saccular ovary (fig. 32). It has a cortex and
germinal epithelium somewhat similar to the compact ovarian variety, but
the area which forms the medulla in the compact ovary is here represented
by a large lymph space. During early development, the amphibian ovary is
a compact structure, but later there is a hollowing out and disappearance of
the compact medullary portion, and the cortical area remains as a relatively
thin, peripheral region (Burns, ’31; Humphrey, ’29).
Histologically the vertebrate ovary is composed of two general cellular
groups, namely:
(1) germ cells, and
(2) general tissue cells of various kinds, such as epithelium, connective
tissue, smooth muscle fibers, and the complex of elements comprising the vascular system of the ovary (figs. 30, 32). Some of the general
cells form the so-called interstitial tissue of the ovary.
The germ cells differ from the general cells in that each of them has a
latent potency for developing a new individual. This latent condition is converted into active potentiality during the differentiation of the primitive germ
cell into the mature egg or ovum.
===2. General Structure of the Accessory Reproductive Organs===
The accessory reproductive structures of the female vertebrate may be
separated into three general types, viz.:
( 1 ) the total absence of or the presence of a pair of short funnel-like
structures which convey the eggs from the peritoneal cavity through
Fig. 31 . Three-dimensional representation of the bird ovary together with the funnel
portion (infundibulum) of the oviduct. Recently ovulated egg is shown in the process of
engulfment by the infundibulum. Various stages of developing eggs are shown.
Fig. 32. Anterior half of the saccular ovary of Necturus maculosus.
an opening into the urogenital sinus and thence to the outside as in
cyclostome fishes,
(2) a short sinus-like tube attached to each ovary and to the urogenital
sinus or to a separate body opening as in many teleost fishes (fig.
28), and
(3) two elongated oviducal tubes variously modified (figs. 29, 33, 34,
35, 36, 37).
Except in the teleost fishes the cephalic end of each oviduct generally is
open and is placed near the ovary but not united directly with it (figs. 29,
33) although in some species, such as the rat, it is united with an ovarian
capsule (fig. 37). In some vertebrates the anterior orifice of the oviduct may
be located a considerable distance from the ovary, as in frogs, toads, and
salamanders. In many vertebrates, as in birds and snakes, there is but one
oviduct in the adult.
In some vertebrates the oviduct is an elongated glandular tube, as in certain
urodele amphibia (fig. 33) and in ganoid fishes; in others, such as frogs,
birds or mammals, it is composed of two main parts: ( 1 ) an anterior glandular
structure and (2) a more caudally placed uterine portion. The latter may
unite directly with the cloaca, as in the frog (fig. 38) or by means of a third
portion, the vaginal canal or vagina located between the uterus and the
cloaca, as in elasmobranch fishes, reptiles, and birds, or between the uterus
and the external urogenital sinus, as in mammals (figs. 35, 36, 37). The
vaginal canal may be single, as in eutherian mammals, or double, as in metatherian mammals (figs. 35, 36). In metatherian (marsupial) mammals it
appears that a third connection with the oviducts is made by the addition
of a birth passageway. This birth canal represents a secondary modification
of a portion of the vaginal canals and associated structures (figs. 34, 35, 114).
(See Nelsen and Maxwell, ’42.) One of the main functions of the vagina or
vaginal canal is to receive the intromittent organ of the male during copulation.
The anterior opening of the oviduct is the ostium tubae abdominale, a
funnel-shaped aperture generally referred to as the infundibulum. In the
transport of the egg from the ovary to the oviduct the infundibulum, in
many species, actually engulfs and swallows the egg.
The portion of the oviduct anterior to the uterus often is called the convoluted glandular part; it is highly twisted and convoluted in many species.
In amphibians, reptiles, birds, and in some mammals the glandular portion
Fig. 33. Diagrammatic representation of the reproductive structures of female urodele,
Necturus maculosus.
Fig. 34. Diagrammatic lateral view of female reproductive system of the opossum,
showing pseudo-vaginal birth canal.
Fig. 35. Reproductive structures of female opossum shown from the ventral view. Observe that the ovary and infundibular portion of the Fallopian tube lie dorsal to the horn
of the uterus.
functions to secrete an albuminous coating which is applied to the egg during
its passage through this region. In amphibians, reptiles, and birds it forms
the major portion of the oviduct, but in mammals it is much reduced in size
and extent. In the latter group it is referred to as the uterine or Fallopian tube.
The uterus is a muscular, posterior segment of the oviduct. Like the
anterior glandular portion of the oviduct, it also has glandular functions, but
these are subservient to its more particular property of expanding into an
enlarged compartment where the egg or developing embryo may be retained.
The protection and care of the egg or of the embryo during a part or all of
its development, is the main function of the uterus in most vertebrates. In the
frogs and toads, however, this structure seems to be concerned with a “ripening” process of the egg. Large numbers of eggs are stored in the uterine sac
of the frog for a period of time before spawning.
Various degrees of union between the uterine segments of the two oviducts are found in mammals. In the primates they fuse to form a single
uterine compartment with two anterior uterine tubes (fig. 29). In carnivores,
there is a caudal body of the uterus with two horns extending forward to
unite with the uterine tubes (fig. 36). In the rat and mouse, the uterine segments may be entirely separate, coming together and joining the single vaginal
chamber (fig. 37). In the opossum the uterine segments are entirely separated,
joining a dual vaginal canal system posteriorly (figs. 34, 35, 114).
===D. Dependency of the Female Reproductive System on General Body===
Conditions
1. Inanition
In the immature female mammal continued underfeeding results in general
retardation of sexual development. The younger follicles may develop, but
the later stages of follicular development are repressed. In the adult female,
inanition produces marked follicular degeneration and atresia as shown by
many records of retarded sexual development, reduced fertility, even cessation
of the cyclic activities of menstruation and estrus occurring in man and
domestic animals during war-produced or natural famine (Mason in Allen,
Danforth, and Doisy, ’39, p. 1153). The ovary thus seems to be especially
susceptible to starvation conditions, even more so than the testis. As the
condition and well-being of the secondary reproductive structures are dependent upon proper ovarian function, this part of the reproductive system
suffers marked changes as a result of ovarian dysfunction during prolonged
starvation.
Fig. 36. Schematic representation of reproductive organs of the female cat. On the left
side of the illustration, the body of the uterus and uterine horn have been cut open, and
the Fallopian tube and ovary are highly schematized. Observe the partial ovarian capsule
around the ovary shown on the right and the relatively fixed condition of the infundibular
opening of the oviduct lateral to the ovary.
Fig. 37. Diagrammatic representation of the reproductive organs of the female rat,
showing the bursa ovarica around each ovary. Observe that uteri open directly into the
vagina. (Modified from Turner, ’48.)
Fig. 38. Diagrammatic representation of reproductive structures of the female frog.
Observe that the ostium of the oviduct is not an open, mouth-like structure. It remains
constricted until the egg starts to pass through.
2. Vitamins
a. Vitamin A
The ovary is not immediately sensitive to a lack in vitamin A in the diet
but general epithelial changes in the reproductive tract occur which may aid
in producing sterility (Mason, ’39).
b. Vitamin B
Ovarian and uterine atrophy occur as a result of deficiency of this vitamin
in monkey, rabbit, mouse and rat (Mason, ’39). This effect may be mediated,
at least partly, through the effect of B-deficiency upon the pituitary gland.
c. Vitamin C
During the earlier stages reproductive activity is maintained, but advanced
stages of C-deficiency produce regressive effects (Mason, ’39).
d. Vitamin E
E-deficiency in the female rat does not upset the ovarian and general reproductive behavior. However, established pregnancies are disturbed and are
terminated by resorption of the embryo (Mason, ’39). In the domestic fowl,
unless sufficient amount of vitamin E is present in the egg, embryonic death
occurs during early incubation periods of the egg.
3. The Hypophysis (Pituitary Gland)
The ovaries experience pronounced atrophy as a result of hypophysectomy
in mammals and non-mammalian species. The earlier stages of follicle formation in the higher mammalian ovary up to the stage of beginning antrum
formation are not so much affected, but later follicular development and interstitial tissue growth are inhibited (Smith, P. E., ’39). (See fig. 40.)
===E. Activities of the Ovary in Producing the Reproductive State===
====1. The Ovary as a "Storehouse"of Oogonia====
The cortex of the ovary contains many young ova in various stages of development. In the human ovary shortly after birth, the number of oogonia
in the cortex of each ovary has been estimated to reach a number as high
as 300,000. This figure should not be taken too literally, as the amount of
variability in the ovary from time to time is great and degeneration of ova
is a common episode. Haggstrdm (’21 ) estimated that each ovary of a 22-yearold woman contained 200,000 young ova. In the ovaries of young rats, Arai
(’20, a and b) estimated that there were on the average around 5,000 ova
under 20 /x in diameter.
Without entering into the controversy (Chap. 3) relative to the rhythmic
origin of germ cells in the ovary, one must accept the conclusion that the
normal ovary has within it at all times during its reproductive life large numbers of oogonia in various stages of development. Thus the ovary, aside from
its other activities, functions as a storehouse and nursery for young oogonia.
Relatively few of these oogonia develop into mature eggs in the mammals.
For example, the reproductive life of the human female occurs from about
the age of 10 or 14 years to about 48 years. If one egg per monthly cycle
is discharged from the ovary which is functional during that cycle, only about
400 eggs would be matured in this way. The number would be less if pregnancies intervened. If one accepts the figures given by Haggstrom, an enormous number of eggs of the human ovary never reach their potential goal.
Similarly, according to Corner (’43): “The most prolific egg producer among
mammals, the sow, might possibly shed a total of 3,000 to 3,500 eggs, allowing ten years of ovarian activity not interrupted by pregnancy, and assuming
the very high average of 20 eggs at each three weekly cycle, but she has vastly
more than this in the ovaries at birth.”
====2 . Position Occupied by the Primitive Female Germ Cells in the Ovarian Cortex====
Within the cortex the definitive germ cells or oogonia are found in or near
the germinal epithelium (figs. 39, 64). Some authors regard the oogonium
as originating from the cells of the germinal epithelium. (See Chap. 3, section
on “germ cell origin.”) The definitive germ cell soon becomes associated
with small epithelial cells (fig. 41). This complex of a germ cell with its
associated epithelial cells is found somewhat deeper in the cortex, within or
below the tunica albuginea. As the oogonium begins to experience the changes
propelling it toward a state of maturity, it is regarded as an oocyte (Chap. 3).
Characteristics of the primitive oocyte are:
# an enlargement of the nucleus,
# changes within the chromatin material of the nucleus pertaining to meiosis (Chap. 3), and
# a growth and increase in the cytoplasmic substances (fig. 41).
Fig. 40. Effects produced by hypophysectomy on the rat ovary and of replacement
therapy utilizing injections of pituitary gonadotrophins. (After Evans, Simpson, and
Penchaez: Symposia of Quantitative Biology, Vol. 5, 1937. The Biological Laboratory,
Cold Spring Harbor, L. 1., N. Y.) (A) Ovary of hypophysectomized animal. Observe
that Graafian follicles are small. They do not proceed further in their development than
the beginning of antral vacuole formation unless replacement therapy is applied. (B)
Ovarian condition of hypophysectomized animal receiving replacement therapy in the
form of injections of the LH (ICSH) gonadotrophic factor of the anterior lobe of the
hypophysis. Interstitial tissue is well developed. (C) Ovarian condition of hypophysectomized animal receiving the FSH gonadotrophic factor. Note follicular growth and antral
vacuole formation; interstitial tissue between the follicles remains somewhat deficient.
(D) Ovarian condition of hypophysectomized animal receiving injections of FSH plus
LH. Corpora lutea are evident (as well as enlarged follicles not shown in the figure).
Interstitial tissue remains deficient.
Fig. 41. Development of primary condition of the Graafian follicle in the opossum
ovary. (A) Young oocyte with associated epithelial (granulosa) cells which in (B)
have encapsulated the oocyte. (C) Encapsulating granulosa cells have increased in
number and are assuming a cuboidal shape. (D) Fully developed condition of the
primary Graafian follicle. Cf. secondary condition shown in fig. 42.
Fig. 42. Secondary conditions of the Graafian follicle in the opossum ovary. Cf. that of the rat ovary in fig. 40.
As these changes are initiated, the associated epithelial cells increase in
number and eventually encapsulate the oocyte (fig. 41B). This complex of
the oocyte with its surrounding layer of follicle cells is known as an egg follicle.
====3. Primary, Secondary, and Tertiary Follicles of de Graaf====
In the mammalian ovary the developing egg with its associated cells is
called the Graafian follicle, so named after the Dutch scientist, Reinier de
Graaf (fig. 1), who first described this structure in mammals in 1672-1673.
De Graaf was in error, partly, for he believed that the whole follicular complex was the egg. The mammalian egg as such was first described in 1827 by Karl Ernst von Baer (1792-1876). The following statement is taken from
de Graaf relative to egg follicles.
We may assert confidently that eggs are found in all kinds of animals, since
they may be observed not only in birds, in fishes, both oviparous and viviparous,
but very clearly also in quadrupeds and even in man himself. Since it is known to
everyone that eggs are found in birds and fishes, this needs no investigation; but
also in rabbits, hares, dogs, swine, sheep, cows, and other animals which we have
dissected, those structures similar to vesicles exhibit themselves to the eyes of the
dissectors like the germs of eggs in birds. Occurring in the superficial parts of
the testicles, they push up the common tunic, and sometimes shine through it, as
if their exit from the testis is impending. (See fig. 48; also Corner, ’43, page 128.)
The mammalian egg with a single layer of epithelial cells surrounding it
is known as a primary Graafian follicle (fig. 41B-D). As the egg and follicle
grow, the number of epithelial cells increase and eventually there are several
Fig. 43. Tertiary conditions of the Graafian follicle in the opossum ovary. Similar conditions are found in other mammalian ovaries. (A) Follicle in which the antral vacuoles
are beginning to form. (B) This is a follicle in which the antral vacuoles are more
numerous and are beginning to coalesce. (C) Condition of the Graafian follicle in the
opossum ovary approaching maturity. Observe that the antral space is large and is filled
with fluid, the liquor folliculi, while the egg and its surrounding cumulus cells are located
at one end of the follicle. The thecal tissue around the follicle is well developed.
Fig. 44. Cellular wall of the mature Graafian follicle in the opossum.
layers of epithelial or granulosa cells surrounding the egg. It may now be
regarded as a secondary Graafian follicle (fig. 42 A, B). When a stage is reached
where the granulosa cells form a layer five to seven or more cells in thickness
extending outward from the egg to the forming thecal layers, small antral
vacuoles begin to appear among the granulosa cells. The latter follicle, which
is capable of forming antral vacuoles, may be regarded as a tertiary Graafian
follicle (fig. 43A).
====4. Hormonal Factors Concerned with the Development of Egg Follicles====
The ovary with its contained egg follicles is greatly affected by the gonadotrophic hormones produced in the pituitary body. The removal of the pituitary body (hypophysectomy) causes profound regression of the ovary and
accessory reproductive structures. Accordingly, the response of the ovarian
tissues to these hormonal substances produced by the hypophysis is responsible
for development of the Graafian follicle beyond the early tertiary stage. (See
fig. 40 A.) The relationships between the pituitary hormones and the ovary
have been studied most intimately in the mammals; the pituitary and eggfollicle relationship in lower vertebrates is more obscure, and probably varies
with the particular group.
a. Effects Produced by the Gonadotrophic Hormones on the Development of the Mammalian Egg Follicle
The follicle-stimulating hormone, FSH, appears to increase the number
of oogonia and to aid the growth and differentiation of the older follicles. It
is possible that some of the effects of FSH upon follicular growth are mediated through its ability, together with small amounts of the luteinizing hormone, LH (ICSH), to cause the formation of estrogen or the female sex hormone, although some investigators believe that estrogen production depends mainly upon the action of LH (ICSH). (See Evans and Simpson in
Pincus and Thimann, ’50, p. 355.) In harmony with the idea that estrogen
is involved in follicular growth there is some evidence which suggests that
introduction of estrogens into the peritoneal cavities of fishes and mammals
results in a stimulation of mitotic activity in the germinal epithelium of the
ovary. It also has been shown that estrogenic substances retard ovarian atrophy
in hypophysectomized immature rats.
When the Graafian follicles of the mammalian ovary reach the proper
morphological and physiological conditions (i.e., when they reach the tertiary
follicular stage) an increased sensitivity of the follicle cells to FSH occurs.
As a result, antral vacuoles filled with fluid appear among the granulosa cells;
these eventually coalesce and form the large antral cavity typical of the
mature Graafian follicle of the mctatherian and eutherian mammal (fig. 43).
The presence of LH (ICSH) is necessary to augment the action of FSH
during the latter part of follicle development. The beneficial action of FSH
and LH together in later follicular development is shown by the fact that the
injection of pure FSH alone is incapable of stimulating growth of the follicle
to its full size or to initiate an increased secretion of estrogen. LH aids the
maturing process of the follicle only when present in very minimal amounts
during the early stages of follicle development and in larger amounts during
the later stages of follicular growth. Large amounts of LH in the earlier phases
of the follicle’s development bring about a premature luteinization of the
follicle with ultimate atresia. A proper quantitative balance of these hormones,
therefore, is necessary, with FSH being in the ascendency during the earlier
phases of follicle development, followed by increased amounts of LH with
decreasing amounts of FSH as the follicle reaches maturity (figs. 22, 53, 59).
(For references, consult Evans and Simpson, ’50; Turner, ’48.)
h. Stimulating Effects of the Pituitary Gonadotrophins on the Ovaries of Other Vertebrates
The hormonal control of the developing follicle of other vertebrate ovaries
follows similar principles to those outlined above for the mammalian ovary,
although data obtained from studies upon other vertebrates in no way compares with the large quantity of information obtained in mammalian studies.
In the hen, FSH and LH injected together cause a rapid development of the
follicles and premature discharge of the egg from the ovary (Fraps, Olsen,
and Neher, ’42). However, in the pigeon. Riddle (’38) reports that another
pituitary hormone, prolactin, appears to decrease the production of these
hormones and stops egg production with a subsequent atrophy of the ovary.
This may be a special means which reduces the number of eggs laid at each
nesting period. In regard to accessory reproductive structures, an estrogenic
hormone is produced in the ovary of the hen which has profound stimulating effects upon the growth of the oviduct (Romanoff and Romanoff, ’49, pp.
242-244). In the frog, Rana pipiens, mammalian pituitary gonadotrophins
are able to effect ovulation (Wright and Hisaw, ’46). Pituitary gonadotrophins
have been shown also to have profound stimulative effects on the ovaries of
fishes, salamanders, and reptiles.
====5. Structure of the Vertebrate, Mature Egg Follicle====
As a result of the differentiation and growth induced by the gonadotrophic
hormones of the anterior lobe of the hypophysis described in the preceding
paragraphs, the egg follicle reaches a state of maturity (fig. 43C). This state
is achieved when the follicle is about to rupture with the resultant discharge
of the egg. The size of the mature egg follicle varies greatly in different metatherian and eutherian mammals, although the size of the follicle is not related
to the size of the egg. On the other hand the size of the mature egg follicle
in prototherian mammals and in other vertebrate species shows great divergences, being dependent in this group upon the size of the egg at the time
of ovulation (fig. 46).
a. Structure of the Mature Follicle in Metatherian and Eutherian
Mammals"'^
The structural pattern of the mature Graafian follicle in the human is
strikingly similar to the follicles in other members of this group. It is a vesicular
structure with a diameter approximating five millimeters. Externally, the follicle is composed of two connective-tissue layers, an inner cellular layer containing blood capillaries, the theca interna, and an external, fibrous layer,
the theca externa (figs. 43C, 44). These two layers are not clearly separable.
Passing inward from the theca interna is the basement membrane. Resting
upon this membrane are several layers of epithelial cells comprising the
membrana granulosa. The latter membrane borders the cavity or antrum of
the follicle, which is filled with the liquor folliculi. This liquid is under considerable pressure in the follicle at the time of egg discharge or ovulation.
Projecting inward into the antrum on one side is a small, mound-like
mass of granulosa cells, the cumulus oophorus (fig. 43C). Within this hillock
of epithelium, is the egg, which measures in the human about 130 /x to 140 fx
in diameter. In the opossum, the fully developed Graafian follicle is about
1.25 by 2 mm. in diameter, while the slightly oval egg approximates 120 by
135 ii. The egg of the rat and mouse is small, having a diameter of 75 ju,
while that of the dog is about 140 /x; sow, 120 to 140 /x; rabbit, 120 to 130 /x;
monkey, 110 to 120 /x; deer, 115 /x; cat, 120 to 130 (x\ mare, 135 /x; armadillo, 80 /X (Hartman, ’29).
* According to Strauss, ’39, the mature Graafian follicle of Erkulus is not a vesicular
structure, as in other higher mammals, but is filled with a loose meshwork of granulosa
cells.
While one Graafian follicle in only one ovary is generally developed in
the human, monkey, cow, ewe, elephant, etc., at each reproductive period,
a multiple condition is found in many other mammals. Each ovary in the
opossum may ripen seven or more follicles, in the bitch (female dog) from
2 to 7 follicles, and in the sow from 4 to 10 follicles at each reproductive period.
b. Structure of the Prototherian Egg Follicle
The follicle of the prototherian mammals contains a relatively large egg,
while the surrounding fluid and follicular tissue in comparison is small in
quantity (fig. 46). In these mammals the egg fills most of the follicular cavity,
with the exception of a small fluid-filled space intervening between it and
the zona pellucida which lies contiguous to the granulosa cells. Internal and
external thecal tissues surround the granulosa cells as in the Graafian follicle
of the higher mammals.
c. Egg Follicles of Other Vertebrates
The fully-developed egg follicle in most vertebrates is similar to that found
in the prototherian mammals in that the egg tends to fill the entire follicle.
The general structural relationships also are similar (figs. 45, 47).
====6. Ovulatory Process; Possible Factors Controlling Ovulation====
The following description of the ovulatory process in the mammal and in
other vertebrates should not be construed as a description of the mechanism,
as the exact mechanism is unknown. However, a certain amount of general
information has been obtained concerning ovulation and the factors involved.
Much of this information has been obtained from studies of the ovulatory
Fig. 45. (A) Young egg follicle of Cryptobranchus alleganiensis, a urodele. (From
Noble: “Biology of the Amphibia,” New York, McGraw-Hill, after Smith.) (B) Diagrammatic representation of ovarian events in the frog resulting in egg discharge. (From Turner: “General Endocrinology,” Philadelphia, W. B. Saunders, slightly modified.)
Fig. 46. Diagrammatic representation of the egg of the prototherian mammal. Echidna.
Fig. 47. Diagrammatic drawings of the pendent egg follicle in the ovary of the hen.
(A) Low magnification of the entire egg follicle. (B) More detailed view of the blastodisc portion of the egg, nearing maturity, in relation to the pedicle. The latter supports
the follicle and permits the blood vessels to pass into and out of the follicle. Compiled
from sections of the developing ovary of the hen.
process in higher mammals, especially the rabbit. Among other vertebrates
ovulation in the hen and frog have been the objects of considerable study.
a. Process of Ovulation in Higher Mammals
1) Changing Tissue Conditions Cuhninating in Egg Discharge from the
Ovary. As the Graafian follicle enlarges and matures under the influence of the follicle-stimulating and luteinizing hormones, it moves closer to the ovarian
surface (fig. 30). The surface of the ovary over the ripening follicle bulges
outward, forming a mound-like protuberance (fig. 30). In the rabbit as shown
by Walton and Hammond (’28) and Hill, Allen, and Cramer (’35) the central part of the original protuberance pushes out still further and forms a
papilla-like swelling (fig. 48A-D). As the papilla develops, it becomes avascuiar, and the underlying tissues become thin and greatly distended. The
tunica albuginea of the ovary and the two thecal layers of the follicle also
are involved in this thinning-out process. As the distended papillary area
continues to grow thinner, a small amount of blood followed by some of the
follicular fluid containing the egg emerges from the follicle and passes into
the surrounding area in close proximity to the infundibulum of the Fallopian
tube (fig. 48 E, F). The entire process is a gradual one and may be described
as gently but not violently explosive (Hill, Allen, and Cramer, ’35). It is of
interest and significance to observe that Burr, Hill, and Allen (’35) were able
to detect a change in electromotive force preceding and during the known
period of ovulation.
Fig. 48. Process of ovulation in the rabbit. (A-C) Early external changes of the
surface of the ovary overlying the bulging Graafian follicle. (D) Formation of a secondary papilla. (E) Rupture of the secondary papilla with discharge of egg and follicular fluid, the latter oozing down over ovarian surface of the follicle. (F) Area of
rupture with oozing follicular fluid and egg greatly magnified. (G) Follicle after egg
discharge. (A-E and G, slightly modified from Walton and Hammond, Brit. J. Exp.
Biol., 6; F, modifier from Hill, Allen, and Kramer, Anat. Rec., 63.)
The process of papillary rupture in the rabbit occupies about five seconds;
egg discharge with the surrounding liquor folliculi occurs in approximately
30 to 60 seconds. After the egg has emerged, the follicle as a whole may
collapse. The slit-like opening through which the egg and follicular fluid
passed during ovulation soon is filled with a clot composed of coagulated
blood and follicular fluid (fig. 48G).
While the foregoing processes, visible on the ovarian surface, are consummated, certain internal changes occur which form a part of the ovulatory procedure. These changes arc as follows: At about the time the egg is to be
extruded, the follicular fluid reaches its maximum in quantity. This increase
produces considerable follicular turgidity which may be associated with an
endosmotic effect due to an increase in the salt content of the contained fluid.
Shortly before the surface of the follicle ruptures, the cumulus begins to disintegrate, and the egg lies free in the antral fluid. At about this time the first
maturation division of the oocyte occurs in the majority of mammals, and
the first polar body is extruded.
Concerning the internal changes accompanying rupture of the mammalian
follicle, passing mention should be made of the theory that bursting blood
vessels discharge their contents into the follicular fluid and thus cause sufficient pressure to rupture the follicle (Heape, ’05). Considerable blood discharge into the follicle seems to be present in some forms, e.g., the mare,
quite absent in others such as the human, and present slightly in the opossum.
2) Hormonal Control of the Ovulatory Process. The hormonal mechanism
involved in ovulation in the spontaneously-ovulating mammals probably is as
follows: The follicle-stimulating hormone causes the growth and development
of the follicle or follicles. Estrogen is released by the growing follicles and
possibly by other ovarian tissues due to the presence of small amounts of LH,
and, in consequence, the estrogenic hormone reaches a higher level in the
blood stream (figs. 53; 59).
In the meantime, it is probable that the corpus luteum hormone, progesterone, is produced in small amounts. The exact source of this hormone is
not clear. It may be produced by old corpora lutea or by the interstitial tissue of the ovary under the influence of luteotrophin, LTH. The presence of
progesterone, in small quantities together with increasing amounts of estrogen, stimulates the anterior lobe to discharge increased amounts of the luteinizing hormone, LH (ICSH). (See figs. 22, 53, 59.) The elevated level of
estrogen, according to this theory also causes a decreased output of FSH until
it reaches a minimal level at the period shortly before egg discharge (figs.
53, 59). As a result, the increased quantity of LH together with FSH has an
added effect upon the follicle which brings about the chain of events leading
to egg discharge. Evans and Simpson in Pincus and Thimann (’50) give the
proportion of 10 parts of FSH to 1 of LH (ICSH) as the proper hormonal
balance in effecting ovulation in the hypophysectomized rat.
In those mammalian species where ovulation is dependent upon the act of
copulation, a nervous stimulus is involved which increases the output from
the pituitary gland of the gonadotrophic factors, particularly LH.
b. Ovulation in Vertebrate Groups Other Than the Higher Mammals
The physical mechanism involved in the ovulatory procedure in the lower
vertebrate classes is different from that found in higher mammals. Two forms,
the hen and the frog, have been studied in detail. These two animals represent
somewhat different types of ovulatory behavior.
1) Hen. As the hen’s egg develops in the ovary, it gradually pushes the
ovarian surface outward; it ultimately becomes suspended from the general
surface of the ovary by means of a narrowing stalk, the pedicle (figs. 31, 47).
When the ovulatory changes are initiated, the musculature of the ovarian
wall overlying the outer surface of the egg appears to contract, and an elongated narrow area along this outer surface becomes avascular. This avascular
area represents the place where the ovarian surface eventually ruptures to
permit the egg to leave the ovary; it is called variously, the rupture area,
stigma, or cicatrix. Gradually, the cicatrix widens and finally a slit-like opening is formed by a tearing apart of tissues in the central region of the cicatrix.
Contractions of the smooth muscle fibers appear to be responsible for this
tearing procedure (Phillips and Warren, ’37). The egg eventually is expelled
through the opening and in many instances it rolls into the infundibular funnel
of the oviduct which at this time is actively engaged in an endeavor to engulf
or “swallow” the egg (fig. 31).
2) Frog. The egg of the frog projects into the ovarian cavity within the
ovary and is attached to the ovarian wall by means of a broad area or stalk
(fig. 45B). As the egg enlarges, it tends to push the ovarian surface outward,
and the egg and its follicle thus forms a mound-like protuberance from the
ovarian surface (figs. 45A, B; 72F). The egg and the surrounding ovarian
tissue thus lies exposed on one aspect to the outer surface of the ovary. The
outer surface of exposure is the stigma or area of rupture, and in the older
follicles this area does not contain blood vessels (fig. 72F). As ovulation
approaches, an opening suddenly appears in the area of rupture. The musculature within the theca interna around the follicle then contracts, and the
egg rolls out through the opening in the rupture area like a big ameba (fig.
45B). As the egg passes through the aperture, it may assume an hourglass
shape (Smith, B. G., T6). After the egg is discharged, the follicle contracts
to a much smaller size (fig. 45B). It has been suggested that the rupture of
the external surface of the follicle might be produced by a digestive enzyme
(Rugh, ’35, a and b).
3) Hormonal Control of Ovulation in Lower Vertebrates. The hormonal
mechanism regulating ovarian rupture and egg discharge in the lower vertebrate groups has not been as thoroughly explored in all of the vertebrate
groups as it has in the mammals. However, sufficient work has been done to
demonstrate that pituitary hormones are responsible in all of the major vertebrate groups, including the fishes. Amphibian pituitary implants under the
skin or macerated anterior-lobe pituitary tissue injected into the peritoneal
cavity of various amphibia have been effective in producing ovulatory phenomena (Rugh, ’35a). More recently, purified mammalian follicle-stimulating
hormone, FSH, and luteinizing hormone, LH, have been used to stimulate
egg discharge in frog ovarian fragments, as well as in normal and hypophysectomized females. However, the follicle-stimulating hormone alone will not
elicit ovulation (Wright, ’45; Wright and Hisaw, ’46). Accordingly, both
factors are necessary in the frog, as in mammals. In the hen, these two pituitary hormones have been shown to bring about ovulation when injected
intravenously (Fraps, Olsen, and Ncher, ’42; Romanoff and Romanoff, ’49,
pp. 208-215). Also, Neher and Fraps (’50) present evidence which suggests
that progesterone plays a part in the physiological chain which elicits ovulation
in the hen. A close relationship between the physiological procedures effecting
ovulation in the hen and the mammal thus appears to exist.
c. Comparison of the Immediate Factors Effecting Egg Discharge in the
Vertebrate Group
In the vertebrates thus far studied contraction of muscle tissue of the follicle following the rupture of surface tissues presumably is the main factor
which brings about egg expulsion. In higher mammals, associated with muscle
contracture, there also may be an increase in follicular turgidity due to endosmotic phenomena associated with the contained follicular fluid (Walton and
Hammond, ’28). In the frog, hen, and mammal the changes involved in the
surface tissues leading to their rupture are associated with the following sequence of events:
( 1 ) avascularity of the surface tissues,
(2) a thinning of the surface tissues, and finally
(3) a rupture of these tissues.
====7. Internal Conditions of the Ovary as an Ovulatory Factor====
Internal conditions of the ovary undoubtedly are important in controlling
follicular growth and ovulation. For example, in the Northern fur seal,
Callorhinus ur sinus, the female begins to breed at the age of two years. These
seals travel north once a year to the Pribilof Islands in the Bering Sea where
they go on land to give birth to the single young and also to breed. Most of
the cows arrive between the middle of June and the middle of July. Heavy
with young, the females give birth to their offspring within a few hours or
days after their arrival. Breeding again takes place about six days after parturition. However, lactation continues, and the young are taken care of during
the summer months.
Accordingly, these seals mate each year and it appears that for any particular year the mating behavior and ovulation of the egg are controlled by
the ovary, which does not have a corpus luteum. As the corpus luteum, which
forms after ovulation in the site of the Graafian follicle, from which the egg
is discharged, remains intact for a considerable portion of the year, the ovary
which does not have the corpus luteum develops the Graafian follicle for the
next summer period. The following year the other ovary will function, and
so on, alternating each year (Enders, et al., ’46). Thus, the corpus luteum
appears to function as a suppressor of follicular growth within the ovary in
which it lies. In the human female, one ovary functions to produce an egg
one month, while the following month the other ovary ovulates its single egg.
It is possible that here also the large corpus luteum suppresses follicular growth
within the particular ovary concerned.
During gestation, the presence of the corpus luteum and its hormone,
progesterone, suppresses follicle growth and ovulation in most of the mammalian group. (The placenta may be the source of progesterone during the
later phases of pregnancy in forms such as the human.) On the other hand,
in the mare, according to Cole, Howell, and Hart (’31 ), ovulation may occur
during pregnancy. Species differences, therefore, exist relative to the control
of ovulation by the corpus luteum and its hormone, progesterone.
====8. Number of Eggs Produced by Different Vertebrate Ovaries====
The number of eggs produced during the lifetime of the female varies with
the species and is correlated generally with the amount of care given to the
young. In many fishes which experience little or no parental care, enormous
numbers of eggs may be produced, as for example, in the cod where several
millions of eggs are spawned in one season. However, in many of the elasmobranch fishes (i.e., the shark group) the eggs develop within the oviduct,
and the young are born alive. Therefore, only six to a dozen eggs produced
each reproductive period is sufficient to keep the shark species plentiful. In
the hen, where careful breeding and selection have been carried out with a view
to egg production, a good layer will lay from 250 to 300 eggs a year. The deer, moose, fur seal, etc., ovulate one egg per year over a life span of a
few years. As stated previously, the human female might ovulate as many
as 400 eggs in a lifetime. In some species the reproductive life is brief. For
example, in the Pacific salmon (Oncorhynchus) females and males die after
their single spawning season, and a similar demise occurs in the eel (Anguilla).
====9. Spontaneous and Dependent Ovulation in the Mammals and in Other Vertebrates====
Spontaneous ovulation without apparent stimulation from external sources
occurs commonly throughout the vertebrate series. However, dependent ovulation conditioned by psychic or other nervous stimuli also is found extensively. In certain mammals ovulation has been shown to be dependent upon
the stimulus induced by copulation, as, for example, the ferret, mink, rabbit,
cat, shrew, etc. The stimulus, carried through the nervous system, affects in
some way the anterior lobe of the pituitary gland which then produces increased amounts of LH in addition to FSH. These females experience estrus
spontaneously, but later follicle growth and egg discharge are dependent upon
the added stimulation afforded by copulation.
The element of nervous stimulation has a fundamental relationship to the
ovulatory phenomena in the vertebrates. Dependent ovulation occurs in certain birds, such as the pigeon, where mating provides a psychic or nervous
stimulation which effects ovulation. The presence of two eggs in the nest tends
to suppress ovulation. The removal of these eggs will arouse the ovulatory
procedures. However, the pigeon may sometimes lay eggs without the presence
of a male. In wild birds in general, the mating reaction is linked to the
stimulus for egg laying. The hen, on the other hand, is not dependent upon
copulation, but in many of the domestic varieties the presence of a number
of eggs in the nest appears to suppress egg laying. In the lower vertebrates
nervous stimuli also appear to have an influence upon ovulation. The mating
antics of many fish and amphibia may be connected with ovulatory phenomena.
====10. Egg Viability after Discharge from the Ovary====
The length of time that the egg may survive and retain its capacity for
fertilization after leaving the ovary depends upon the nature of the egg and
its membrane and the surrounding environment. In the urochordate, Styela,
the egg may remain for 3 to 4 hours after it is discharged into the sea water
and still be capable of fertilization. In the elasmobranch fishes, reptiles, and
birds the conditions of the oviduct are such that fertilization must take place
in the upper part of the oviduct within a few seconds or minutes after the
egg reaches the infundibular portion. In Fundulus hetewclitus and possibly
many other teleost fishes, the egg must be fertilized within 15 to 20 minutes
after spawning. In the frog, the egg passes to the uterus at the lower end of
the oviduct shortly after it leaves the ovary. Under ordinary reproductive temperatures which obtain in the spring, the egg may remain there for 3 to 5
days without producing abnormalities. If kept at very cool temperatures,
the period may be extended. Among the mammals the viability after ovulation
varies considerably. In the mare, fertilization must occur within about 2 to 4
hours; rabbit, 2 to 4 hours (Hammond and Marshall, ’25); rat, about 10
hours; mouse, 12 to 24 hours (Long, ’12; Charlton, ’17); opossum, probably
within the first hour or so because of the deposition of the albuminous coating
in the oviduct; fox, probably only a few hours; sow, about 24 hours or less;
man, probably 24 hours or less. In the guinea pig, functional degeneration
may begin within 4 to 8 hours after ovulation (Blandau and Young, ’39) .
====11. History of the Egg Follicle after Ovulation====
a. Follicles Which Do Not Develop a Post-ovulatory Body
The changes which occur within the egg follicle after the egg has departed
are most variable in different vertebrate species. In most of the fish group
the ovary as a whole shrinks to a fraction of its previous size, and many
very small, immature eggs, interstitial tissue, and collapsed, contracted, empty
follicles make up its composition. Similarly, in frogs, toads, and salamanders
the collapsed follicle which follows ovulation does not develop an organized
structure. The thecal tissue contracts into a small rounded form within which
are a few follicle cells (fig. 45B). These bodies soon disappear.
In many snakes and in turtles, the follicle collapses after ovulation, and it
is questionable whether organized bodies develop in the site of the ovulated
follicle. A similar condition appears to be the case in birds. However, Pearl
and Boring (’18) described an abbreviated form of a corpus luteum in the
hen in both discharged and atretic follicles. Also, Rothschild and Traps (’44)
found that the removal of the recently ruptured follicle or of this follicle together with the oldest maturing follicle, at a time when the egg which originated from the ruptured follicle is in the oviduct, retarded the laying of the
egg from 1 to 7 days. Removal of other portions of the ovary in control
hens “practically never” resulted in egg-laying retardation. The ruptured follicle, therefore, is believed, by these investigators, to have some influence on
the time of lay of the egg. Whether the hormone progesterone or something
similar to it may be produced by the ruptured follicle of the hen is questionable, although present evidence appears to suggest that it does (Neher
and Traps, ’50).
b. Follicles Which Develop a Post-ovulatory Body; Formation of the
Corpus Luteum
Post-ovulatory bodies or corpora lutea (yellow bodies) develop in the
ovaries of elasmobranch fishes which give birth to their young alive. Also
in viviparous snakes of the genera Natrix, Storeria, and Thamnophis, it has been shown that the removal of the ovaries with their corpora lutea invariably
results in resorption of the young during the first part of gestation and abortion
of the young during the midgestational period, while their removal during
the close of gestation permits normal birth to occur (Clausen, ’40). The
differentiation of the corpus luteum in the snake involves the granulosa cells
of the follicle and possibly the theca interna. The differentiated organ appears
similar to that of the mammal (Rahn, ’39).
The function of the corpus luteum which develops in the site of the ruptured follicle in all mammals, including the Prototheria (fig. 49), has been
the subject of a long series of studies. (See Brambell, ’30, Chap. 9; Corner,
’43, Chap. V.) Its function during the reproductive period of the female
mammal is described below under the section of the ovarian hormones.
The events leading to the formation of the corpus luteum in the mammalian
ovary may be described as follows: After the discharge of the egg, the follicle
collapses. The opening of the follicle at the ovarian surface through which
the egg emerged begins to heal. A slight amount of blood may be deposited
within the antrum of the follicle during the ovulation process in some mammals. If so, the follicle in this condition is known as the corpus hemorrhagicum.
Fig. 49. (A) Luteal cells of the corpus luteum of the opossum. The cellular conditions
in other higher mammals are similar. The centsal core has not yet been invaded and resorbed by the phagocytes accompanying the ingrowing luteal cells and blood vessels. This
central core is composed of coagulated blood, blood cells, and connective tissue fibrils.
(B) Corpus luteum of the platypus (Ornithorhynchiis).
Then, under the influence of the luteinizing hormone, LH, the granulosa cells
of the follicle and also cells from the theca interna, together with blood capillaries, proliferate and grow inward into the antral space (figs. 22, 30, 49).
Phagocytes remove the blood clot within the antral space if present, during
the inward growth of these structures. As the ingression of cells and capillaries into the follicle continues, the granulosa cells begin to form large, polyhedral lutein cells, while the epithelioid cells of the theca interna form a
mass of smaller cells which resemble the true lutein cells; the latter are formed
in the peripheral area of the corpus luteum and are called paralutein cells.
The small spindle-shaped cells of the theca interna, together with blood capillaries, become dispersed between the lutein cells, forming a framework for
the latter.
If the egg is fertilized, the corpus luteum persists and is known as the
corpus luteum of pregnancy; if fertilization does not take place, it is called
the corpus luteum of ovulation. The latter body soon degenerates. Histologically, both types of corpora are identical when first formed. Eventually the
corpus luteum undergoes involution, and its site becomes infiltrated with
connective tissue. The latter structure is sometimes referred to as the corpus
albicans.
====12. Hormones of the Ovary and Their Activities in Effecting the Reproductive Condition====
The ovary produces two important hormones which have a profound effect
upon the reproductive process. These two hormones are the female sex hormone, estrogen, and the gestational hormone, progesterone.
a. Estrogenic Hormone
1) Definition and Source of Production. The induction of estrus (see p.
93 ) or conditions simulating this state is a property of a relatively large number
of organic compounds. Because of this estrus-inducing power, they are spoken
of as estrogenic substances or estrogens. Estrogens are widely distributed in
nature. Two of the most potent natural estrogens are estradiol and estrone
(theelin). Both have been extracted from the mammalian ovary and are
regarded as primary estrogenic hormones. The most powerful estrogen is
estradiol, and it is regarded at present as the compound secreted by the ovary.
During pregnancy it also is found in the placenta. These structures are not
the only sources of estrogens, however, for it is possible to extract them from
urine after ovariectomy, and they occur in the urine of males as well as that
of females. The urine of the stallion is one of the richest sources of estrogens,
and the testis contains a high estrogenic content (Pincus and Thimann, ’48,
p. 381 ). Estrogens are found also in various plants, such as the potato, pussy
willow, etc.
The structural formulae of estradiol and of estrone are as follows:
OH ()
Estradiol Estrone
2) The Ovary as the Normal Source of Estrogen in the Non-pregnant
Female. Aside from the fact that estradiol and estrone are readily extracted
from the ovary, certain experiments tend to focus attention on the ovary as
an important site of estrogen production. For example, the removal of the
ovaries of a normal, adult female mammal causes the accessory reproductive
organs to undergo profound atrophy. The administration of appropriate
amounts of estrogen will restore the accessories of such a female to the condition normal for the resting state. (Consult Pincus, ’50, in Pincus and
Thimann, Chap. I.) The injection of follicle-stimulating hormone with small
amounts of the luteinizing hormone into the diestrous (i.e., sexually-resting)
female with intact ovaries results in follicular development within the ovaries,
accompanied by hypertrophy of the accessory reproductive organs to the full
estrous condition (Nelsen and White, ’41 ; Pincus, ’50, in Pincus and Thimann) .
These and similar experiments point to the ovary as the main site of estrogen
formation in the body of the non-pregnant female.
The exact structures of the ovary responsible for estrogen elaboration are
not easily determined. Estrogen is found in all parts of the ovary, but certain
observations and experimental results suggest that it is formed in relation
to the follicular tissues and also by the so-called interstitial tissue of the
ovary. For example, when tumors occur within the thecal tissue of the egg
follicle in women who have experienced the menopause, there is often an
accompanying hypertrophy of the accessory organs. This relationship suggests
that thecal gland tissue of the follicle may have the ability to elaborate estrogen (Geist and Spielman, ’43). On the other hand, the normal hypertrophy
of the granulosa cells of the egg follicle during the normal reproductive cycle,
with the presence of follicular fluid containing estrogen in the antral space
of the follicle, points to the granulosa cells as a possible source of estrogen.
Also, it has been observed that tumorous growths of the granulosa cells of
the follicle produce an excess of estrogenic substance (Geist and Spielman,
’43). Thus, these observations point to the granulosa cells of the egg follicle
of the ovary as being capable of estrogen formation. Another possible source
of estrogen secretion in the ovary is the interstitial cells, derived in part
from theca interna tissue and atretic follicles. These cells are large polyhedral
epithelioid cells scattered between the follicles. Their growth appears to be
directly stimulated by the injection of pure luteinizing hormone (LH; ICSH) in hypophysectomized rats (fig. 40). A rapid production of estrogen results
from such injections and this may mean that these cells are involved in
estrogen production within the ovary (Evans and Simpson in Pincus and
Thimann, ’50).
In the pregnant female mammal the placenta appears to be a source of
estrogen production (Pincus and Thimann, ’48, p. 380; Turner, ’48, p. 422).
This is suggested by the successful extraction of estrogen from the placenta
of the human and the mare and also by the fact that in these females removal
of the ovaries during the middle or latter phase of gestation does not result
in estrogen diminution in urinary excretion.
3) Pituitary Control of Estrogen Formation. The removal of the anterior
lobe of the pituitary gland of the female results in marked atrophy of ovarian
structures (figs. 40, 50) and of the accessory reproductive organs. Replacement therapy (i.e., the injections of the pituitary gonadotrophins, FSH and
LH) produces a normal reconstitution of the ovarian and reproductive duct
tissues, effecting a normal appearance and functioning of these structures
Fig. 50. Follicular atresia in guinea pig ovary. (Redrawn from Asdell, ’46.) This atresia
is a sporadic but not uncommon event in the normal ovary of the mammal. However,
after removal of the pituitary gland, marked atresia and degeneration of the more mature
follicles occur. (A) Fragmentation of granulosa cells is shown. (B) Beginning invasion of the antral space by theca interna tissue is depicted. (Cf. fig. 40A.) (C) Late stage of atresia with invasion of the antral space by internal thecal cells.
Fig. 51. Effects of estradiol (estrogen) upon the female genital tract of the opossum.
(After Risman, J. Morphol., 81.) (A) Reproductive tract of an ovariectomized female.
(B) Hypertrophied condition of a female experiencing the normal estrous changes. (C)
Reproductive tract of an ovariectomized female injected with estradiol (0.9 mm.) 36
days after the ovaries were removed.
(fig. 40). This evidence suggests that the pituitary gonadotrophins, FSH and
LH, control the development of the ovary and, through their influence upon
the ovarian tissues, promote the secretion of estrogen with the subsequent
hypertrophy of the female accessory reproductive structures. It is to be observed that it is not at all clear that FSH in pure form is able to elicit estrogen
production without the presence of LH (ICSH). (See Evans and Simpson
in Pincus and Thimann, ’50, p. 355.)
4) Effect of Estrogen upon the Female Mammal. The changes in the
mammalian accessory reproductive organs produced by estrogen are marked.
An increase in vascularity and great hypertrophy of the accessory structures
result from its injection into ovariectomized females. (See figs. 51, 52, 53.)
Increased irritability and activity of the accessory structures also occur. This
increased activity appears to be an important factor in the transportation of
sperm upward within the female accessory organs to the region where the
egg awaits the sperm’s arrival.
The alterations in behavior of the female as a result of estrogen stimulation
may be considerable. Females actually seek the presence of a male during
the period of strong estrogenic influence. The long journey of the female fur
seal to the mating grounds in the Bering Sea, the bellowing and tireless search
of the cow moose, the almost uncontrollable demeanor of seeking the male
on the part of the female dog or of the cow in “heat” — these are a few illustrations of the regnant power of this stimulant upon the female mammal.
The culmination of these changes in behavior, resulting in a receptive attitude
toward the male, is reached at about the time when the egg is discharged
from the ovary in many mammalian species. In certain other mammals the
period of heat may precede the ovulatory phenomena.
5) Effects of Estrogen in Other Vertebrates. In the hen, estrogenic hormone causes enlargement and functional activity of the oviduct. Estrogenic
substance, when injected into female chicks from the eighteenth to the fortieth
day, causes an enlargement of the oviduct to about 48 times the natural size.
Estrogen also has a profound effect upon the activities of the full-grown hen
and aids in egg production (Romanoff and Romanoff, ’49; Herrick, ’44).
Estrogen has a pronounced effect upon the oviducts of other vertebrate forms.
b. Progesterone - The Hormone of the Corpus Luteum
1) Production of Progesterone. The luteinizing hormone, LH, of the anterior lobe of the pituitary gland is concerned not only with the development of the egg follicle, but also, after ovulation or the discharge of the egg from
the egg follicle, the remaining granulosa cells, and also, some of the theca
interna cells of the follicle are induced by the LH factor to form the corpus
luteum (figs. 30, 49). Corpora lutea also may be induced by estrogens. This,
however, appears to be an indirect stimulus aroused through estrogenic stimulation of the pituitary gland to secrete added amounts of the LH factor (Evans
and Simpson in Pincus and Thimann, ’50, p. 359).
Fig. 52. Characteristic histological changes in the female reproductive tract under the
influence of estrogen and progesterone. (A-C) Vaginal cyclic changes in the rat. In
(A) is shown the condition of the vaginal wall in the diestrus (resting) condition; (B)
shows changes in vaginal wall structure during estrus. Observe cornification of outer layer
of cells; (C) shows vaginal wall tissue immediately following estrus, i.e., during metestrus.
The presence of progesterone tends to suppress the action of estrogen. (After Turner:
General Endocrinology, Philadelphia, Saunders.) (D, E) Cyclic changes of the Fallopian tube of the human female during the reproductive cycle. In (D) is shown the midinterval of the cycle, i.e,, at a time paralleling estrus in mammals in general; (E) shows
the cellular condition of the lining tissue of the Fallopian tube just before menstruation.
In (D) the tissue has responded to the presence of estrogen; (E) effect of progesterone
is shown. (After Maximow and Bloom: A Textbook of Histology, Philadelphia, Saunders.)
(F, G) Cyclic changes in the uterine-wall tissue during the reproductive cycle in the
human female. In (F) is shown general character of the uterine wall during the follicular
phase, i.e., responses to estrogen; (G) shows the general condition of the uterine wall
following ovulation. The uterus is now responding to the presence of progesterone added
to the follicular or estrogenic stimulation. (After Maximow and Bloom: A Textbook of
Histology, Philadelphia, Saunders.)
A further pituitary principle, however, seems to be involved in the functional behavior of the corpus luteum. This principle, referred to as luteotrophin
(LTH), is associated with the lactogenic-hormone complex produced by the
anterior lobe of the pituitary body; it induces the morphologically developed
corpus luteum to secrete progesterone. (Consult Evans and Simpson in Pincus
and Thimann, ’50, pp. 359, 360; Turner, ’48, p. 379, for references.)
The structural formula of progesterone is as follows:
2) Effects of Progesterone. Progesterone reduces the irritability of the accessory structures and stimulates the mucosa of the uterus to undergo further
development. This increased developmental and functional condition of the
Fig. 53. Relationship of the pituitary gonadotrophins and ovarian hormones to the developing Graafian follicle and reproductive-duct change in a polyestrous female mammal.
The Graafian follicle responds to the pituitary gonadotrophins, FSH and LH, with the
subsequent growth and ultimate rupture of the follicle and ovulation. Ovulation terminates the follicular phase of the cycle. Under the influence of the LH factor the corpus
luteum is established. The latter becomes functional as a result of stimulation by the
luteotrophic (lactogenic) hormone. The progestational hormone (progesterone) then is
elaborated by the luteal cells. The activity of the latter together with estrogen controls
the luteal phase of the cycle.
The rising level of estrogen in the blood suppresses FSH secretion, and together possibly with small amounts of progesterone stimulates LH secretion. Estrogen and small
amounts of progesterone also probably stimulate the secretion of large quantities of LTH,
and the latter stimulates the secretion of progesterone from the recently formed corpus
luteum. When the estrogen level falls, FSH again is secreted.
When the estrogen level rises, the endometrium of the uterus and vaginal mucosa are
stimulated. The presence of progesterone suppresses vaginal development, but the uterine
mucosa is stimulated to greater activity. Observe that the involution of the endometrial
lining in most mammals is gradual but in primates it is precipitous and violent, resulting
in menstruation (Cf. fig. 59). (The diestrous period on this chart is shown as a relatively brief period compared to the other aspects of the reproductive cycle. However,
it may be very long in females which do not experience a polyestrous condition and in
some species it may last a good portion of a year.) (Compiled from various sources in
the literature. The portion of the chart showing pituitary and gonadal hormonal relationships is based on data obtained from The Schering Corporation, Bloomfield, N. J.)
accessory reproductive structures added normally to the estrogenic effects
during the reproductive cycle constitutes the luteal phase of the cycle. In this
phase of the cycle the uterine glands elongate and begin secretion, and the
uterus as a whole is prepared for gestation as a result of the action of the
progestational hormone, progesterone, associated with estrogen. (See figs.
53, 59.)
===F. Reproductive State and Its Relation to the Reproductive Cycle in Female Vertebrates===
The changes in the female reproductive organs resulting in structural growth
and development referred to above (70-74, 85-88) are consummated in the
ability of the female to fulfill the reproductive functions. The phase of the reproductive events characterized by the ability to reproduce is known as the reproductive climax. This period of culmination remains for a brief period, to be
followed by recession and involution once again to a resting condition. This
developmental progression to a state of reproductive climax followed by regression to a resting condition constitutes a cycle of changing events. When
conditions again are right, the cycle is repeated. Each of these cyclic periods
is known as a reproductive or sexual cycle (figs. 53-59). The reproductive
life of all female vertebrates is characterized by this series of cyclic changes.
In most vertebrate species, the female experiences one sexual cycle per
year, which corresponds to the seasonal cycle in the male. However, in various
mammals and in certain birds, such as the domestic hen, several or many
reproductive cycles may occur during the year. The male, under these conditions, is a continuous breeder; that is, he produces sperm continuously throughout the year.
1. Sexual Cycle in the Female Mammal
a. Characteristics and Phases of the Reproductive Cycle
The estrous cycle in mammals is a complex affair composed of a number
of integrated subcycles. The changes occurring in the ovary are called the
ovarian cycle; the cellular changes in the uterine (Fallopian tube) form a
cycle; the responses in the mammary glands constitute the mammary cycle;
the cyclic events in the uterus make up the uterine cycle, while those in the
vagina form the vaginal cycle (figs. 53, 54, 57).
The entire estrous cycle may be divided by ovarian changes into two main
phases: the follicular phase and the luteal phase (fig. 53). The former is under
the immediate influence of the enlarging Graafian follicle, which in turn is
stimulated by the follicle-stimulating and luteinizing hormones of the pituitary
gland, with the subsequent production of estrogen. It is probable that the
luteinizing hormone, LH, is mainly responsible for estrogen secretion. (See
Evans and Simpson in Pincus and Thimann, ’50, p. 355.) The luteal phase on the other hand is controlled by the activities of the corpus luteum, which
has replaced the Graafian follicle under the influence of the luteinizing hormone. The production of progesterone by the corpus luteum is effected as
stated previously by the pituitary hormone, luteotrophin (LTH). Ovulation
is the pivotal point interposed between these two phases. The follicular phase
may occur without ovulation, but the true luteal phase of a normal or fertile
reproductive cycle is dependent upon the ovulatory phenomena. Certain luteal
conditions may be elaborated in an anovulatory cycle, but we are here concerned with the normal events of the fertile reproductive cycle.
The follicular phase includes that portion of the reproductive cycle known
as proestrus and a considerable part of estrus. Proestrus is the period of
rapid follicular growth and elaboration of the estrogenic substance which
precedes the period of estrus. Estrogen stimulates developmental changes in
the cellular structure of the accessory reproductive organs, particularly the
vagina and the uterus (figs. 52, 53). Estrus represents the climax of the follicular phase. As such, it is a period of sexual receptivity of the male, and,
in spontaneously ovulating forms, of ovulation. During other periods of the
cycle the female is indifferent or even antagonistic to the male. The period
of estrus is often called period of heat, or period of rut. Estrus is followed
by pregnancy if mating is allowed and is successful, or, in many species, by
a period of pscudopregnancy if mating is not permitted or if the mating is
sterile (figs. 53-57). In some animals, such as the dog, pseudopregnancy is
a prolonged normal event even if mating does not occur, continuing over a
period almost as long as that of normal pregnancy (fig. 54). In other animals,
such as the opossum, pseudopregnancy forms but a brief episode.
Pseudopregnancy is, generally speaking, intermediate in duration between
that of a normal luteal phase of the cycle and that of gestation. In those female
mammals where it does not occur normally, it is aroused by such procedures
as sucking of the nipples, stimulation of the vagina and cervix by the natural
mating process, or by artificially stimulating these structures. In some forms,
such as the rabbit, pseudopregnancy is aroused by mere handling or even by
sight of a male. (For discussion, see Selye, ’48, p. 813.)
The general changes of growth and development of the accessory organs
which occur during pregnancy and pseudopregnancy are controlled largely
by the secretions of the corpus luteum. The conditions thus imposed by the
corpus luteum comprise the luteal or progestational phai^e of the cycle (fig. 57).
In most mammals, if pregnancy does not occur, the ovary and accessory organs again gradually return to the sexually-resting condition known
as diestrus (fig. 53). In man and other primates the changes within the uterus
are not gradual but are precipitous, and most of the endometrial lining, together with considerable amounts of blood, is discharged to the outside (figs.
53, 59). This phenomenon is called menstruation. The causes of menstruation
are largely problematical; it is related to the fall of the level of either or both of the ovarian hormones, progesterone and estrogen. Why certain mammals
should experience violent endometrial changes evident in menstruation and
others a gradual involution and resorption is a question for the future. The
general period of change following estrus in a non-fertile cycle is known as
metestrus (fig. 53). In the rat and mouse, metestrus is short, about one or
two days; in the human and opossum it occupies approximately ten days to
two weeks of the cycle; in the dog, about 40 to 50 days, depending upon
the pseudopregnant conditions experienced in different females. The word
anestrus is applied to a prolonged diestrus or sexual quiescence between two
sexual cycles. However, the involution experienced by the sexual organs in
anestrus is somewhat more profound than that prevailing during a brief
diestrus. The term lactational diestrus is used to refer to the prolonged diestrous condition in forms such as the rat, wherein estrus is suppressed in the
mother while suckling the young.
The length of the sexual cycle varies with the species. When females of
the rat or mouse are kept away from a male, the estrous or sexual cycle will
repeat itself every 4 to 5 days. In the sow it occurs every 17 to 20 days. In
the opossum there is a prolonged anestrous period during the summer and
autumn months followed by a polyestrous period during the winter and spring
when the estrous cycle reoccurs about every 28 days. In the human female,
the sexual cycle occupies about 28 days, and there arc probably about ten
normal ovulatory cycles in a year. Some human females may have more,
while others experience a slightly smaller number of true ovulatory cycles
per year.
Many mammals have one estrous cycle per year. This condition, known as
monestrus, is true of most wild mammals, such as the deer, wolf, fox, moose,
and coyote. In the shrew, mink, and ferret the moncstrous period may be
prolonged if the female is kept away from the male.
Various types of polyestrous conditions exist. In the female dog, for example, there are two or three estrous periods per year about 4 to 6 months
apart. In the cat there are several cycles about two weeks apart during the
autumn, winter, and spring. In the domestic sheep there is a polyestrous period
from September to February in which the cycles occur about every 17 days,
followed by an anestrous period from early March to September. In the mare
in North America, estrous cycles of about 19 to 23 days occur from March
to August. In South America the breeding season is reversed, corresponding
to the reversed seasonal conditions south of the equator. In England many
mares breed in autumn and winter (Asdell, ’46).
In some mammals estrus may follow immediately after parturition or birth
of the young. This may occur occasionally in the rat. Under normal conditions
in the fur seal, the female lactates and gestates simultaneously. It is not a
common procedure.
It should be observed that there are two aspects of the female reproductive cycle of the mammal relative to fertilization or the bringing together of the
male and female reproductive cell. One aspect is the sexual receptivity of
the female; the other is the time of ovulation of the egg. In most female mammals sexual receptivity and ovulation are intimately associated and occur
spontaneously in the cycle; in others the two events may be separated. In
the former group, the development of “heat” and the maturing of the egg
follicle are closely associated, while in the latter the conditions favoring sexual
receptivity or heat are developed considerably in advance of the maturation
of the follicle, as noted in the table below.
b. Relation of Estrus and Ovulation in Some Common Mammals
1) Spontaneously Ovulating Forms (Sexual Receptivity of Male Occurs at
or near Time of Ovulation):
Length of Estrus or Period
of Heat
Time of Ovulation
Dog
True period of heat about
5-10 days in the middle of
a 21 -day estrous period
Variable: 1st day; 2nd day; 5th day;
etc., of true period of heat
Guinea pig
6-1 1 hrs.
Views vary: 1-2 hrs. after heat or
estrus begins; 10 hrs. after; at end
of estrus
Man
Receptivity not always related
to cyclic events
12-17 days after onset of preceding menstruation; average around
14th day
Mare
2-11 days; average length 5-6
days
About 1-2 days before end of estrus; best breeding about 3 days
after heat begins
Sheep
About 36 hrs.
Late in estrus or just after estrus
ends; presumably about 20-36
hrs. after estrus begins
Sow
Silver fox
Rat
15 days
1-5 days; occurs once a year
in February
One determination estimates
estrus to be 9-20 hrs.; most
receptive to male about first
3 hrs. of heat. Another determination estimates estrus
to be 12-18 hrs.
About 1-3 days after onset of estrus
1st or 2nd day of estrus
8 -11 hrs. after beginning of heat
2) Dependent Ovulatory Forms (Sexual Receptivity (Heatl Occurs Previous to Time of Ovulation);
Length of Estrus or Period
of Heat
Time of Ovulation
Cat
2-3 days
Time of ovulation uncertain but is
dependent upon copulation
Length of Estrus or Period
of Heat
Time of Ovulation
Rabbit (tame)
Estrus prolonged indefinitely
during the breeding season
from spring to summer; a
series of different sets of
egg follicles matured; each
series lasts about a week,
then becomes atretic
Ovulation 10-14 hrs. after mating
Shrew
Estrus prolonged
About 55 -70 hrs. after mating
Ferret
Estrus prolonged
About 30 hrs. after mating
If ovulation and subsequent pregnancy are not permitted by mating, ovarian
involution occurs, and an anestrous interlude is established. Anestrus in the
common rabbit, Oryctolagus cuniculus, occurs from October to March, but
is not absolute.
c. Non-ovulatory (Anovulatory) Sexual Cycles
Not all of the cyclic changes referred to above in those species which normally experience spontaneous ovulation are related to definite egg discharge.
Some cycles occur, more or less abortively, without ovulation of the egg. This
may happen in the human or in other mammals, such as the dog and monkey.
Cycles without ovulations are called non-ovulatory cycles. Menstruation may
follow non-ovulatory cycles in the human female.
d. Control of the Estrous Cycle in the Female Mammal
In the control of a reproductive cycle in the vertebrate animal, three main
categories of factors appear to influence its appearance and course. These are:
(1) external environmental factors, such as light and temperature,
(2) external factors governing food supply, and
(3) internal factors resulting from an interplay of the activities of the pituitary gland, the ovary, general body health, and of the particular
hereditary constitution of the animal.
These factors should be considered not alone in terms of the immediate
production of fertile conditions in the parent, but rather, in view of the total
end to be achieved, namely, the production of a new individual of the species.
For example, the reproductive cycle in the deer reaches its climax or estrus
in the autumn after a long period of lush feeding for the mother. The young
are born the next spring amid favorable temperatures, followed by another
period of bountiful food supply for the mother during lactation and for the
fawn as it is weaned. A receding light factor in the late summer and early
fall thus may be correlated with the period of heat, which in turn proves to
be an optimum time of the year for conception with the resulting birth the
following spring. Similarly, light ascendency is a factor in producing fertility in many birds. Here the incubation period for the young is short and a
plentiful supply of food awaits the parents and young when it is needed. In
other words, the factors which induce the onset of the reproductive state
are correlated with the conditions which enhance the end to be achieved,
namely, the production of a new individual.
Let us consider next the internal factors which induce the breeding state
in the female mammal. The commonly held theory regarding the pituitaryovarian relationship governing the control of the reproductive periods in the
mammal which ovulates spontaneously is as follows (figs. 53 and 59) :
( 1 ) FSH of the pituitary gland stimulates later follicular growth. This factor
probably is aided by small amounts of the luteinizing factor, LH, to
effect an increased production by the ovarian tissues of the estrogenic
hormone. Early follicle growth probably occurs without FSH.
(2) Estrogen output by the ovary rises steadily during the period previous
to ovulation.
(3) Old corpora lutea or other ovarian tissue possibly secrete minimal
amounts of progesterone under the influence of lutcotrophin, LTH.
(4) As the quantity of estrogen rises in the blood stream, it inhibits the
production of FSH and together with small quantities of progesterone,
increases the output of LH from the pituitary gland. This combination
also may cause an increased outflow of the luteotrophic factor.
(5) An increased amount of LH aids in effecting ovulation and the subsequent luteinization of the follicle. As the follicle becomes converted
into the corpus luteum, the presence of the luteotrophic factor brings
about the formation of increased quantities of progesterone and maintains for a time the corpus luteum and the functional luteal phase of
the cycle.
(6) In those mammals possessing a scries of repeating sexual cycles, it
is assumed that the fall of estrogen in the blood stream after ovulation
suppresses the LH outflow and permits a fresh liberation of FSH
from the anterior lobe of the pituitary gland, thus starting a new cycle.
The lowering of the estrogen level may be particularly and immediately effective in forms such as the rat and mouse, which have a
short metestrus or luteal phase in the estrous cycle.
e. Reproductive Cycle in Lower Vertebrate Females
While the words estrus, heat, or rut are generally applied to the mammalian
groups, the recurrent periods of sexual excitement in lower vertebrates are
fundamentally the same sort of reaction, although the changes in the reproductive tract associated with ovarian events are not always the same as in
mammals. However, similar cyclic changes in the ovary and reproductive tract
are present in the lower vertebrates, and their correlation with the activities of the pituitary gland is an established fact. Consequently, the words estrus,
rut, sex excitement, and heat basically designate the same thing throughout
the vertebrate series — namely, a period during which the physiology and
metabolism of the parental body is prepared to undertake the reproductive
functions. In this sense, the words estrus, anestrus, heat, etc. also may be
applied to the male as well as to the female when the male experiences periodic expressions of the sexual state.
Although the reproductive cycle in all vertebrates represents basically a
periodic development of the reproductive functions, there is a marked difference between the estrous cycle in the female mammal and the reproductive
cycle in most of the other female vertebrates with the exception of viviparous
forms among the snakes, lizards, and certain fishes. This difference is due
to the absence of a true luteal phase in the cycle. The follicular phase and
elaboration of estrogen appears to be much the same in birds, amphibia, and
fishes as in the mammals, but the phase of the cycle governed by progesterone
secretion, associated with a gestational condition in the accessory reproductive
organs, is found only among those vertebrates which give birth to their
young alive.
The reproductive cycles in certain vertebrates may be changed by selective
breeding and domestication. For example, the domestic hen is derived from
the wild jungle fowl. The jungle fowl conform to the general stimuli of nature
as do most wild birds, and the reproductive cycle is associated with a particular season of the year. However, domestication and selection by man of
certain laying strains have altered the original hereditary pattern of seasonal
laying. Consequently, good layers will lay eggs over an extended period of
the year, although there is a strong tendency to follow the ancestral plan by
laying most of the eggs during the spring and summer months; during the
fall and winter months, a smaller number of eggs are laid. Some of the varieties of the domestic hen conform more closely to the ancestral condition
than do other strains. Similar changes may be produced in the buffalo, which
in nature breeds in middle to late summer but in captivity has estrous periods
three weeks apart throughout the year (Asdell, ’46).
G. Role of the Ovary in Gestation (Pregnancy)
1. Control of Implantation and the Maintenance of
Pregnancy in Mammals
The ruling power of the ovary over the processes involved in pregnancy
is absolute, particularly during its earlier phases. In the first place, the corpusluteum hormone, progesterone, is necessary to change the uterus already conditioned by the estrogenic hormone into a functionally active state. The latter
condition is necessary for the nutrition and care of the embryo. A second
change which the gestational hormone imposes upon the genital tract of the
female is to quiet the active, irritable condition aroused by the estrogenic
factor. Progesterone thus serves to neutralize or antagonize the effects of
the estrogenic hormone. A placid condition of the uterus must be maintained
during the period immediately following copulation if the fertilized egg is to
be cared for within the uterine structure. Large doses of estrogens injected
into mammals shortly after copulation prevent implantation of the embryo
in all species thus far studied. (See Selye, ’48, p. 822.)
A third effect of the presence of progesterone is the inhibition of the
copulatory responses. Immediately following estrus and ovulation, the female
dog will fight off the aggressiveness of the male — an aggressiveness which
she invited a day or two previously. This change in behavior is introduced
by the development of the corpora lutea and the initiation of the luteal phase
of the reproductive cycle. Similar anaphrodisiac changes are sometimes mentioned in the behavior of the human female during the luteal phase of the
cycle. Progesterone injections also inhibit the copulatory responses in the
ferret (Marshall and Hammond, ’44). All of the above-mentioned activities
of progesterone thus inhibit or antagonize the condition aroused by estrogenic
stimulation.
However, aside from these immediate metestrous and post-ovulatory changes
in behavior induced by progesterone, one of its most essential aetivities is
concerned with the maintenance of gestation or pregnancy. Ovariectomy or
the removal of the ovaries at any time during the gestational period in the
rat, mouse, and goat results in death and abortion of the embryo. During
the first part of pregnancy in the rabbit, the ovaries must be left intact but
may be removed in the closing phase without endangering the gestational
process. In the human female, and also in the mare, cat, dog, guinea pig,
and monkey, the ovaries may be removed during the latter half of pregnancy
without danger to the offspring. However, ovariectomy performed in the
early stages of pregnancy in these animals, as well as in all other mammals
thus far studied, produces abortion (Pincus, ’36; Selye, ’48, p. 820). The
corpus luteum hormone, therefore, is essential in the early phases of gestation
in all mammals, and it appears to be necessary during most of the pregnant
period in many other mammals.
It is highly probable that the placenta takes over the elaboration of progesterone in those mammals where ovariectomy is possible after the first part
of pregnancy has elapsed. In the human female the corpus luteum normally
involutes at about the third month of pregnancy, but progesterone may be
extracted from the placenta after this period.
Although certain effects of the estrogenic hormone appear to be neutralized
(or antagonized) by progesterone during the early phases of reproduction,
other effects of estrogen in relation to progesterone are important for the
maintenance of the pregnant condition. In this connection the estrogenic
hormone appears to suppress some of the growth-promoting effects of progesterone. The two hormones thus work together to promote a gradual development of the uterine tissue and maintain a regulated, balanced condition
throughout pregnancy. The placenta, through its ability to elaborate progesterone and estrogen during the latter phases of pregnancy, is an important
feature regulating pregnancy in some mammals.
It should be emphasized in connection with the above statements that the
presence of the fertilized egg and its subsequent development in some manner
affects the maintenance of the corpus luteum. The mechanism by which this
influence is conveyed to the ovary is unknown.
2. Gestation Periods, in Days, of Some Common Mammals*
* Adapted from Asdell, ’46; Cahalane, ’47; Kenneth, ’43.
Armadillo (Dasypus novemcinctus)
150
Bear, black (Ursiis americanus)
210
Bear, polar (Thalarctos maritimus)
240
Beaver, Canadian (Castor canadensis)
94-100
Bison (Bison bison)
276
Cat, domestic (Felis catus)
60
Cattle (Bos taurus)
282
Chimpanzee (Pan satyrus)
250
Deer, Virginian (Odocoileus virginianus)
160-200
Dog, domestic (Canis familiaris)
58-65
Donkey, domestic (Eqiius asinus)
365-380
Elephant (Elephas africanus)
641
Elephant (Elephas indicus)
607-641
Elk (A Ices alces)
250
Ferret (Putorius faro)
42
Fox, arctic (Alopex lagopus)
60
Fox, red (Vulpes vulpes and V. fulva)
52-63
Giraffe (Giraffa Camelopardalis)
450
Goat, domestic (Capra hircus)
140-160
Guinea pig (Cavia porcellus)
68-71
Horse (Equus cabaltus)
330-380
Man (Homo sapiens)
270-295
Lion (Felis leo)
106
Lynx (Lynx canadensis)
63
Marten, American (Martes americana)
267-280
Mink (Mustela vison)
42-76
Mole (Talpa europaea)
30
Monkey, macaque (Macaca mulato)
160-179
Mouse, house (Mas rnusculus)
20-21
Opossum (Didelphis virginiana)
13
Pig (Sus scrofa)
115-120
Rabbit (Lepus; Sylvilagus; Oryctolagus)
30-43
Rats (Various species)
21-25
Seal, fur (Callorhinus sp.)
340-350
Sheep, domestic (Ovis aries)
144-160
Skunk, common (Mephitis mephitis)
63
Squirrel, red (Tamiasciurus sp.) 30-40
Tiger (Felis tigris) 106
Whale (Various species) 334-365
Wolf (Canis lupus) 63
Woodchuck (Marmota monax) 35-42
Zebra, mountain (Equus zebra) 300-345
3. Maintenance of Pregnancy in Reptiles and Other
Vertebrates
In certain viviparous species of the genera Storeria, Matrix and Thamnophis,
Clausen (’40) reports that ovariectomy during gestation results in resorption
of the embryo when performed during the earlier phases of gestation and
abortion during the middle of gestation, but during the terminal portion of
pregnancy the process is unaffected and the young are born normally. These
results are similar to those obtained from the rabbit as noted previously.
While experimental evidence is lacking in other vertebrate groups which
give birth to the young alive, the evidence obtained from reptilian and mammalian studies suggests that hormones are responsible for the maintenance
of pregnancy. In harmony with this statement, it may be pointed out that
in the viviparous elasmobranch fishes (e.g., sharks) corpora lutea are developed in the ovaries.
H. Role of the Ovary in Parturition or Birth of the Young
The real factors bringing about parturition are not known, and any explanation of the matter largely is theoretical. However, certain aspects of
the subject have been explored. For example, it was observed above that
progesterone appears to antagonize the action of estrogen with the result
that the uterus stimulated to irritability and contractility under the influence
of estrogen is made placid by the action of progesterone. In harmony with
this action studies have shown that estrogen tends to increase during the
final stages of normal gestation, while progesterone appears to decrease, accompanied by an involution of the corpora lutea. Consequently, the foregoing
facts have suggested the “estrogen theory,” which postulates that activities
of the uterine musculature are increased by the added amounts of estrogen
in the presence of decreasing amounts of progesterone during the latter phases
of pregnancy. In confirmation of this theory, it has been shown that progesterone injected into a pregnant rabbit near the end of the gestation period
will tend to prolong gestation. A second theory of parturitional behavior
assumes that the posterior lobe of the pituitary gland elaborates oxytocin
which induces increased uterine activity, resulting in birth contractions
(Waring and Landgrebe in Pincus and Thimann, ’50). Again, a third concept
emphasizes Ihe possibility that the placenta may produce substances which
bring about contractions necessary for the expulsion of the young (Turner,
’48, p. 428). Oxytocic substances have been extracted from the placenta,
which suggests the validity of this theory.
Fig. 54. Changes occurring in the reproductive organs and mammary glands of the
bitch during the reproductive cycle. The student is referred to Asdell (’46), pp. 150-156
and Dukes (’43), pp. 678-682, for detailed description and references pertaining to the
data supporting this chart. The gestation period is based upon data supplied by Kenneth
(’43) and the author’s personal experience with dogs.
Fig. 55. Reproductive and pregnancy cycles in the sow. (Modified from data supplied
by Corner, Carnegie Inst., Washington, pub. 276, Contrib. to Embryol., 13; the parturition
data derived from Kenneth, ’43.)
The specific functions of the ovary in parturition probably are more pronounced in those forms where it is essential throughout most of the gestational period, such as the viviparous snakes, and among the mammals, such
forms as the opossum, rat, mouse, and rabbit. The waning of corpus-luteum
activity in these species may serve to lower the level of progesterone in the
body and thus permit some of the other factors, such as estrogen or the
pituitary principle, to activate the uterus.
Another factor associated with the ovary and parturition is the hormone
relaxin. This substance was first reported by Hisaw and further studied by
this investigator and his associates (Hisaw, ’25, ’29; and Hisaw, et al., ’44).
Fig. 56. Reproductive and pregnancy cycles in the mare. (Parturition period based
upon data supplied by Kenneth (’43); other data supplied by Asdell (’46) and Dukes
(’43).) It is to be noted that the first corpus luteum of pregnancy degenerates after
about 35 days; the second “crop of corpora lutea” (Asdell) degenerate by 150 days. The
ovaries may be removed after 200 days of pregnancy without causing abortion of young.
Relaxin aids in the production of a relaxed condition of the pelvic girdle, a
necessity for the formation of a normal birth passageway for the young.
Relaxin somehow is associated in its formation with the presence of progesterone in the blood stream and also with the intact reproductive system.
Relaxin together with estrogen and progesterone establishes a relaxed condition of the tissues in the pubic area of the pelvic girdle.
I. Importance of the Ovary in Mammary-Gland Development
and Lactation
Estrogen and progesterone together with the lactogenic hormone, luteotrophin, of the pituitary gland are necessary in mammary-gland development.
The entire story of the relationship of these and of other factors in all mammals or in any particular mammal is not known. However, according to one
theory of mammary-gland development and function, the suggestive roles
played by these hormones presumably are as follows (fig. 58): Estradiol and
other estrogens bring about the development of the mammary-gland ducts;
as a result a tree-like branching of the ducts is effected from a simple im
Fig. 57. Reproductive and pregnancy cycles in the cow. (Parturition period based upon
data supplied by Kenneth (’43), also by Asdell (’46), Other data for chart derived from
Asdell (’46).
Three main characteristics of heat or estrous period are evident: (1) A duration of
heat of only about 10 to 18 hours; (2) abundant secretion during heat of a “stringy
mucus,’’ derived from mucoid epithelium of vagina and from sealing plug of cervix when
cow not in estrus (Asdell); and (3) ovulation occurs from 13Vi to 15Vi hours after termination of estrus (Asdell), Variation in time of ovulation may be considerable, from
2 hours before end of estrus to 26 hours after (Asdell).
mature pattern established during earlier development (fig. 5 8 A, A', B). The
male mammary gland may remain similar to the condition shown in fig. 58A.
The maturing of the egg follicles within the ovary and the concomitant formation of estrogen which accompanies sexual maturity is linked with the
more complex state of the mammary-gland system shown in fig. 58B.
The next step of mammary-gland development is carried out under the
influence of progesterone. Progesterone is necessary for the development
of the terminal glandular tissue or alveoli associated with these ducts (fig.
58C, D). Finally, the pituitary lactogenic hormone (luteotrophin [LTH];
prolactin) stimulates the actual secretion of milk (fig. 58E). Recent research
also has shown that the lactogenic hormone collaborates in some way with
estrogen and progesterone in the development of the mammary-gland tissue.
Fig. 58. Mammary gland changes in relation to reproduction. (Figures are a modification of a figure by Corner: Hormones in Human Reproduction, Princeton, Princeton
University Press. The figure in the latter work was based on a figure by C. D. Turner:
Chap. XI of Sex and Internal Secretions, by Allen, et al., Baltimore, Williams & Wilkins,
1939.) Factors involved in mammary gland development and secretion are somewhat as
follows: (A, A') Condition of the young, infantile gland. (B) Development from a
simple, branched, tubular gland of the immature animal (A') into a compound tubular
gland presumably under the direct stimulation of estrogen, according to one theory, or
by the action of estrogen upon the pituitary gland which then releases mammogen I,
producing these changes, according to Turner, et al.: Chap. XI, Sex and Internal Secretions, by Allen, et al., Baltimore, Williams & Wilkins. (C) Transformation of the compound tubular gland into a compound tubulo-alveolar gland under the influence of progesterone, during the first part of pregnancy, or, according to Turner, et al., by the influence
of estrogen plus progesterone which causes the pituitary to release a second mammogen
which produces the alveolar transformation. (D) Effect of the latter part of pregnancy
is to bring about a development of the cells of the acini of the acinous or alveolar system.
The unit shown in (D) represents a simple, branched, acinous gland, in which there are
six alveoli or acini associated with the duct. (E) Affect of parturition is to release the
lactogenic hormone (prolactin; luteotrophin) from the pituitary gland which brings about
milk secretion. During pregnancy the high levels of estrogen presumably inhibit milk
secretion. However, following pregnancy the level of estrogen is lowered permitting
lactogenic-hormone action upon the alveoli of the gland.
The removal of the placenta and embryo at any time during gestation permits milk
flow, provided the mammary glands are sufficiently developed. In the human, any remains
of the placenta after birth inhibit milk secretion, probably because the estrogenic hormone
is elaborated by the placental remnants. (See Selye, ’48, p. 829.)
In the rabbit, estrogen and progesterone are necessary for the elaboration of the duct
and secretory acini; in the guinea pig and goat, and to some extent in the primates,
including the human female, estrogen alone is capable of producing the development of
the entire duct and acinous system. (See Turner, ’48, p. 430.)
During pregnancy, the actual secretion of milk is inhibited by the estrogenic
hormone produced by the ovary and the placenta. The role of estrogen as
an inhibitor of lactation is suggested by the fact that, after lactation has started
following normal parturition, it is possible in the cow and human to suppress
milk flow by the administration of estrogens. After parturition, however,
estrogen is no longer present in^sufflcient amounts to suppress the secretion
of milk, and the mammary gland begins to function. (In the fur seal a postpartum estrus with ovulation follows a short time after parturition. However,
the amount of estrogen produced by this reproductive cycle is not sufficient
to curb lactation.) The neurohumoral reflex, or “suckling reflex,” produced
by the sucking young appears to maintain the flow of milk over a period of
time. Probably this reflex causes a continuous discharge of the lactogenic hormone from the anterior lobe of the hypophysis.
Another theory of mammary-gland development maintains that estrogen
stimulates the anterior pituitary gland to release mammogen, which causes
development of the duct system, and estrogen plus progesterone induce a
second mammogen which stimulates lobule-alveolar development. The lactogenic hormone produces the actual secretion of milk. The ovary thus assumes considerable importance in controlling the (morphological) development of the mammary glands in mammals, particularly in those forms in
which the functional condition of the ovary is maintained throughout most of the gestational period, e.g., rat, rabbit, dog, etc. In other species, such as
the human, mare, etc., the placenta through its ability to duplicate the production of the ovarian hormones, assumes a role during the latter phase of
pregnancy. (For further details, consult Folley and Malpress in Pincus and
Thimann, ’48; Selye, ’48, pp. 828-832; and Turner, ’48, pp. 428-448.)
Fig. 59. Stages in the reproductive cycle of the human female and its pituitary-ovarianendometrial relationships (Cf. fig. 53). (Compiled from various sources in the literature.)
(a) As shown at the extreme right of the figure, a fall in the level of estrogen and progesterone in the blood stream, either or both, is associated with endometrial necrosis, bleeding, and discharge (menstruation), (b) The lowering of the estrogen level is associated
with a new outflow of the follicle-stimulating hormone (FSH), as shown at the right of
the figure, (c) In the left side of the figure, the influence of FSH induces egg follicles,
probably several, to grow. Antral spaces appear and enlarge. The presence of a small
amount of the luteinizing hormone (LH) together with FSH stimulates the secretion of
estrogen by the ovarian tissues, possibly by the follicles and interstitial tissue between
the follicles, (d) In consequence, the estrogen level rises in the blood stream, and
menstruation subsides by the fourth day. (e) The continued influence of estrogen produces endometrial growth, and probably increases the outflow of LH from the pituitary
(fig. 53). It is probable, also, that the increased estrogen level stimulates a release of
the luteotrophic hormone from the pituitary, which in turn stimulates the formation of
a small quantity of progesterone by either the interstitial tissue of the ovary or in old
corpora lutea. (f) Some of the developing egg follicles degenerate, while one continues
to develop, (g) The elevation of estrogen suppresses the outflow of FSH as indicated
by the heavy broken line to the left, (h) The elevated level of estrogen together possibly
with small amounts of progesterone evokes an increased outflow of LH and LTH as
indicated by the heavy broken line to the right, (i) LH and FSH bring about ovulation
at about the fourteenth day. (j) LH causes development of corpus luteum. (k) LTH
elicits secretion of progesterone by corpus luteum. Possibly some estrogen is secreted
also by corpus luteum. (1) Progesterone and estrogen stimulate added development of
endometrium, (m) In the absence of fertilization of the egg, the corpus luteum regresses,
with a subsequent fall of progesterone and estrogen levels in the blood stream, terminating
the cycle and permitting a new menstrual procedure.
In the dog or opossum during each reproductive cycle, the mammary glands
are stimulated to grow and may even secrete milk (dog). These changes
closely parallel the ovarian activities, particularly the luteal phase of the cycle.
In the human, functional growth changes occur in pregnancy, but, pending
the events of the ordinary cycle, alterations in the duct system are slight although the breasts may be turgid due to increased blood flow and connectivetissue development.
J. Other Possible Developmental Functions Produced by the Ovary
As the eggs of the opossum and rabbit travel through the uterine (Fallopian) tube toward the uterus, they are coated with an albuminous, jelly-like
coating. Similar jelly coatings are added to the eggs of the bird, reptile, frog,
toad, and salamander. These coatings or membranes added to the egg as it
travels through the oviduct are known as tertiary egg membranes.
In the toad, the secretion of the protective jelly by the oviduct can be
elicited by the lactogenic hormone present in beef pituitary glands. The secretion of the albuminous jelly coatings around the eggs of frogs, salamanders,
reptiles, and birds may be related to this hormone. The formation of the
crop milk of pigeons has been shown by Riddle and Bates (’39) to be dependent upon the presence of the lactogenic hormone.
The function of the ovary in influencing the outflow of the lactogenic hormone from the pituitary, if present in the above cases of glandular secretion,
must be an indirect one. Evans and Simpson in Pincus and Thimann (’50)
ascribe the outflow of the “lactogenic hormone (luteotrophic hormone)” of
the mammalian pituitary to estrin produced by the ovary. It is possible that
in the salamanders, frogs, toads, and the birds an indirect ovarian influence
may similarly induce secretion of the lactogenic hormone which in turn governs the elaboration of the albuminous jelly deposited around the egg in
transit through the oviduct.
K. Determinative Tests for Pregnancy
Various tests have been used to determine the probability of pregnancy
in the human female. These tests are discussed in Chapter 22.
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Enders, R. K., Pearson, O. P., and Pearson,
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Humphrey, R. R. 1929. Studies on sex
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==The Development of the Gametes or Sex Cells==
==The Development of the Gametes or Sex Cells==

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

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Part I The Period of Preparation

Part I - The Period of Preparation: 1. The Testis and Its Relation to Reproduction | 2. The Vertebrate Ovary and Its Relation to Reproduction | 3. The Development of the Gametes or Sex Cells

The events which precede the initiation of the new individual's development are:

  1. The preparation of the male and female parents and their reproductive structures for the act of reprcxluction (Chaps. 1 and 2).
  2. The preparation of the gametes (Chap. 3).

The anterior lobe of the pituitary gland, because of its secretion of the gonadotrophic (gonad-stimulating) hormones, is the pivotal structure in the reproductive mechanism.

The gonadotrophic hormones are:

  1. Follicle-stimulating hormone, FSH;
  2. Luteinizing hormone, LH (ICSH), and
  3. Luteotrophin, LTH.

The Development of the Gametes or Sex Cells

A. General Considerations

In the two preceding chapters the conditions which prepare the male and female parents for their reproductive responsibilities are considered. This chapter is devoted to changes which the male and female germ cells must experience to enable them to take part in the processes involved in the reproduction of a new individual.

The gamete is a highly specialized sex cell or protoplasmic entity so differentiated that it is capable of union (fertilization; syngamy) with a sex cell 6T the opposite sex to form the zygote from which the new individual arises. TE^ process of differentiation whereby the primitive germ cell is converted into the mature gamete is called the maturation of the germ cell.

The main events which culminate in the fully-developed germ cell are possible only after the primitive or undifferentiated germ cell has reached a certain condition known as the definitive state. When this stage is reached, the germ cell has acquired the requisite qualities which make it possible for it to differentiate into a mature gamete. Before the definitive state is reached, germ cells pass through an eventful history which involves:

  1. their so-called "origin" or first detectable appearance among the othercells of the developing body, and
  2. their migration to the site of the future ovary or testis.

After entering the developing substance of the sex gland, the primitive germ cells experience a period of multiplication. If the sex gland is that of the male, these undifferentiated sex cells are called spermatogonia; if female, they are known as oogonia.

B. Controversy Regarding Germ-cell Origin

The problems of germ-cell origin in the individual organism and of the continuity of the germ plasm from one generation to the next have long been matters of controversy. Great interest in these problems was aroused by the ideas set forth by Waldeyer, Nussbaum, and Weismann during the latter part of the nineteenth century. Waldeyer, 1870, as a result of his studies on the chick, presented the “germinal epithelium” hypothesis, which maintains that the germ cells arise from the coelomic epithelium covering the gonad. Nussbaum, 1880, championed the concept of the extra-gonadal origin of the germ cells. According to this view, derived from his studies on frog and trout development, the germ cells arise at an early period of embryonic development outside the germ-gland area and migrate to the site and into the substance of the germ gland.


At about this time the speculative writings of August Weismann aroused great interest. In 1885 and 1892 Weismann rejected the popular Darwinian theory of pangenesis, which held that representative heredity particles or “gemmules” passed from the body cells (i.e., soma cells) to the germ cells and were there stored in the germ cells to develop in the next generation (Weismann, 1893). In contrast to this hypothesis he emphasized a complete independence of the germ plasm from the somatoplasm. He further suggested that the soma did not produce the germ plasm as implied in the pangenesis theory, but, on the contrary, the soma resulted from a differentiation of the germ plasm.


According to the Weismannian view, the germ plasm is localized in the chromosomal material of the nucleus. During development this germ plasm is segregated qualitatively during successive cell divisions with the result that the cells of different organs possess different determiners. However, the nuclear germ plasm (Keimplasma) is not so dispersed or segregated in those cells which are to become the primitive sex cells; they receive the full complement of the hereditary determiners for the various cells and organs characteristic of the species. Thus, it did not matter whether the germ cells were segregated early in development or later, so long as the nucleus containing all of the determinants for the species was kept intact. In this manner the germ plasm, an immortal substance, passed from one generation to the next via the nuclear germ plasm of the sex or germ cells. This continuity of the nuclear germ plasm from the egg to the adult individual and from thence through the germ cells to the fertilized egg of the next generation, constituted the Weismann “Keimbahn” or germ-track theory. The soma or body of any particular generation is thus the “trustee” for the germ plasm of future generations.

The Weismannian idea, relative to the qualitative segregation of the chromatin materials, is not tenable for experimental and cytological evidence suggests that all cells of the body contain the same chromosomal materials. However, it should be pointed out that Weismann was one of the first to suggest that the chromosome complex of the nucleus acts as a repository for all of the hereditary characteristics of the species. This suggestion relative to the role of the nucleus has proved to be one of the main contributions to biological theory in modern times.


Fig. 60. Representation of the concept of the early embryonic origin of the primordial germ cells and their migration into the site of the developing germ gland. (A-C are adapted from the work of Allen, Anat. Anz. 29, on germ cell origin in Chrysemys; D-F are diagrams based on the works of Dustin, Swift, and Dantschakoff, etc., referred to in the table of germ-cell origins included in the text.) (A~C) Germ cells arising within the primitive entoderm and migrating through the dorsal mesentery to the site of the primitive gonad, shown in (D), where they become associated in or near the germinal epithelium overlying the internal mesenchyme of the gonad. (E, F) Increase of the primitive gonia within the developing germ gland, with a subsequent migration into the substance of the germ gland of many germ cells during the differentiation of sex.



Fig. 61 Diagrammatic representation of the process of chromatin diminution in the nematode worm, Ascaris cquorum (A. megalocephala), and of the “Keimbahn” (in black, E). One daughter cell shown by the four black dots of each division of the germcell line (i.e., the stem-cell line) is destined to undergo chromatin diminution up to the 16-cell stage. At the 16-cell stage, the germ-cell line ceases to be a stem cell (e.g., P 4 ), and in the future gives origin only to sperm cells (E). (A-D, copied from King and Beams (’38); E, greatly modified from Durkin (’32).)

Animal pole of the cleaving egg (A) is toward the top of the page. (B) Metaphase conditions of the second cleavage. Observe the differences in the cleavage planes of the prosomatic cell, Si, and that of the stem cell, P,. (C) Anaphase of the second cleavage of Si. Observe that the ends of the chromosomes in this cleaving cell are left behind on the spindle. (D) It is to be noted that the ends of the chromosomes are not included in the reforming nuclei of the two daughter cells of S,. thus effecting a diminution of the chromatin substance. In P„ P., and E.M. ST. of (D), the chromosomes are intact. E.M. ST. = second prosomatic cell. MST = mesoderm-stomodaeal cell.


A second contributory concept to the germ-cell (germ-plasm) theory was made by Nussbaum, 1880; Boveri, 1892, ’10, a and b, and others. These investigators emphasized the possibility that a germinal cytoplasm also is important in establishing the germ plasm of the individual. A considerable body of observational and experimental evidence derived from embryological studies substantiates this suggestion. Consequently, the modern view of the germ cell (germ plasm) embodies the concept that the germ cell is composed of the nucleus as a carrier of the hereditary substances or genes and a peculiar, specialized, germinal cytoplasm. The character of the cytoplasm of the germ cell is the main factor distinguishing a germ cell from other soma cells.

The matter of a germinal cytoplasm suggests the necessity for a segregation of the germinal plasm in the form of specific germ cells during the early development of the new individual. As a result, great interest, as well as controversy, has accumulated concerning this aspect of the germ-cell problem: namely, is there a separate germinal plasm set apart in the early embryo which later gives origin to the primordial germ cells, and the latter, after migration (fig. 60), to the definitive gonia; or according to an alternative view, do some or all of the definitive germ cells arise from differentiated or relatively undifferentiated soma cells? The phrase primary primordial germ cells often is used to refer to those germ cells which possibly segregate early in the embryo, and the term secondary primordial germ cells is employed occasionally to designate those which may arise later in development.


The dispute regarding an early origin or segregation of the germinal plasm in the vertebrates also occurs relative to their origin in certain invertebrate groups, particularly in the Coelenterata and the Annelida (Berrill and Liu, ’48). In other fnvertebrata, such as the dipterous insects and in the ascarid worms, the case for an early segregation is beyond argument. An actual demonstration of the continuity of the Keimbahn from generation to generation is found in Ascaris megalocephala described by Boveri in 1887. (See Hegner, ’14, Chap. 6.) In this form the chromatin of the somatic cells of the body undergoes a diminution and fragmentation, whereas the stem cells, from which the germ cells are ultimately segregated at the 16-cell to 32-cell stage, retain the full complement of chromatin material (fig. 61). Thus, one cell of the 16-cell stage retains the intact chromosomes and becomes the progenitor of the germ cells. The other 15 cells will develop the somatic tissues of the body. The diminution of the chromatin material in this particular species has been shown to be dependent upon a certain cytoplasmic substance (King and Beams, ’38).

In some insects the Keimbahn also can be demonstrated from the earliest stages of embryonic development. In these forms a peculiar polar plasm within the egg containing the so-called “Keimbahn determinants’’ (Hegner, ’14, Chap. 5) always passes into the primordial germ cells. That is, the ultimate formation and segregation of the primordial germ cells are the result of nuclear migration into this polar plasm and the later formation of definite cells from this plasm (fig. 62). The cells containing this polar plasm are destined thus to be germ cells, for they later migrate into the site of the developing germ glands and give origin to the definitive germ cells.

Many investigators of the problem of germ-cell origin in the vertebrate group of animals have, after careful histological observation, described the germ cells as taking their origin from among the early entodermal cells (see table, pp. 121-124). On the other hand, other students have described the origin of the germ cells from mesodermal tissue — some during the early period of embryonic development, while others suggest that the primordial germ cells arise from peritoneal (mesodermal) tissue at a much later time.

In more recent years much discussion has been aroused relative to the origin of the definitive germ cells in mammals, particularly in the female. According to one view the definitive germ cells which differentiate into the mature gametes of the ovary arise from the germinal epithelium (peritoneal covering) of the ovary during each estrous cycle (figs. 39A, 63, 64). For example, Evans and Swezy (’31) reached the conclusion that all germ cells in the ovaries of the cat and dog between the various reproductive periods degenerate excepting those which take part in the ovulatory phenomena. Accordingly, the new germ cells for each cycle arise from the germinal epithelium. A similar belief of a periodic proliferation of new germ cells by the germinal epithelium has been espoused by various authors. (See Moore and Wang, ’47; and Pincus, ’36, Chap. II.) More recent papers have presented views which are somewhat conflicting. Vincent and Dornfeld, ’48 (fig. 63) concluded that there is a proliferation of germ cells from the germinal epithelium of the young rat ovary, while Jones (’49), using carbon granules as a vital-marking technic, found no evidence of the production of ova from the germinal epithelium in rat ovaries from 23 days until puberty. In the adult rat, she concedes that a segregation of a moderate number of oogonia from the germinal epithelium is possible.



Fio. 62. Early development of the fly, Miflj/o/’. {A) Miastor metraloas. (B) Miastor americana. In (A) the division figures I and III (II not shown) are undergoing chromatin diminution, while nucleus IV divides as usual. In (B) one segregated germ is shown at the pole of the egg. This cell will give origin to the germ cells. Other division figures experiencing chromatin diminution.


Fig. 63. Cells proliferating inward from germinal epithelium of the ovary of a oneday-old rat. Observe cords of cells (Pfliiger’s cords) projecting into the ovarian substance. Within these cords of cells are young oogonia. (After Vincent and Dornfeld, ’48.)

Fig. 64. Cellular condition near the surface of the ovary of a young female opossum. This section of the ovary is near the hilar regions, i.e., near the mesovarium. Observe young oocytes and forming Graafian follicles. Primitive germ cells may be seen near the germinal epithelium.


Aside from the above studies of carefully-made, histological preparations relative to the time and place of origin of the primordial and definitive germ cells, many experimental attacks have been made upon the problem. Using an x-ray-sterilization approach, Parkes (’27); Brambell, Parkes and Fielding (’27, a and b), found that the oogonia and oocytes of x-rayed ovaries of the mouse were destroyed. In these cases new germ cells were not produced from the germinal epithelium. Brambell (’30) believed that the destruction of the primitive oogonia was responsible for the lack of oogenesis in these x-rayed ovaries. However, this evidence is not conclusive, for one does not know what injurious effects the x-rays may produce upon the ability of the various cells of the germinal epithelium to differentiate.


An experimental study of the early, developing, amphibian embryo relative to the origin of the primordial germ cells also has been made by various investigators. Bounoure (’39) applied a vital-staining technic to certain anuran embryos. The results indicate that the germinal plasm in these forms is associated with the early, entodermal, organ-forming area located at the vegetal pole of the cleaving egg. This germinal plasm later becomes segregated into definite cells which are associated with the primitive entoderm. At a later period these cells migrate into the developing germ gland or gonad. On the other hand, experimental studies of the urodele embryo indicate that the early germinal plasm is associated with the mesoderm (Humphrey, ’25, ’27; Nieuwkoop, ’49). Existence of an early germinal plasm associated with the entoderm in the Anura and with the mesoderm in the Urodela thus appears to be well established for the amphibia.


The evidence derived from amphibian studies together with the observations upon the fish group presented in the table (see pp. 121-124) strongly suggests that an early segregation of a germinal plasm (germ cells) occurs in these two major vertebrate groups. Also, in birds, the experimental evidence presented by Benoit (’30), Goldsmith ('35), and Willier ('37) weighs the balance toward the conclusion that there is an early segregation of germ cells from the entoderm. Similar conditions presumably are present in reptiles. In many vertebrates, therefore, an early segregation of primordial germ cells and their ultimate migration by: (1) active ameboid movement, (2) by the shifting of tissues, or (3) through the blood stream (see table, pp. 121-124) to the site of the developing gonad appears to be well substantiated.

The question relative to the origin of the definitive ova in the mammalian ovary is still in a confused state as indicated by the evidence presented above and in the table on pp. 121-124. Much more evidence is needed before one can rule out the probability that the primordial germ cells are the progenitors of the definitive germ cells in the mammals. To admit the early origin of primordial germ cells on the one hand, and to maintain that they later disappear to be replaced by a secondary origin of primitive germ cells from the germinal epithelium has little merit unless one can disprove the following position, to wit: that, while some of the primordial germ cells undoubtedly do degenerate, others divide into smaller cells which become sequestered within or immediately below the germinal epithelium of the ovary and within the germinal epithelium of the seminiferous tubules of the testis, where they give origin by division to other gonial cells. Ultimately some of these primitive gonia pass on to become definitive germ cells.

However, aside from the controversy whether or not the primordial germ cells give origin to definitive germ cells, another aspect of the germ-cell problem emphasizing the importance of the primitive germ cells is posed by the following question: Will the gonad develop into a functional structure without the presence of the primordial germ cells? Experiments performed by Humphrey (’27) on Ambystoma, and the above-mentioned workers — Benoit (’30), Goldsmith (’35), and Willier (’37) — on the chick, suggest that only sterile gonads develop without the presence of the primordial germ cells.

Finally, another facet of the germ-cell problem is this; Are germ cells completely self differentiating? That is, do they have the capacity to develop by themselves; or, are the germ cells dependent upon surrounding gonadal tissues for the influences which bring about their differentiation? All of the data on sex reversal in animals, normal and experimental (Witschi, in Allen, Danforth, and Doisy, ’39), and of other experiments on the development of the early embryonic sex glands (Nieuwkoop, ’49) suggest that the germ cells are not self differentiating but are dependent upon the surrounding tissues for the specific influences which cause their development. Furthermore, the data on sex reversal shows plainly that the specific chromosome complex (i.e., male or female) within the germ cell does not determine the differentiation into the male gamete or the female gamete, but rather, that the influences of the cortex (in the female) and the medulla (in the male) determine the specific type of gametogenesis.

The table given on pp. 121-124 summarizes the conclusions which some authors have reached concerning germ-cell origin in many vertebrates. It is not complete; for more extensive reviews of the subject see Everett ('45), Heys ('31 ), and Nieuwkoop ('47, '49).


Species

Place of Origin, etc.

A uthor

Entospheniis wihieri (brook lamprey)

Germ cells segregate early in the embryo; definitive germ cells derived from "no other source"

Okkelberg. 1921.

J. Morphol. 35

Petromyzon marinus unicolor (lake lamprey)

Definitive germ cells derive from: a) early segregated cells, primordial germ cells, and b) later from coelomic epithelium. Suggests that primordial germ cells may induce germ-cell formation in peritoneal epithelium

Butcher. 1929.

Biol. Bull. 56

Squalus acanthias

Germ cells segregate from primitive entoderm; migrate via the mesoderm into site of the developing gonad

Woods. 1902.

Am. J. Anat. 1

Amia and

Lepidosteiis

Germ cells segregate early from entoderm; continue distinct and migrate into the developing gonad via the mesoderm (see fig. 60)

Allen. 1911.

J. Morphol. 22


Species

Place of Origin, etc.

Author

Lophius piscatorius

Germ cells segregate from primitive entoderm; migrate through mesoderm to site of gonad; migration part passive and part active

Dodds. 1910.

J. Morphol. 21

Fundulus

heteroclitus

Germ cells segregate from peripheral entoderm lateral to posterior half of body; migrate through entoderm and mesoderm to the site of the developing gonad

Richards and Thompson. 1921.

Biol. Bull. 40

Cottus bairdii

Primordial germ cells derive from giant cells, in the primitive entoderm; migrate through the lateral mesoderm into the site of the developing gonad

Hann. 1927.

J. Morphol. 43

Lebistes reticulatus (guppy)

Germ cells segregate early in development; first seen in the entodermmesoderm area; migrate into the sites of the developing ovary and testis, giving origin to the definitive germ cells

Goodrich, Dee, Flynn, and Mercer. 1939. Biol. Bull. 67

Rana tempo raria

Germ cells segregate from primitive entoderm; migrate into developing genital glands

Witschi. 1914.

Arch. f. mikr. Anat. 85

Rana temporaria

Primordial germ cells from entoderm discharged at first spawning. Later, the definitive germs cells of adults originate from peritoneal cells

Gatenby. 1916.

Quart. J. Micr. Sc. 61

Rana catesbiana

Primordial germ cells segregate from primitive entoderm; definitive germ cell derives from primordial cells according to author’s view but admits possibility of germinal epithelium origin

Swingle. 1921.

J. Exper. Zool. 32

Rana temporaria, Triton alpestris, Bufo vulgaris

Primordial germ cells segregate from entoderm

Bounoure. 1924. Compt. rend. Acad. d. Sc. 178, 179

Rana sylvatica

Primordial germ cells originate from entoderm and migrate into the developing gonads. They give origin to the definitive sex cells

Witschi. 1929.

J. Exper. Zool. 52

H emidactylium scutatum

Primordial germ cells arise in mesoderm between somite and lateral plate; move to site of gonad by shifting of tissues

Humphrey. 1925.

J. Morphol. 41


Species

Place of Origin, etc.

Author

Ambystorna

maculatiirn

Most germ cells somatic in origin from germinal epithelium, although a few may come from primordial germ cells of entodermal origin

McCosh. 1930.

J. Morphol. 50

Triton, and Ambystorna mexicanum

Germ cells differentiate from lateral plate mesoderm

Nieuwkoop. 1946.

Arch. Neerl. de zool. 7

Chrysemys

marginata

(turtle)

Primordial germ cells from entoderm; most of definitive germ cells arise from peritoneal cells

Dustin. 1910.

Arch, biol., Paris. 25

Sternotherus

odoratus (turtle)

Primordial cells segregate early from entoderm; later definitive cells derive from these and from peritoneal epithelium

Risley. 1934.

J. Morphol. 56

Callus ( domesticus) gallus (chick)

Germ cells arise from primitive cells in entoderm of proamnion area and migrate by means of the blood vessels to the site of the developing gonad. Definitive germ cells of sex cords and later seminiferous tubules derive from primordial germ cells

Swift. 1914, 1916.

Am. J. Anat. 15, 20

Callus (domesticus) gallus (chick)

Primordial germ cells arise from enlodermal cells

Dantschakoff. 1931. Zeit. f. Zellforsch., mikr. Anat. 15

Chick and albino rat

Early primordial cells degenerate; definitive cells from peritoneal epithelium

Firket. 1920.

Anat. Rec. 18

Didelpliys

virginiana

(opossum)

Germ cells arise from germinal epithelium

Nelsen and Swain. 1942.

J. Morphol. 71

Mils musculiis (mouse)

Oogonia derived from primordial germ cells; spermatogonia from epithelial cells of testis cords

Kirkham. 1916.

Anat. Rec. 10

Mus musculus (mouse)

Primordial germ cells of ovary arise from germinal epithelium during development of the gonads. These presumably give origin to the definitive sex cells

Brambell. 1927.

Proc. Roy. Soc. London, s.B. 101

Felis dornestica (cat)

Primordial cells segregate early but do not give origin to definitive germ cells which derive from germinal epithelium

de Winiwarter and Sainmont. 1909. Arch, biol., Paris. 24


Species

Place of Origin, etc.

A uthor

Felis dome Stic a (cat)

Definitive ova derived from germinal epithelium of the ovary at an early stage of gonad development

Kingsbury. 1938.

Am. J. Anat. 15

Cavia porcellus (guinea pig)

Primordial germ cells from entodermal origin degenerate; the primordial germ cells derived from the germinal epithelium give rise to the definitive germ cells in the testis

Bookkout. 1937.

Zeit. f. Zellforsch, mikr. Anat. 25

Homo sapiens (man)

Primordial germ cells found in entoderm of yolk sac; migrate by ameboid movement into developing gonad

Witschi. 1948. Carnegie Inst., Washington Publ. 575. Contrib. to Embryol.' 32


C. Maturation (Differentiation) of the Gametes

1. General Considerations

Regardless of their exact origin definitive germ cells as primitive oogonia or very young oocytes are to be found in or near the germinal epithelium in the ovaries of all vertebrates in the functional condition (figs. 39B, 64). In the testis, the primitive spermatogonia are located within the seminiferous tubules as the germinal epithelium, in intimate association with the basement membrane of the tubule (figs. 65, 66).

The period of coming into maturity (maturation) of the gametes is a complicated affair. It involves profound transformations of the cytoplasm, as well as the nucleus. Moreover, a process of ripening or physiological maturing is necessary, as well as a morphological transformation. The phrase “maturation of the germ cells” has been used extensively to denote nuclear changes. However, as the entire gamete undergoes morphological and physiological change, the terms nuclear maturation, cytosomal maturation, and physiological maturation are used in the following pages to designate the various aspects of gametic development.

One of the most characteristic changes which the germ cell experiences during its maturation into a mature gamete is a reduction of chromatin material. Because of this, the germ cell which begins the maturing process is called a meiocyte. This word literally means a cell undergoing diminution and it is applied to the germ cell during meiosis or the period in which a reduction in the number of chromosomes occurs. The word haplosis is a technical name designating this reduction process.


Fig. 65. Semidiagrammatic representation of a part of the seminiferous tubule of the cat testis.

The word meiocyte thus is a general term applicable to both the developing male and female germ cells. On the other hand, the word spermatocyte is given to the developing male gamete during the period of chromosome diminution, whereas the word oocyte is applied to the female gamete in the same period. When, however, the period of chromosome diminution is completed and the chromosome number is reduced to the haploid condition, the developing male gamete is called a spermatid while the female gamete is referred to as an ootid or an egg. {Note: the word egg is applied often to the female gamete during the various stages of the oocyte condition as well as after the maturation divisions have been accomplished.)

The reduction of chromatin material is not the only effect which the meiotic process has upon the chromatin material, or possibly upon the developing cytosomal structures as well. This fact will become evident during the descriptions below concerning the meiotic procedures.

Another prominent feature of the gametes during the meiocyte period is their growth or increase in size. This growth occurs during the first part of the meiotic process when the nucleus is in the prophase condition and it involves both nucleus and cytoplasm. The growth phenomena are much more pronounced in the oocyte than in the spermatocyte. Due to this feature of growth, the oocyte and spermatocyte also are regarded as auxocytes, that is growing cells, a name introduced by Lee, 1897. The words meiocyte and auxocyte thus refer to two different aspects of the development of the oocyte and the spermatocyte.

2. Basic Structure of the Definitive Sex Cell as It Starts to Mature or Differentiate into the Male Meiocyte (i.e., the Spermatocyte) or the Female Meiocyte (i.e., the Oocyte)

The definitive sex cells of both sexes have a similar cytological structure.

The component parts are (fig. 68):

  1. nucleus,
  2. investing cytoplasm,
  3. idiosome,
  4. Golgi substance, and
  5. chondriosomes.


The nucleus is vesicular and enlarged, and the nuclear network of chromatin may appear reticulated. A large nucleolus also may be visible. The investing cytoplasm is clearer and less condensed in appearance than that of ordinary cells. The idiosome (idiozome) is a rounded body of cytoplasm which, in many animal species, takes the cytoplasmic stain more intensely than the surrounding cytoplasm. Within the idiosome it is possible to demonstrate the centrioles as paired granules in some species. Surrounding the idiosome are various elements of the Golgi substance, and near both the idiosome and Golgi elements, is a mass of chondriosomes (mitochondria) of various sizes and shapes. The idiosome and its relationship with the Golgi material, the mitochondria, and the centrioles varies considerably in different species of animals.


Much discussion has occurred concerning the exact nature of the idiosome. Some investigators have been inclined to regard the surrounding Golgi substance as a part of the idiosome, although the central mass of cytoplasm containing the centrioles is the “idiosome proper” of many authors (Bowen, ’22). Again, when the maturation divisions of the spermatocyte occur, the idiosome and surrounding Golgi elements are broken up into small fragments. However, in the spermatids the Golgi pieces (dictyosomes) are brought together once more to form a new idiosome-like structure, with the difference that the latter “seems never to contain the centrioles” (Bowen, ’22). It is, therefore, advisable to regard the idiosome as being separated into its various components during the maturation divisions of the spermatocyte and to view the reassemblage of Golgi (dictyosomal) material in the spermatid as a different structure entirely. This new structure of the spermatid is called the acroblast (Bowen, ’22; Leuchtenberger and Schrader, ’50). (See fig. 68B.) A similar breaking up of the idiosome occurs in oogenesis (fig. 68F, G). However, all meiocytes do not possess a typical idiosome. This fact is demonstrated in insect spermatocytes, where the idiosomal material is present as scattered masses to each of which some Golgi substance is attached.

The various features which enter into the structure of the definitive germ cell do not behave in the same way in each sex during gametic differentiation. While the behaviors of the chromatin material in the male and female germ cells closely parallel each other (fig. 67), the other cytosomal features follow widely divergent pathways, resulting in two enormously different gametic entities (fig. 68A-H).


Fig. 66. Section of part of a seminiferous tubule of human testis. (Redrawn from Gatenby and Beams, ’35.)



Fig. 67. Diagrammatic representation of the nuclear changes occurring during meiosis in spermatocyte and oocyte. Six chromosomes, representing three homologous pairs, are used. Observe the effects of the crossing over of parts of chromatids. The diplotene condition of oocyte depicted by arrows and the enlarged nucleus. The haploid condition is shown in each of the spermatids or in the egg and its three polocytes.


Fig. 68. Possible fate of the primitive meiocyte and its cytoplasmic inclusions when exposed to testicular or ovarian influences. Particular attention is given to the idiosome. Under male-forming influences the idiosome components are dispersed during the maturation divisions and are reassembled into three separate component structures, namely, (1) Acroblast of Golgi substance, (2) centriolar bodies, and (3) mitochondrial bodies (see B). Each of these structures, together with the post-nuclear granules of uncertain origin, play roles in spermatogenesis as shown. Under ovarian influences the idiosome is dispersed before the maturation divisions. The Golgi substance and mitochondria play (according to theory, see text) their roles in the formation of the deutoplasm.


3. Nuclear Maturation of the Gametes

Most of our information concerning the maturation of the nucleus pertains to certain aspects of chromosome behavior involved in meiosis, particularly the reduction of the chromosome number together with some activities of “crossing over” of materials from one chromosome to another. But our information is vague relative to other aspects of nuclear development. For example, we know little about the meaning of growth and enlargement of the nucleus as a whole during meiosis, an activity most pronounced in the oocyte. Nor do we know the significance of nuclear contraction or condensation in the male gamete after meiosis is completed. Therefore, when one considers the nuclear maturation of the gametes, it is necessary at this stage of our knowledge to be content mainly with observations of chromosomal behavior.

a. General Description of the Chromatin Behavior During Somatic and Meiotic Mitosis

As the maturation behavior of the chromatin components in the spermatocyte and oocyte are similar, a general description of these activities is given in the following paragraphs. Before considering the general features and details of the actions of the chromosomes during meiosis, it is best to recall some of the activities which these structures exhibit during ordinary somatic and gonial mitoses.

Cytological studies have shown that the chromosomes, in most instances, are present in the nucleus in pairs, each member of a pair being the homologue or mate of the other. Homologous chromosomes, therefore, are chromosomal pairs or mates. During the prophase condition in ordinary somatic and gonial mitoses, the various chromosomal mates do not show an attraction for each other. A second feature of the prophase stage of ordinary cell division is that each chromosome appears as two chromosomes. That is, each chromosome is divided longitudinally and equationally into two chromosomes. At the time when the metaphase condition is reached and the chromosomes become arranged upon the metaphase plate, the two halves or daughter chromosomes of each original chromosome are still loosely attached to each other. However, during anaphase, the two daughter chromosomes of each pair are separated and each of the two daughter nuclei receives one of the daughter chromosomes. Reproduction of the chromatin material and equational distribution of this material into the two daughter cells during anaphase is a fundamental feature of the ordinary type of somatic and gonial mitoses. The two daughter nuclei are thus equivalent to each other and to the parent nucleus. In this way, chromosomal equivalence is passed on ad infinitum through successive cell generations.

On the other hand, a different kind of chromosomal behavior is found during meiosis, which essentially is a specialized type of mitosis, known as a meiotic mitosis. In one sense it is two mitoses or mitotic divisions with only one prophase; that is, two metaphase-anaphase separations of chromosomes preceded by a single, peculiar prophase. The peculiarities of this meiotic prophase may be described as follows: As the prophase condition of the nucleus is initiated, an odd type of behavior of the chromosomes becomes evident — a behavior which is entirely absent from ordinary somatic mitosis: namely, the homologous pairs or mates begin to show an attraction for each other and they approach and form an intimate association. This association is called synapsis (figs. 67, 69, zygotene stage). As a result, the two homologous chromosomes appear as one structure. As the homologous chromosomes are now paired together and superficially appear as one chromosome, the number of “chromosomes” visible at this time is reduced to one-half of the ordinary somatic or diploid number. However, each “chromosome” is in reality two chromosomes and, therefore, is called a bivalent or twin chromosome.



Fig. 69. Steps in spermatogenesis in the grasshopper. In the center of the chart is represented a longitudinal section of one of the follicles of a grasshoper testis with its various regions of spermatogenic activity. In the upper right of the chart the apical-cell complex is depicted with its central apical cell, spermatogonia, and surrounding epithelial cells. The primary spermatogonia lie enmeshed between the extensions of the apical cell and the associations of these extensions with the surrounding epithelial elements of the complex. (Also see Wenrich, 1916, Bull. Mus. Comp. Zool. Harvard College, 60.)



While the homologous chromosomes are intimately associated, each mate reproduces itself longitudinally just as it would during an ordinary mitosis (fig. 67, pachytene stage). (The possibility remains that this reproduction of chromatin material may have occurred even before the synaptic union.) Hence, each bivalent chromosome becomes transformed into four potential chromosomes, each one of which is called a chromatid. This group of chromatids is, collectively speaking, a tetrad chromosome. (As described below, interchange of material or crossing over from one chromatid to another may take place at this time.) As a result of these changes, the nucleus now contains the haploid number of chromosomes, (i.e., half of the normal, diploid number) in the form of tetrads (fig. 67, pachytene stage). However, as each tetrad represents four chromosomes, actually there is at this time twice the normal number of chromosomes present in the nucleus (fig. 67; compare leptotene, pachytene, diplotene and diakinesis).


The next step in meiosis brings about the separation of the tetrad chromosome into its respective chromatids and it involves two divisions of the cell. These divisions are known as meiotic divisions. As the first of these two divisions begins, the tetrad chromosomes become arranged in the mid- or metaphase plane of the spindle. After this initial step, the first division of the cell occurs, and half of each tetrad (i.e., a dyad) passes to each pole of the mitotic spindle (fig. 67, first meiotic division). Each daughter cell (i.e., secondary spermatocyte or oocyte) resulting from the first maturation (meiotic) division thus contains the haploid or reduced number of chromosomes in the dyad condition, each dyad being composed of two chromatids. A resting or interphase nuclear condition occurs in most spermatocytes, following the first maturation division, but in the oocyte it usually does not occur (fig. 69, interkinesis).

As the second maturation division is initiated, the dyads become arranged on the metaphase plate of the mitotic spindle. As division of the cell proceeds, half of each dyad (i.e., a monad) passes to the respective poles of the spindle (fig. 67, second meiotic division; fig. 69). As a result of these two divisions, each daughter cell thus contains the haploid or reduced number of chromosomes in the monad (monoploid) condition (fig. 67, spermatid or egg). Meiosis or chromatin diminution is now an accomplished fact.

It is to be observed, therefore, that the meiotic phenomena differ from those of ordinary mitosis by two fundamental features:

(1) In meiosis there is a conjugation (synapsis) of homologous chromosomes during the prophase stage, and while synapsed together each of the homologues divides equationally; and

(2) following this single prophase of peculiar character, two divisions follow each other, separating the associated chromatin threads.

While the meiotic prophase is described above as a single prophase preceding two metaphase-anaphase chromosome separations, it is essentially a double prophase in which the process of synapsis acts to suppress one of the equational divisions normally present in a mitotic division; a synapsed or double chromosome, therefore, is substituted for one of the longitudinal, equational divisions which normally appears during a somatic prophase. It is this substitution which forms the basis for the reduction process, for two mitotic divisions follow one after the other, preceded by but one equational splitting, whereas in ordinary mitosis, one equational splitting of the chromosomes always precedes each mitotic division.

b. Reductional and Equational Meiotic Divisions and the Phenomenon of

Crossing Over

In the first meiotic division (i.e., the first maturation division), if the two chromatids which are derived from one homologous mate of the tetrad are separated from the two chromatids derived from the other homologous mate the division is spoken of as reductional or disjunctional. In this case the two associated chromatids of each dyad represent the original chromosome which synapsed at the beginning of meiotic prophase (fig. 67, tetrads B and C, first meiotic division). If, however, the separation occurs not in the synaptic plane but in the equational plane, then the two associated chromatids of each dyad come, one from one synaptic mate and one from the other; such a division is spoken of as an equational division (fig. 67, tetrad A, first meiotic division). There appears to be no fixity of procedure relative to the separation of the tetrads, and great variability occurs. However this may be, one of the two meiotic divisions as far as any particular tetrad is concerned is disjunctional (reductional) and the other is equational, at least in the region of the kinetochore (see p. 135 and fig. 70). If the first division is reductional, the second is equational and vice versa. Disjunction in the first maturation division is often referred to as pre-reduction, while that in the second maturation division is called post-reduction.



Fig. 70. Some of the various possibilities which may occur as a result of the exchanges of parts of chromatids during the crossing-over phenomena associated with meiosis. Two chiasmata (singular, chiasma) are shown in (A), (C), (E). Observe that homologous chromosome A has split equationally into chromatids A and A', while homologous chromosome B has divided equationally into B and B'. The resulting interchanges between respective chromatids of the original homologous chromosomes are shown in (B), (D), (F). The kinetochore (place of spindle-fiber attachment) is indicated by the oval or circular area to the left of the chromatids. (Modified from White: Animal Cytology and Evolution, London, Cambridge University Press, 1943.)


The foregoing statement regarding disjunctional and equational divisions should be considered in the light of the phenomenon of crossing over. In the latter process, a gene or groups of genes may pass from one chromatid to the other and vice versa during their association at the four strand stage (fig. 70). In the region of the centromere or kinetochore (i.e., the point) of the achromatic, spindle-fiber attachment) and nearby regions, cross overs are thought not to occur (fig. 70, kinetochore). Consequently, in the regions of the kinetochore, the statements above regarding disjunctional and equational divisions of the chromosomes appear to be correct. However, the terms disjunctional and equational may mean little in other regions of the chromosomes of a tetrad during the meiotic divisions. For example, let us assume as in fig. 70 (see also fig. 67), that we have chromatids A and A', B and B', A and B representing the original homologues or synaptic chromosomes which have divided into these chromatids respectively. Then during the tetrad stage of association or slightly before, let us assume that there has been a crossing over of genes from chromatid A to chromatid B and from chromatid B to chromatid A in a particular area (fig. 70A). (It is to be observed that chromatids A' and B' are not involved in this particular instance.) Further, let us assume that AA' and BB' as a whole are separated at the first maturation division, the kinetochore and immediate regions would represent a disjunctional division, but for the particular area where crossing over is accomplished, the division would be equational (fig. 70A, B; central portions of chromatids A and B in fig. 70B). Thus, it would be for other regions where cross overs may have occurred. Other cross-over possibilities are shown in fig. 70C-F.

c. Stages of Chromatin Behavior During the Meiotic Prophase in Greater Detail

The following five stages of chromatin behavior within the prophase nucleus during meiosis are now in common usage. They are based on the stages originally described by H. von Winiwarter, ’00. The substantive form is presented in parentheses.

1) Leptotene (Leptonema) Stage. The leptotene stage (figs. 69, 71) represents the initial stage of the meiotic process and is seen especially well in the spermatocyte. At this time the nucleus of the differentiating germ cell begins to enlarge, and the diploid number of very long, slender chromatin threads make their appearance. (Compare “resting” and leptotene nuclei in figs. 69, 71.) The chromatin threads may lie at random in the nucleus or they may be directed toward one side, forming the so-called “bouquet” condition (fig. 69, leptotene stage) . The nucleolus is evident at this time (fig. 7 IB) .

2) Zygotene or Synaptene (Zygonema) Stage. The zygotene stage (figs. 69, 71, 85) is characterized by a synapsis of the chromatin threads. This synapsis or conjugation occurs between the homologous chromosomes, that is, the chromosomes which have a similar genic constitution. Synapsis appears to begin most often at the ends of the threads and progresses toward the middle (fig. 67, zygotene). At this stage the chromatin threads may show a strong tendency to collapse and shrink into a mass toward one end of the nucleus (fig. 85C, D). This collapsed condition, when present, is called synizesis. The zygotene stage gradually passes into the pachytene condition.


Fig. 71. Certain aspects of the oocyte nucleus during the meiotic prophase. (A-G) Chromatin and nuclear changes in the oocyte of the cat up to the diplotene condition when the germinal vesicle is fully developed. (After de Winiwarter and Sainmont, Arch, biol., Paris, 24.) (H, I) Germinal vesicle in the dogfish, Scy Ilium canicula, and in Amphioxus. (After Marechal, La Cellule, 24.) Observe the typical “lamp-brush” chromosome conditions in the germinal vesicle of the shark oocyte. These lamp-brush chromosomes are developed during the diplotene stage of meiosis by great attenuation of the chromosomes and the formation of lateral extensions or loops from the sides of the chromosomes.



3) Pachytene (Pachynema) Stage. Gradually, the synapsis of the homologous chromosomes becomes more complete, and the threads appear shorter and thicker. The contracted threads in this condition are referred to as pachynema (figs. 69, 71, 85E). The nucleus in this manner comes to contain a number of bivalent chromosomes, each of which is made up of two homologous mates arranged side by side in synaptic union, known technically as parasynapsis. (Telosynapsis probably is not a normal condition.) Consequently, the number of chromosomes now appears to be haploid. Each pachytene chromosome (i.e., each of the pair of homologous chromosomes) gradually divides equationally into two daughter thread-like structures, generally referred to as chromatids. The exact time at which division occurs during meiosis is questionable. The entire group of four chromatids which arise from the splitting of the synapsed homologues is called a tetrad.

4) Diplotene (Diplonema) Stage. In the diplotene stage (figs. 67, 69, 71, 85F, G), two of the chromatids tend to separate from the other two. (See fig. 70A, C, E.) The four chromatids in each tetrad may now be observed more readily, at least in some species, because the various chromatids of each tetrad show a repulsion for one another, and the chromatids move apart in certain areas along their length. This condition is shown in both the male and female meiocyte, but in the latter, the repulsion or moving apart is carried to a considerable degree and is associated with a great lengthening and attenuation of the chromatids. (See fig. 67.) In the female meiocyte at this stage, the chromosomes become very diffuse and are scattered throughout the nucleus, somewhat resembling the non-mitotic condition (figs. 71F-T, 72B-E). The peculiar behavior of the chromosomes and nucleus of the oocyte in the diplotene stage of meiosis is described more in detail on p. 141.

Although there is a tendency for the chromatids to widen out or separate from each other at this time, they do remain associated in one or more regions. In these regions of contact, the paired chromatids appear to exchange partners. This point of contact is called a chiasma (plural, chiasmata). Hence, a chiasma is the general region where the chromatids appear to have exchanged partners when the tetrad threads move apart in the diplotene state. (See fig. 70, chiasmata.)

5) Diakinesis. The diplotene stage gradually transforms into the diakinesis state (figs. 67, 69, 72F, 85H) by a process of marked chromosomal contraction. There also may be an opening up of the tetrads due to a separation of the homologous mates in the more central portions of the tetrad, with the result that only the terminal parts of the chromatids remain in contact. This latter process is called “terminalization.” Coincident with this partial separation, a further contraction of the tetrads may occur. As a result, at the end of diakinesis the tetrads may assume such curious shapes as loops, crosses, rings, etc., scattered within the nucleus of the female and male meiocyte (fig. 69, diakinesis). The nuclear membrane eventually undergoes dissolution, and



Fig. 72. Growth of the nucleus during meiosis in the amphibian egg, showing the enlarged germinal vesicle and diplotene lamp-brush chromosomes with lateral loops. (A) Early diplotene nucleus of the frog. (B, C, E) Different phases of the diplotene nucleus in this form. These figures are based upon data provided by Duryee (’50) and sections of the frog ovary. (D) Drawing of the unfixed germinal vesicle of Triturus. Some aspects of the attenuate chromatin threads with lateral loops are shown. The nucleoli are numerous and occupy the peripheral region of the germinal vesicle. (F) Semidiagrammatic drawing of the later phases of the developing frog egg. It shows the germinal vesicle assuming a polar condition, with the initial appearance of germinal vesicle shrinkage before the final dissolution of the nuclear membrane. Observe that the chromosomes are contracting and now occupy the center of the germinal vesicle.




Fig. 73. Various aspects of Sertoli-cell conditions in the fowl. (Redrawn from Zlotnick, Quart. J. Micr. Sc., 88.) (A) Resting Sertoli cell, showing mitochondria. (B) Sertoli element at the beginning of cytoplasmic elongation. (C) Sertoli cell with associated late spermatids.



Fig. 74. Types of chordate sperm. All the chordate sperm belong to the flagellate variety. (A) Amphioxus (protochordate). (B) Salmo (teleost). (C) Perea (teleost). (D) Petromyzon (cyclostome). (E) Raja (elasmobranch). (F) Bufo (anuran). (G) Rana (anuran). (H) Salamandra (urodele). (I) Anguis (lizard). (J) Crex (bird). (K) Fringilla (bird). (L) Turdus (bird). (M) Echidna (monotrematous mammal). (N) Mus (eutherian mammal). (O, P) Man (full view and side view, respectively).


the tetrads become arranged on the metaphase plate of the first maturation division. (See figs. 69, first maturation division; 72F, 119A, B.) This division is described on pp. 132 and 133.

d. Peculiarities of Nuclear Behavior in the Oocyte During Meiosis; the Germinal Vesicle

Although the movements of the chromosomes during meiosis in the developing male and female gamete appear to follow the same general behavior


Fig. 75. Non-flagellate sperm. (A-C) Ameboid sperm of Polyphemus. (After Zacharias.) (D) Lobster, Homarus. (After Herrick.) (E) Decapod Crustacea, Galathea (Anomura). (After Koltzoff.) (F) Nematode woim, Ascaris.


Fig. 76. Conjugate sperm of grasshopper associated temporarily to form the “sperm boat.”


pattern (fig. 67), some differences do occur. For example, in the female when the diplotene stage is reached, the repulsion of the tetrad threads is greater (figs. 67, $ and 9 \ 12). Furthermore, the chromatids elongate and become very attenuate although they appear to retain their contacts or chiasmata (fig. 72). Side loops and extensions from the chromatids also may occur, especially in those vertebrates with large-yolked eggs (e.g., amphibia, fishes, etc.). (See figs. 71H, 72B-D.) When these lateral extensions are present, the chromosomes appear diffuse and fuzzy, taking on the characteristics which suggest their description as “lamp-brush” chromosomes. Another difference of chromatic behavior is manifested by the fact that the chromosomes in the developing female gamete during the diplotene stage are not easily stained by the ordinary nuclear stains, whereas the chromosomes in the spermatocyte stain readily.



Fig. 77. Spatula-type sperm of various mammals. (Compiled from Bowen; Gatenby and Beams; Gatenby and Woodger; see references in bibliography.) Observe the vacuole inside the head of the sperm. Gatenby and Beams found that this vacuole, in some instances, stains similar to a nucleolus, but suggest it may be a hydrostatic organ, or respiratory structure. (P. 20, Quart. J. Micr. Sc., 78.)



Aside from the differences in chromosomal behavior, great discrepancies in the amount of growth of the nucleus occur in the two gametes during meiosis. The nucleus of the oocyte greatly increases in size and a large quantity of nuclear fluid or sap comes to surround the chromosomes (figs. 7 IF, G; 72C, ^ F). Correlated with this increase in nuclear size, the egg grows rapidly.


and deutoplasmic substance is deposited in the cytoplasm (fig. 68F-H). As differentiation of the oocyte advances, the enlarged nucleus or germinal vesicle assumes a polar position in the egg (figs. 68H, 70F). When the oocyte has finished its growth and approaches the end of its differentiation, the



Fig. 78. Different shapes and positions of the acrosome. (A) Type of acrosome found in Mollusca, Echinodermata, and Annelida. (B) Reptilia, Aves, and Amphibia. (C) Lepidoptera. (D) Mammalia. (E) Many Hemiptera and Coleoptera. (After Bowen, Anat. Rec., 28.) (F) Sperm of certain birds, i.e., finches. (After Retzius, Biol.

Untersuchungen, New Series 17, Stockholm, Jena.) Observe the well-developed acrosome in the form of a perforatorium. The spiral twist of the acrosome shown in this drawing is characteristic of passerine birds.


Fig. 79. Sperm of urodele amphibia. (After Meves, 1897, Arch. f. mikr. Anat. u. Entwichlingsgesch., 50; McGregor, 1899, J. Morphol., 15. (A~E) Stages in the morphogenesis of the sperm of Salamandra. (F) Diagram of head, middle piece, etc. of the sperm of the urodele.


chromosomes within the germinal vesicle condense once again, decrease in length (fig. 72F), and assume conditions more typical of the diakinesis stage (figs. 67; 1 19A). The tetrad chromosomes now become visible. Following the latter chromosomal changes, the nuclear membrane breaks down (fig. 119A), and the chromatin elements pass onto the spindle of the first maturation division (fig. 119B). The nuclear sap, membrane, nucleolus, and general framework pass into the surrounding cytoplasmic substance (figs. 119A; 132A-C). This nuclear contribution to the cytoplasm appears to play an important part in fertilization and development, at least in some species (fig. 132C; the clear protoplasm is derived from the nuclear plasm).

e. Character of the Meiotic (Maturation) Divisions in the Spermatocyte Compared with Those of the Oocyte

1) Dependent Nature of the Maturation Divisions in the Female Meiocyte.

The maturation divisions in the developing male gamete occur spontaneously and in sequence in all known forms. But in most oocytes, either one or both of the maturation divisions are dependent upon sperm entrance. For example, in Ascaris, a nematode worm (fig. 133), and in Nereis, a marine annelid worm (fig. 130), both maturation divisions occur after the sperm has entered and are dependent upon factors associated with sperm entrance. A similar condition is found in the dog (van der Stricht, ’23; fig. 115) and in the fox (Asdell, ’46). In the urochordate, Styela, the germinal vesicle breaks down, the nuclear sap and nucleolus move into the surrounding protoplasm, and the first maturation spindle is formed as the egg is discharged into the sea water (fig. 116A, B). Further development of the egg, however, awaits the entrance of the sperm (fig. 116C--F). Somewhat similar conditions are found in other Urochordata. In the cephalochordate, Amphioxus, and in the vertebrate group as a whole (with certain exceptions) the first polar body is formed and the spindle for the second maturation division is elaborated before normal sperm entrance (figs. 117C, D; 119D). The second maturation division in the latter instances is dependent upon the activities aroused by sperm contact with the oocyte. In the sea urchin, sperm can penetrate the egg before the maturation divisions occur; but, under these conditions, normal development of the egg does not occur. Normally in this species both maturation divisions are effected before sperm entrance, while the egg is still in the ovary. When the egg is discharged into the sea water, the sperm enters the egg, and this event affords the necessary stimulus for further development (fig. 131).

2) Inequality of Cytoplasmic Division in the Oocyte. When the first maturation division occurs, the two resulting cells are called secondary spermatocytes in the male and secondary oocytes in the female (figs. 67, 69). The secondary spermatocytes are smaller both in nuclear and cytoplasmic volume. They also form a definite nuclear membrane. Each secondary spermatocyte then divides and forms two equal spermatids. In contrast to this condition of equality in the daughter cells of the developing male gamete during and following the maturation divisions, an entirely different condition is found in the developing female gamete. In the latter, one of the secondary oocytes is practically as large as the primary oocyte, while the other or first polar body (polocyte) is extremely small in cytoplasmic content although the nuclear material is the same (fig. 117D). During the next division the secondary oocyte behaves in a manner similar to that of the primary oocyte, and a small second polocyte is given off, while the egg remains large (fig. 117E, F). Unlike the secondary spermatocyte, the secondary oocyte does not form a nuclear membrane. The polar body first formed may undergo a division, resulting in a total of three polar bodies (polocytes) and one egg (ootid).

/. Resume of the Significance of the Meiotic Phenomena

In view of the foregoing data with regard to the behavior of the male and female gametes during meiosis, the significant results of this process may be summarized as follows:

(1) There is a mixing or scrambling of the chromatin material brought about by the crossing over of genic materials from one chromatid to another.

(2) Much chromatin material with various genic combinations is discarded during the maturation divisions in the oocyte. In the latter, two polar bodies are ejected with their chromatin material as described above. The egg thus retains one set of the four genic combinations which were present at the end of the primary oocyte stage; the others are lost. (A process of discarding of chromatin material occurs in the male line also. For although four spermatids and sperm normally develop from one primary spermatocyte, great quantities of sperm never reach an egg to fertilize it, and much of the chromatin material is lost by the wayside.)

(3) A reduction of the number of chromosomes from the diploid to the haploid number is a significant procedure of all true meiotic behavior.

(For more detailed discussions and descriptions of meiosis, see De Robertis, et al., ’48; Sharp, ’34, ’43; Snyder, ’45; White, ’45.)

4. Cytosomal (Cytoplasmic) Maturation of the Gametes a. General Aspects of Cytoplasmic Maturation of the Gametes

During the period when the meiotic prophase changes occur in the nucleus of the oocyte, the cytoplasm increases greatly and various aspects of cytoplasmic differentiation are effected. That is, differentiation of both nuclear and cytoplasmic materials tend to occur synchronously in the developing


Fig. 80. Morphogenesis of guinea-pig and human sperm. (A) Spermatocyte of guinea pig before first maturation division. The Golgi complex with included proacrosomic granules and centrioles is shown. (After Gatenby and Woodger, ’21.) (B) Young sister

spermatids of guinea pig. (C) Later spermatid of guinea pig showing acroblast with proacrosomic granules. (D) Young human spermatocyte, showing Golgi apparatus with proacrosomic granules similar to that shown in (A). (After Gatenby and Beams, ’35.) (E) Spermatid of guinea pig later than that shown in (C), showing acroblast with Golgi substance being discarded from around the acroblast. (F) Later human spermatid, showing Golgi substance surrounding acroblast with acrosome bead. (After Gatenby and Beams, ’35.) (G) Later human spermatid, showing acroblast, with acrosome bead

within, surrounded by a vacuole. (After Gatenby and Beams, ’35.) (H) Later spermatid

of guinea pig, showing outer and inner zones of the acrosome. The inner zone corresponds somewhat to the acrosome bead shown in (G) of the human spermatid. (After Gatenby and Wigoder, Proc. Roy. Soc., London, s.B., 104.)

female gamete. In the male gamete, on the other hand, the meiotic processes are completed before morphological differentiation of the cytoplasm is initiated.

Another distinguishing feature in the morphogenesis of the sperm relative to that of the egg is that the cytoplasmic differentiation of the sperm entails a discarding of cytoplasm and contained cytoplasmic structures, whereas the oocyte conserves and increases its cytoplasmic substance (fig. 68). In regard to the behavior of the cytoplasms of the two developing gametes, it is interesting to observe that the idiosome-Golgi-mitochondrial complex behaves very differently in the two gametes (fig. 68).

A third condition of egg and sperm differentiation involves the possible function of the “nurse cells.” In the vertebrate ovary the follicle cells which surround the egg have much to do with the conditions necessary for the differentiation of the oocyte. The latter cannot carry the processes of differentiation to completion without contact with the surrounding follicle cells. Spermiogenesis also depends upon the presence of a nurse cell. In the vertebrate seminiferous tubule, the Sertoli cell is intimately concerned with the transformation of the spermatid into the morphologically adult sperm, and a close contact exists between the developing sperm element and the Sertoli cell during this period (figs. 65, 66, 73). In the discharge of the formed sperm elements into the lumen of the tubule, the Sertoli cell also is concerned (Chap. 1).

b. Morphogenesis (Spermiogenesis; Spermioteleosis) of the Sperm

1) Types of Sperm. There are two main types of sperm to be found in animals, namely, flagellate and non-flagellate sperm (figs. 74, 75). Flagellate sperm possess a flagellum or tail-like organelle; non-flagellate sperm lack this structure. The flagellate type of sperm is found quite universally among animals; non-flagellate sperm occur in certain invertebrate groups, particularly in the nematode worms, such as Ascaris, and in various Crustacea, notably the lobster, crab, etc. (fig. 75). Flagellate sperm may be either uniflagellate or biflagellate. Single flagellate sperm occur in the majority of animals, while a biflagellate form is found in the platode, Procerodes. However, biflagellate sperm may be found as abnormal specimens among animals normally producing uniflagellate sperm.

Conjugate sperm are produced in certain animal species. For example, two sperm heads adhere closely together in the opossum (fig. 125), also in the beetle, Dytiscus, and in the gastropod, Turritella. Many sperm heads become intimately associated in the grasshopper to form the so-called “sperm boat” (fig. 76). However, all conjugate sperm normally separate from each other in the female genital tract.

2) Structure of a Flagellate Sperm. The flagellate sperm from different species of animals vary considerably in size, shape, and morphological details. Some possess long, spear-shaped heads, some have heads resembling a hatchet, in others the head appears more or less cigar-shaped, while still others possess a head which resembles a spatula (fig. 74). The spatula-shaped head is found in the sperm of the bull, opossum, man, etc. The description given below refers particularly to the spatula-shaped variety. Although all flagellate sperm resemble one another, diversity in various details is the rule,


Fig. 81. Later stages of human spermatogenesis. (Redrawn from Gatenby and Beams,

1935.)

Fig. 82. Stages of guinea-pig spermatogenesis. Observe dual nature of the acrosome; also, middle-piece bead (kinoplasmic droplet). (A-C redrawn from Gatenby and Beams, 1935; D redrawn from Gatenby and Woodger, ’21.)

and the description given below should be regarded as being true of one type of sperm only and should not be applied to all flagellate sperm.

A fully differentiated spatulate sperm of the mammals possesses the following structural parts (fig. 77).

a) Head. Around the head of the sperm there is a thin, enveloping layer of cytoplasm. This cytoplasmic layer continues posteriad into the neck, middle piece, and tail. Within the cytoplasm of the head is the oval-shaped nucleus. Over the anterior half of the nucleus the apical body or acrosome is to be found, forming, apparently, a cephalic covering and skeletal shield for the nucleus. The caudal half of the nucleus is covered by the post-nuclear cap. This also appears to be a skeletal structure supporting this area of the nucleus; moreover, it affords a place of attachment for the anterior centrosome and the anterior end of the axial filament.

In human and bull sperm the acrosome is a thin cap, but in some mammalian sperm it is developed more elaborately. In the guinea pig it assumes the shape of an elongated, shovel-shaped affair (fig. 82), while in the mouse and rat it is hatchet or lance shaped (fig. 74N). In passerine birds the acrosome is a pointed, spiral structure often called the perforatorium (fig. 78). On the other hand, in other birds, reptiles, and amphibia it may be a simple, pointed perforatorial structure (figs. 74, 78, 79). In certain invertebrate species, it is located at the caudal or lateral aspect of the nucleus (figs. 75, 78).

b) Neck. The neck is a constricted area immediately caudal to the posterior nuclear cap and between it and the middle piece. Within it are found the anterior centriole and the anterior end of the axial filament. In this particular region may also be found the so-called neck granule.

c) Connecting Body or Middle Piece. This region is an important portion of the sperm. One of its conspicuous structures is the central core composed of the axial filament and its surrounding cytoplasmic sheath. At the distal end of the middle piece, the central core is circumscribed by the distal, or ring centriole. Investing the central core of the middle piece is the mitochondrial' sheath. The enveloping cytoplasm is thicker to some degree in this area of the sperm than that surrounding the head.

d) Flagellum. The flagellum forms the tail or swimming organ of the sperm. It is composed of two general regions, an anterior principal or chief piece and a posterior end piece. The greater part of the axial filament and its sheath is found in the flagellum. A relatively thick layer of cytoplasm surrounds the filament and its sheath in the chief-piece region of the flagellum, but, in the caudal tip or end piece, the axial filament seems to be almost devoid of enveloping cytoplasm. The end piece often is referred to as the naked portion of the flagellum.

In figure 79 is shown a diagrammatic representation of a urodele amphibian sperm. Two important differences from the mammalian sperm described above are to be observed, namely, the middle piece is devoid of mitochondria and is composed largely of centrioles 1 and 2, and the tail has an elaborate undulating or vibratile filament associated with the chief piece.

3) Spermiogenesis or the Differentiation of the Spermatid into the Morphologically Differentiated Sperm. The differentiation of the spermatid into the fully metamorphosed sperm is an ingenious and striking process. It involves changes in the nucleus, during which the latter as a whole contracts and in some forms becomes greatly elongated into an attenuant structure. (See figs. 79B-F; 85L-P.) It also is concerned with profound modifications of the cytoplasm and its constituents; the latter changes transform the inconspicuous


Spermatid into a most complicated structure. Some of these changes are outlined below.

a) Golgi Substance and Acroblast; Formation of the Acrosome. The Golgi substance or parts thereof previously associated with the idiosome of the spermatocyte (fig. 80A) proceeds to form the acrosome of the developing spermatid as follows: In the differentiating human sperm, the Golgi substance of the spermatocyte (fig. SOD) becomes aggregated at the future anterior end of the nucleus, as shown in fig. 80F, where it forms an acroblast within a capsule of Golgi substance. This acroblast later forms a large vacuole within which is the acrosomal “bead” (figs. 68B; 80G). The acrosomal bead proceeds to form the acrosomal cap, shown in figure 81 A, and the latter grows downward over the anterior pole of the nucleus (fig. 81 A, B). Most of the Golgi substance in the meantime is discarded (fig. 81 A, B). (Sec Gatenby and Beams, ’35.)

In the guinea pig the acroblast together with other Golgi substance, migrates around the nucleus toward the future anterior pole of the latter where the acroblast takes up its new position (fig. 80B, C, E). (See Gatenby and Woodger, ’21.) As shown in figure 80E, the acroblast is composed of inner and outer acrosomal substances. These inner and outer areas of the acroblast give origin respectively to the inner and outer zones of the acrosome (fig. 82). The peripheral or surrounding Golgi material of the acroblast detaches itself meanwhile from the developing acrosome (fig. 80E, H) and drifts downward toward the posterior end of the sperm. Eventually it is discarded with the excess cytoplasm and some mitochondrial material. In some animal species (e.g., grasshopper) the acrosomal substance arises from a multiple type of acroblast (Bowen, ’22). (See fig. 83.) Nevertheless, the general process of acrosome formation is similar to that outlined above.

b) Formation of the Post-nuclear Cap. All spatulate sperm of mam


Fig. 83. Formation of the acrosome from a multiple acroblast in the grasshopper. (After Bowen, Anat. Rec., 24.)


Fig. 84. The mitochondrial nebenkern and its elaborate development in Brachynema. (After Bowen, J, Morphol., 37 and Biol. Bull., 42.) (B-I) Division of the nebenkern

(A) and its elaboration into two attenuant strands extending posteriad into the flagellum.

mals possess a nucleus which has an acrosomal cap over its anterior aspect and a post-nuclear cap covering its posterior area. Both of these caps tend to meet near the equator of the nucleus (fig. 77).

The exact origin of the post-nuclear cap is difficult to ascertain. In the human sperm it appears to arise from a thickened membrane in association with centriole 1 (fig. 80G, post-nuclear membrane). This membrane grows anteriad to meet the acrosomal cap (fig. 81A-C). In the sperm of the guinea pig, a series of post-nuclear granules in the early spermatid appear to coalesce to form the post-nuclear cap (fig. 82A-C).

c) Formation of the Proximal and Distal Centrioles; Axial Filament. While the above changes in the formation of the acrosome are progressing, the centriole (or centrioles) of the idiosome move to the opposite side of the nucleus from that occupied by the forming acrosome, and here in this position the proximal and distal centrioles of the future sperm arise. In this area the neck granules also make their appearance (figs. 68B; 80F-H). The axial filament arises at this time and it probably is derived from the two centrioles simultaneously (fig. 80F, H). The centrioles soon become displaced along the axial filament, the caudal end of which projects from the surface of the cell membrane (fig. 80F-H). The axial filament grows outward posteriorly from the cell membrane in line with the two centrioles and the acrosome-forming material. The anterior-posterior elongation of the sperm thus begins to make its appearance (fig. 80H). The anterior centriole retains a position close to the nuclear membrane, but the posterior or ring centriole moves gradually posteriad toward the cell surface (figs. 81, 82A-C).

d) Mitochondrial Material and Formation of the Middle Piece OF THE Sperm. The behavior of the mitochondria in the formation of sperm varies greatly. In the spatulate sperm described above, a portion of the mitochondrial substance becomes aggregated around the axial filament in the middle-piece area (figs. 77, 82D). In certain amphibian sperm the middle piece appears to be formed mainly by centrioles 1 and 2 (fig. 79D-F). In certain insects the mitochondrial body or nebenkern, divides into two masses which become extended into elongated bodies associated with the flagellum (fig. 84). Some of the mitochondrial substance is discarded with the Golgi substance and excess cytoplasmic materials.

e) The Cytoplasm, Axial Filament, Mitochondria, and Tail Formation. Synchronized with the above events, the cytoplasm becomes drawn out in the posterior direction, forming a thin cytoplasmic layer over the sperm head, and from thence posteriad over the middle piece and the chief piece of the flagellum. However, the end piece of the flagellum may be devoid of investing cytoplasm (fig. 77). As the cytoplasm is elongating posteriorly over the contained essential structures of the forming sperm, much of the cytoplasm and Golgi substance and some mitochondria are discarded and lost from the sperm body. It may be that these discarded bodies form a part of the essential substances of the spermatic (seminal) fluid. (See Chap. 1.) (See figs. 66; 68B-E; 81; 82; 85M-0.)

The centralized core of the tail is the axial filament which arises in relation to centrioles 1 and 2 and grows posteriad through the middle piece and tail

(figs. 80F-H; 81A-C; 82A-~C; 85M-P). A considerable amount of mitochondrial material may also enter into the formation of tail (fig. 84).

A peculiar, highly specialized characteristic of many sperm tails is the development of a vibratile membrane associated with the axial filament (fig. 79E, F). Its origin is not clear, but it probably involves certain relationships with the mitochondrial material as well as the cytoplasm and axial filament.

In the formation of the human and guinea-pig sperm, the nucleus experiences only slight changes in shape from that of the spermatid. However, in many animal species, spermiogenesis involves considerable nuclear metamorphosis as well as cytoplasmic change (figs. 69, 79, 85).

In summary it may be stated that while the various shapes and sizes of mature flagellate sperm in many animal species, vertebrate and invertebrate,


Fig. 85. Spermatogenesis in the common fowl. Observe extreme nuclear metamorphosis. (After Miller, Anat. Rec., 70.) (A) Resting spermatocyte. (B) Early leptotene stage. (C, D) Synaptene stage. (E) Pachytene stage. (F, G) Diplotene stage. (H) Diakinesis. (I) First division, primary sperm. (J-P) Metamorphosing sperm.



are numerous, there is a strong tendency for spermiogenesis to follow similar lines of development. Deviations occur, but the following comparisons between mammalian and insect spermiogenesis, somewhat modified from Bowen (’22), illustrate the uniformity of transformation of the basic structures of the primitive meiocyte:


Mammalian Sperm

Insect Sperm

Nucleus — head

Nucleus — head

Centrioles — originally double and arranged in a proximal-distal formation. The axial filament arises from both centrioles

Centrioles — same as in mammals

Mitochondria — form an elaborate sheath for the anterior portion of the axial filament

Mitochondria — form a somewhat similar sheath for the axial filament

Idiosome and Golgi apparatus (acroblast portion) — gives origin to a vesicle which contains a granule, the acrosome granule, which is involved in the production of the acrosome

Idiosome and Golgi apparatus — much the same as in mammals

Excess Golgi substance — cast off with excess cytoplasm

Excess Golgi substance — cast off with excess cytoplasm

Excess cytoplasm — cast off — may be part of seminal fluid or possibly may be engulfed by Sertoli cells

Excess cytoplasm — cast off — may be part of seminal fluid or possibly may be engulfed by epithelial cells of the sperm cyst wall


c. Cytoplasmic Differentiation of the Egg

The cytoplasmic differentiation of the egg involves many problems. These problems may be classified under three general headings, viz.:

(1) Formation of the deutoplasm composed of fats, carbohydrates and proteins,

(2) development of the invisible organization within the true protoplasm or hyaloplasm, and finally,

(3) formation of the vitelline or egg membrane or membranes.

In view of the complexity of these three problems and of their importance to the egg in the development of the new individual, the mature oocyte or egg is in a sense no longer a single cell. Rather, it is a differentiated mass of protoplasm which is capable, after proper stimulation, to give origin to a new individual composed of many billions of cells. As such, the differentiation of the oocyte within the ovary represents a relatively unknown period of embryological development.


Fig. 86. Young oogonia of the fowl entering the growth (oocyte) stage. (A) Idiosome from which the Golgi substance has been removed and stained to show the centrosphere (archoplasm). The centrosome has two centrioles. (B) Idiosome with surrounding Golgi substance. The mitochondria surround the Golgi substance and the nucleus. (After Brambell, ’25.)


Fig. 87. The so-called mitochondrial yolk body in the developing egg of the fowl. (A) Oocyte from 11 -week-old chick, showing mitochondrial cloud and Golgi substances I and II. (B) Oocyte from ovary of adult fowl, showing both types of Golgi substance and mitochondrial cloud. (C) Oocyte from ovary of adult fowl, showing the appearance of the mitochondrial yolk body within the mitochondrial cloud. (D) Oocyte from ovary of adult fowl, showing fragmentation of Golgi substance 1 and the association of the resulting Golgi granules around the mitochondrial yolk body. (After Brambell, ’25.)



Fig. 88. Portion of follicle and periphery of oocyte from ovary of the adult bird, showing the mitochondria and their transformation into the M-yolk spheres of Brambell. (After Brambell, ’25.)

Before considering the various aspects of cytoplasmic differentiation of the oocyte, it is best for us to review the types of vertebrate and other chordate eggs in order to be able to visualize the various goals toward which the developing oocyte must proceed.

1) Types of Chordate Eggs. Eggs may be classified according to the amount of deutoplasm (yolk, etc.) present in the cytoplasm as follows:

a) Homolecithal (Isolecithal) Eggs. True homolecithal eggs in the phylum Chordata are found only in the mammals, exclusive of the Prototheria. Here the deutoplasm is small in amount, and is present chiefly in the form of fat droplets and small yolk spherules, distributed in the cytoplasm of the egg (figs. 118A, B; 147A).

b) Telolecithal Eggs. In the telolecithal egg the yolk is present in considerable amounts and concentrated at one pole. Telolecithality of the egg in the phylum Chordata exists in various degrees. We shall arrange them in sequence starting with slight and ending with very marked telolecithality as follows:

(1) Amphioxus and Styela. In Amphioxus and Styela from the subphyla Cephalochordata and Urochordata, respectively, the yolk present is centrally located in the egg before fertilization but becomes concentrated at one pole at the time of the first cleavage where it is contained for the most part within the future entoderm cells (figs. 132D, 167A).

(2) In many Amphibia, such as the frogs and toads, and also in the Petromyzontidae or fresh-water lampreys among the cyclostome fishes, the yolk present is greater in amount than in the preceding eggs. As such, it is concentrated at one pole, the future entodermal or vegetal pole, and a greater degree of telolecithality is attained than in the eggs of Amphioxus or Styela (fig. 141 A).

(3) In many Amphibia, such as Necturus, also in Neoceratodus and Lepidosiren among the lung fishes, and in the cartilaginous ganoid fish, Acipenser, yolk is present in considerable amounts, and the cytoplasm of the animal pole is smaller in comparison to the yolk or vegetal pole (figs. 150, 151, 152).

(4) In the bony ganoid fishes, Amia and Lepisosteus, as well as in the Gyrnnophiona (legless Amphibia) the yolk is situated at one pole and is large in quantity (figs. 153B-F; 154).

(5) Lastly, in a large portion of the vertebrate group, namely, in reptiles, birds, prototherian mammals, teleost and elasmobranch fishes, and in the marine lampreys, the deutoplasm is massive and the protoplasm which takes part in the early cleavages is small in comparison. In these eggs the yolk is never cleaved by the cleavage processes, and development of the embryo is confined to the animal pole cytoplasm (figs. 46, 47).

2) Formation of the Deutoplasm. The cytoplasm of the young oocyte is small in quantity, with a clear homogeneous texture (figs. 68A; 86A, B). As the oocyte develops, the cytoplasmic and nuclear volumes increase (fig. 68F), and the homogeneity of the cytoplasm is soon lost by the appearance of deutoplasmic substances (fig. 68G, H). In the oocyte of the frog, for example, lipid droplets begin to appear when the oocyte is about 50 /x in diameter (fig. 72A). (See Brachet, ’50, p. 53.) A little later glycogen makes its appearance, and finally yolk protein arises.

The origin of fat droplets and yolk spherules has been ascribed variously to the activities of chondriosomes (mitochondria and other similar bodies), Golgi substance, and of certain vacuoles. Most observers place emphasis upon the presence of a so-called “yolk nucleus” or “yolk-attraction sphere” situated near the nucleus of many oocytes as a structure associated with fat and yolk formation. In general, two types of yolk bodies have been described. One is the yolk nucleus of Balbiani and the other the mitochondrial yolk body of Brambell. The yolk nucleus of Balbiani (fig. 86A, B) consists of the following:

  1. a central body, the centrosphere or archoplasmic sphere within which one or more centriole-like bodies are found, and
  2. surrounding this central body, a layer of Golgi substances and chondriosomes (i.e., mitochondria, etc.).

This cytoplasmic structure probably is related to the idiosome of the oogonia (fig. 68A).

The formation of the deutoplasm, according to the theory associated with the Balbiani type of yolk nucleus is as follows: The surrounding pallial layer of Golgi substance and mitochondria moves away from the central portion (i.e., away from the centrosphere) of the yolk nucleus and becomes scattered and dispersed as small fragments within the cytosome (fig. 68F, G). The yolk nucleus as an entity thus disappears, and its fragments become immersed within the substance of the cytoplasm. Coincident with this dispersion of yolk nuclear material, rapid formation of small yolk spherules and fat droplets occur (fig. 68H). It appears thus that the formation of the deutoplasm composed of fat droplets and yolk spherules is directly related to the activities of the Golgi substance and chondriosomes.


Fig. 89. (A) Cytoplasm of oocyte, showing formation of a second kind of yolk (the M-C-yolk) in a vacuole surrounding the M-yolk sphere. (After Brambell, ’25.) (B)

Passing of Golgi substance from the follicle cells into the ooplasm of developing oocyte of the fowl. (After Brambell, '25.)



Fig. 90. Diagrams showing contrasting theories explaining the organization of polarity of the cytoplasm of the fully developing egg or oocyte. Diagram at left shows polarity explained according to quantitative differences, while the diagram to the right shows qualitative differences. A = animal pole; V = vegetal pole. E represents a substance or a factor, while EN-1, EN-2, etc., represent different quantities of substance E distributed from pole to pole. SEC, SEN and SM are different chemical substances assumed to be responsible for the determination of the ectoderm, entoderm, and mesoderm of the developing embryo. (After Barth: Embryology, New York, Dryden Press.)


On the other hand, the interpretation and description of the yolk body and its subsequent activities given by Brambell (’25) present a different view. According to the latter author, the yolk body is composed entirely of mitochondria; the Golgi substance and centrosphere are absent. Yolk formation proceeds as follows: As the young oocyte grows, the mitochondria increase in number and form the mitochondrial cloud (fig. 87A, B). The transitory mitochondrial yolk body differentiates within this cloud (fig. 87C). The mitochondrial yolk body ultimately breaks up into a mass of mitochondria, and the latter becomes dispersed in the cytoplasm of the oocyte (figs. 68F, G; 87D). Some of these dispersed mitochondria transform directly into yolk spheres (figs. 68H, 88, 89). Following this, another kind of yolk is formed in vacuoles surrounding these original yolk spheres (figs. 68H, 89 A, yolk spheres plus vacuoles). The fat droplets (C-yolk) within the ooplasm are formed according to Brambell “possibly under the influence of Golgi elements” (fig. 68H, fat droplets). Relative to the function of the yolk nucleus and its mitochondria, Brachet (’50), p. 57, considers it significant at the beginning, but its real importance is still to be understood.

The relationship, if any, of the oocyte nucleus to the deposition of yolk materials is not apparent. One must not overlook the real probability that the germinal vesicle (i.e,, the enlarged nucleus of the oocyte) may be related to the increase and growth of the cytoplasm and to yolk formation, for it is at this time that the chromatin threads surrender their normal diplotene appearance and become diffusely placed in the germinal vesicle. They also lose much of their basic chromatin-staining affinities while the Feulgen reaction is diminished (Brachet, ’50, p. 63). With regard to the possible function of the germinal vesicle in yolk synthesis, the following quotation is taken from a publication by Brachet (’47):

It is well worth pointing out that Puspiva (1942), using a very delicate and precise technique, found no correlation between the dipeptidase content of the nucleus and the onset of vitellus synthesis: such a correlation exists, however, in the case of the cytoplasm where dipcptidase increases markedly when the first yolk granules make their appearance. These results suggest that there is not evidence that the nucleus is the site of an especially active metabolism; cytoplasmic dipeptidase probably plays a part in yolk protein synthesis; if the nucleus controls such a synthesis, it works in a very delicate and still unknown way.

However, the means by which protein synthesis is effected still is a problem which awaits explanation (Northrop, ’50). (The interested student should consult Brachet, ’50, Chap. Ill, for a detailed discussion of the cytochemistry of yolk formation.)

Another aspect of the problem of cytoplasmic growth and differentiation of the oocyte presents itself for further study. Brambell (’25) concluded from his observations that Golgi substance passes from the follicle cells into the ooplasm of the growing bird oocyte and contributes to the substance of the peripheral layer (fig. 89B). Palade and Claude (’49) suggest that at least some of the Golgi substance be identified as myelin figures which develop “at the expense of lipid inclusions.” Thus it may be that the Golgi substance which Brambell observed (fig. 89B) passing from the follicle cells to the oocyte represents lipid substance. In the growing oocyte of the rat, Leblond (’50) demonstrated the presence of small amounts of polysaccharides in the cytoplasm of the oocyte, while the surrounding zona pellucida and follicle cells contained considerable quantities. These considerations suggest that the blood stream using the surrounding follicle cells as an intermediary may contribute food materials of a complex nature to the growing cytoplasm of the oocyte.

The localization of the yolk toward one pole of the egg is one of the movements which occurs during fertilization in many teleost fishes. In these forms, the deutoplasmic materials are laid down centrally in the egg during oogenesis, but move poleward at fertilization (fig. 122). A similar phenomenon occurs also during fertilization in Amphioxus and Styela among the protochordates. In many other fishes and in the amphibia, reptiles, birds, and monotrematous mammals, the yolk becomes deposited or polarized toward one pole of the oocyte during the later stages of oocyte formation, as the cytoplasm and the germinal vesicle move toward the other pole (figs. 68H, 72F). The polarization of the deutoplasmic substances thus is a general feature of the organization of the chordate egg.

3) Invisible Morphogenetic Organization Within the Cytoplasm of the Egg.

Two general categories of substances are developed within the cytoplasm of the oocyte during its development within the ovary, viz.:

(1) the visible or formed cytoplasmic inclusions, and

(2) an invisible morphogenetic ground substance.

The former group comprises the yolk spherules, fats, and other visible, often pigmented bodies which can be seen with the naked eye or by means of the microscope. The morphogenetic ground substance probably is composed of enzymes, hormones, and various nucleocytoplasmic derivatives enmeshed within the living cytoplasm. However, although we may assume that the basic, morphogenetic ground substance is composed of enzymes, hormones, etc., the exact nature of the basic substance or its precise relationship to the various formed inclusions of the cytoplasm is quite unknown (see Fankhauser, ’48, for discussion). More recent experiments demonstrate that the yolk or deutoplasmic material not only serves as a reservoir of energy for embryonic development but also is in some way connected with the essential, basic organization of the egg.

Although we know little concerning the exact nature of the morphogenetic organization of the egg or how it forms, studies of embryological development force upon us but one conclusion, to wit, that, during the period when the oocyte develops in the ovary, basic conditions are elaborated from which the future individual arises (Fankhauser, ’48). Within the cytoplasm of the mature egg of many chordates, this inherent organization is revealed at the time of fertilization by the appearance of definite areas of presumptive organ-forming substances. For example, in the egg of the frog and other amphibia, the yolk pole is the stuff from which the future entodermal structures take their origin; the darkly pigmented animal or nuclear pole will eventually give origin to epidermal and neural tissues; and from the zone between these two areas mesodermal and notochordal tissues will arise (fig. 119K). Similar major organ-forming areas in the recently fertilized egg have been demonstrated in other chordates, as in the ascidian, St ye la, and in the cephalochordate, Amphioxus. In the eggs of reptiles, birds, and teleost and elasmobranch fishes, while the relationship to the yolk is somewhat different, major organ-forming areas of a similar character have been demonstrated at a later period of development (Chaps. 6-9). This suggests that these eggs also possess a fundamental organization similar, although not identical, to that in the amphibian egg.

4) Polarity of the Egg and Its Relation to Body Organization and Bilateral Symmetry of the Mature Egg. One of the characteristic features of the terminal phase of egg differentiation in the chordate group is the migration of the gerrninal vesicle toward the animal pole of the egg (figs. 72F, 119A). As stated aboveTTn many vertebrate eggs the deutoplasmic jnaterial becomes situated at the opposite pole, known as the vegetal (vegetative) or yolk pole, either before fertilizatloiToTsEortty after. The relatively yolk-free protoplasm aggregates at the animal pole. Consequently the maturation divisions of the egg occur at this pole (fig. 1 19 A, B, D). The formation of a definite polarity of the egg, therefore, is one of the main results of the differentiation of the oocyte.

Various theories have been suggested in an endeavor to explain polarity in the fully developed egg or oocyte. All these theories emphasize qualitative and quantitative differences in the cytoplasmic substances extending from one pole of the egg to the other (fig. 90).

The animal and vegetal poles of the egg have a definite relationship to the organization of the chordate embryo. In Amphioxus, the animal pole becomes the ventro-anterior part of the embryo, while in the frog the animal pole area becomes the cephalic end of the future tadpole, and the yolk pole comes to occupy the posterior aspect. In teleost and elasmobranch fishes the yolk-laden pole lies in the future ventral aspect of the embryo, and it occupies a similar position in the reptile, bird, and prototherian mammal (see fig. 215). Studies have shown that the early auxiliary or trophoblastic cells in eutherian mammals lie on the ventral aspect of the future embryo. Consequently, it is to be observed that the various substances in mature vertebrate and protochordate eggs tend to assume a polarized relationship to the future embrydnic axis and body organization.


Many vertebrate and protochordate eggs possess a bilateral symmetry which becomes evident when the fertilization processes are under way or shortly after their conclusion. The appearance of the gray crescent in the frog’s egg (fig. 119K) and in other amphibian eggs during fertilization and the similar appearance of the yellow crescent in the fertilized egg of the ascidian, Styela (fig. 132D) serve to orient the future right and left halves of the embryo. Conditions similar to that of Styela, but lacking the yellow pigment, are present in Amphioxus. Similarly, in the chick, if one holds the blunt end of the egg to the left, and the pointed end to the right, the early embryo appears most often at right angles, or nearly so, to the axis extending from the broader to the smaller end of the egg, and in the majority of cases the cephalic end of the embryo will appear toward the side away from the body of the observer. There is some evidence that the “yolk” or egg proper is slightly elongated in this axis. It appears, therefore, that the general plane of bilateral symmetry is well established in the early chick blastoderm, although the early cleavages do not occur in a manner to indicate or coincide with this plane. In prototherian mammals, a bilateral symmetry and an anteroposterior orientation is established in the germinal disc at the time of fertilization, soon after the second polar body is discharged (fig. 136).

5) Membranes Developed in Relation to the Oocyte; Their Possible Sources of Origin. A series of membranes associated with the surface of the oocyte are formed during its development within the ovary. Three general types of such membranes are elaborated which separate from the oocyte’s surface at or before fertilization, leaving a perivitelline space between the egg’s surface and the membrane. They are:

( 1 ) A true vitelline membrane which probably represents a specialization or product of the ooplasmic surface. For a time this membrane adheres closely to the outer boundary of the ooplasm, but at fertilization it separates from the surface as a distinct membrane.

(2) A second membrane in certain chordates is elaborated by the follicle cells. It is known as a chorion in lower Chordata but is called the zona pellucida in mammals.

(3) A zona radiata or a thickened, rather complex, membrane is formed in many vertebrates; it may be considered to be a product of the ooplasm or of the ooplasm and the surrounding follicle cells.

All of the above membranes serve to enclose the egg during the early phases of embryonic development and therefore may be considered as primary embryonic membranes. As such, they should be regarded as a definite part of the egg and of the egg’s differentiation in the ovary. A description of these membranes in relation to the egg and possible source of their origin in the various chordate groups is given below.

a) Chorion in Styela. A previously held view maintained that the chorion


Fig. 91. Formation of the chorion in the egg of Styela. (A) Chorion is shown along the inner aspect of the follicular epithelium. The test cells lie in indentations of the peripheral ooplasm. (B) Optical section of an ovulated egg. (Redrawn and modified from Tucker, ’42.)


Fig. 92. Developing vitelline membranes of Scyllium canicula. Observe that two membranes are present in the young egg; later these membranes fuse into one membrane. (A) Surface area of young oocyte with a vitelline membrane and zona radiata. (B) Slightly older oocyte with the radiate zone not as prominent. (C) Older oocyte with a single, relatively thick, vitelline membrane. (D) Nearly mature oocyte with a thin vitelline membrane. (After Balfour, Plate 25, The Works of Francis Maitland Balfour, ed. by Foster and Sedgwick, London, Macmillan, 1885.)


and “test” cells of the egg of Styela were ejected from the surface cytoplasm at the time of ovulation (Conklin, ’05). A recent view, however, maintains that the test cells arise from follicle cells and come to lie in indentations of the periphery of the egg outside of the thin vitelline membrane (Tucker, ’42). The chorion is formed by the inner layer of follicle cells and comes to lie between the test cells and the inner layer of follicle cells in the mature egg (fig. 91A). At ovulation the chorion moves away from the surface of the oocyte. At this time also, the test cells move outward from their indentations in the peripheral ooplasm and come to lie in the perivitelline space between the egg surface and the chorion (fig. 9 IB). An ooplasmic membrane which represents the thin surface layer of ooplasm is present. However, it does not separate from the periphery of the egg at fertilization. During its early development, the embryo remains within this chorionic shell. The chorion thus represents the primary embryonic membrane of this species.


Fig. 93. Vitelline membranes of certain teleost fishes. (After Eigenmann, 1890.) (A)

Pygosteus pungtius. Radial section through micropyle of egg about 0.4 mm. in diameter. (B) Radial section through micropyle of egg of Perea, the perch. (C) Vitelline membranes of Fundulus heteroclitus about 0.8 mm. in diameter.



b) Egg Membranes of Amphioxus. Two surface membranes are formed and eventually separate from the egg of Amphioxus. The outer vitelline membrane is elaborated on the surface of the egg and remains in contact with this surface until about the time of the first maturation division. It then begins to separate from the egg’s surface. (See Chap. 5.) After the sperm enters and the second maturation division occurs, a second, rather thick, vitelline membrane also separates from the egg. The first and second vitelline membranes then fuse together and become greatly expanded to form the primary embryonic membrane. (See Chap. 5.) A thin ooplasmic membrane remains at the egg’s surface.

c) Vitelline Membrane and Zona Radiata of Elasmobranch Fishes. In the egg of the shark, Scy Ilium canicula, two egg membranes are formed, an outer and an inner membrane. The outer membrane is a homogeneous vitelline membrane, while the membrane which comes to lie beneath this outer membrane has a radiate appearance and hence may be called a zona radiata. This latter membrane soon loses its radiate appearance and becomes a thin membrane along the inner aspect of the vitelline membrane (fig. 92 A, B). In the mature egg both of these membranes form a thin, composite, vitelline membrane (fig. 92C, D). At about the time of fertilization the latter membrane separates from the egg’s surface; a perivitelline space then lies between these structures and the surface ooplasm of the egg.

d) Zona Radiata of Teleost Fishes. The surface ooplasm in teleost fishes gives origin to a membrane which in many cases has a radiate appearance. In some species this membrane appears to be composed of two layers. This radiate membrane which forms at the surface of the egg of teleost fishes appears to be the product of the ooplasm, and, therefore, should be regarded as a true vitelline membrane. In the perch a true chorion also is formed as a gelatinous or filamentous layer produced external to the radiate membrane by the follicle cells (fig. 93B). In Fundulus heteroclitus there are apparently three distinct parts to the membrane which surrounds the ooplasm of the egg:

( 1 ) a zona radiata,

(2) a thin structureless membrane external to the zona, and,

(3) the filamentous layer whose filaments are joined to the thin membrane around the zona (fig. 93C).

These three layers are probably derived from the ooplasm of the egg (Eigenmann, ’90). Consequently, the filamentous chorion or gelatinous layer, if derived from the egg itself, is not a true chorion in this particular egg.


Fig. 94. Vitelline membrane of an almost mature egg of the frog.


Fig. 95. Zona radiata (zona pellucida) or vitelline membrane of Chrysemys picta. (After Thing, ’18.)


Fig. 96. Zona radiata of the egg of the fowl. (After Brambell, ’25.)


At one end of the forming egg, a follicle cell sends an enlarged pseudopodiumlike process inward to the surface of the egg. As a result of this enlarged extension of the follicle cell to the ooplasmic surface, an enlarged pore-like opening in the zona radiata is formed. This opening persists as the micropyle after the egg leaves the ovary (fig. 93A).

As the teleost egg is spawned, the chorionic layer hardens when it comes in contact with the water. If fertilization occurs, the surface of the egg emits a fluid and shrinks inward from the zona radiata. In this manner, a perivitelline space is formed between the egg, and the zona is filled with a fluid. The egg is thus free to revolve inside of the zona (Chap. 5).

e) Vitelline Membrane (Zona Radiata) in Amphibia. In the amphibia, a vitelline membrane is formed probably by the surface ooplasm, although there may be contributions by the follicle cells of the ovary (Noble, ’31, p. 281). This membrane separates from the egg at the time of fertilization, forming a perivitelline space (fig. 94). The latter space is filled with fluid. Later the vitelline membrane expands greatly to accommodate the developing embryo. A delicate surface layer or membrane forms the outer portion of the ooplasm below the vitelline membrane. In some amphibia the vitelline membrane may have a radiate appearance.

f) Zona Radiata (Zona Pellucida) of the Reptile Oocyte. In the turtle group, the development of the zona radiata (pellucida) appears to be the product of the follicle cells (Thing, ’18). Filamentous prolongations of the follicle cells extend to the surface ooplasm of the developing egg (fig. 95). A homogeneous substance produced by the follicle cells then fills the spaces between these prolongations. The filamentous extensions of the follicle cells in this way produce a radiating system of canals passing through the homogeneous substance; hence the name, zona radiata. Bhattacharya describes Golgi substance as passing from the follicle cells through the canals of the zona radiata into the egg’s ooplasm in the developing eggs of Testudo graeca and Uromastix hardwicki. (See Brambell, ’25, p. 147.)

In contradistinction to the above interpretation, Retzius (’12) describes the homogeneous substance which forms the zona radiata of the lizard, Lacerta viridis, as originating from the ooplasm of the egg.

g) Vitelline Membrane (Zona Radiata) of the Hen’s Egg. The vitelline membrane, as in the turtle groups, appears to form about the young oocyte as a result of contributions from the surrounding follicle cells although the superficial ooplasm of the oocyte may contribute some substance. This occurs before the rapid deposition of yolk within the developing oocyte. It is probable that the follicle cells send small pseudopodium-like strands of cytoplasm through the numerous perforations of the very thin vitelline membrane around the oocyte’s surface into the superficial ooplasm in a similar manner to that which occurs in reptiles. The vitelline membrane (zona radiata) thus assumes a radiate appearance as it increases in thickness (figs. 47, 96).



Fig. 97. Kinoplasmic bead or droplet upon the middle piece of mammalian sperm.

(A) Pig sperm. (After Retzius, Biol. Untersuchungen, New Serfes, 10; Stockholm: Jena.)

(B) Cat sperm. (After Retzius, Biol. Untersuchungen, New Series, 10; Stockholm: Jena.) (C-D) Dog sperm. (C) Upper part of epididymis. (D) Lower or caudal part of epididymis.

When the vitelline membrane thickens, the loci where the cytoplasmic strands from the follicle cells pass through the membrane become little canals or canaliculi. As the oocyte increases in size, a thin space forms between the vitelline membrane or zona radiata and the follicle cells; it is filled with fluid and forms the follicular space. The egg now is free to rotate within the follicle. In consequence, the pole of the egg containing the blastodisc always appears uppermost. Due to the increasing pendency of the egg follicle as the egg matures, the blastodisc comes to rest, a short while previous to ovulation, at the base of the pedicle where the blood vessels are most abundant (fig. 47B). During the latter phases of oocyte development, the vitelline membrane constitutes an osmotic membrane through which all nourishment must pass to the oocyte, particularly in its later stages of growth. Tlie surface ooplasm forms a delicate surface membrane beneath the zona radiata.

h) Membranes of the Mammalian Oocyte. All mammalian oocytes possess a membrane known as the zona pellucida. It is a homogeneous layer interposed between the ooplasm and the follicle cells. By some investigators it is regarded as a product of the oocyte, while others regard it as a contribution of the ooplasm and follicle cells. The majority opinion, however, derives the zona pellucida from the follicle cells. In addition to the zona pellucida, the oocyte of the prototherian mammals has a striate layer lying close to the surface of the oocyte. This striated layer probably is derived from the surface ooplasm. This membrane later disappears, and a perivitelline space occupies the general area between the surface of the oocyte and the zona pellucida (fig. 46; Chap. 5). The zona pellucida separates from the egg surface after sperm contact.

5. Physiological Maturation of the Gametes a. Physiological Differentiation of the Sperm

Added to the nuclear and cytoplasmic transformations of the sperm described above, a further process of sperm ripening or maturing appears to be necessary. In the mammal, for example, the sperm cell must pass through the epididymis to achieve the ability to fertilize the egg. This is shown by the fact that sperm taken from the seminiferous tubules will not fertilize, although, morphologically, two sperm, one from the testis and one from the epididymis cannot be distinguished other than by the presence in some mammals of the so-called “kinoplasmic droplet” (figs. 82D, 97). These droplets do not appear in great numbers upon ejaculated sperm but are found on sperm, particularly in epididymides. It is possible that these droplets may arise from a secretion from the epididymal cells (CoIIery, ’44). In the dog, these droplets are attached to the neck of the sperm in the caput epididymidis but are found at the posterior end of the middle piece of the sperm in the cauda epididymidis and vas deferens and are probably lost at the time of ejaculation (Collery, ’44). Investigators differ greatly in interpreting the significance of this body. However, these droplets do seem in some way to be directly or indirectly concerned with the physiological maturing of the sperm. In this connection Collery (’44) notes that sperm are probably motile on leaving the seminiferous tubules, but active forward movement is not seen until the bead has reached the junction of middle piece and tail.

In the fowl, Domm (’30, p. 318) suggests the probability that the sperm may undergo an aging or ripening process essential for reproduction somewhere in the reproductive system other than the seminiferous tubules. The work of Lipsett quoted in Humphrey (’45) suggests that the accessory reproductive system also is necessary for a ripening process of the sperm in urodele amphibia.

On the other hand, in the frog, sperm taken from the testis have the ability to fertilize eggs. In this case, the sperm probably undergo a physiological ripening in the testis along with morphological differentiation.

The foregoing considerations suggest that a physiological maturation of the sperm is necessary to enable the sperm to take part in the fertilization process.


b. Physiological Ripening of the Female Gamete

The physiological maturing of the oocyte is linked to factors which influence the developing egg at about the time the maturation divisions occur. Sea-urchin sperm may penetrate the egg before the maturation divisions occur (Chap. 5). However, development does not take place in such instances. On the other hand, sperm entrance after both maturation divisions are completed initiates normal development. In the protochordate, Styela, marked cortical changes transpire at about the time the egg leaves the ovary, and as it reaches the sea water, the germinal vesicle begins to break down. The oocyte becomes fertilizable at about this time. In Amphioxus, although the first polar body is given off within the adult body, the egg apparently is not fertilizable until it reaches the external salt-water environment. The secondary oocyte of the frog presumably must remain within the uterus for a time to ripen in order that ensuing development may be normal. These and other instances suggest that physiological changes — changes which are imperative for the normal development of the egg — are effected at about the time that the maturation divisions occur.

D. Summary of Egg and Sperm Development

From the foregoing it may be seen that the development of the gametes in either sex involves a process of maturation. This maturation entails changes in the structure and constitution of the nucleus and cytoplasm, and, further, a functional or physiological ripening must occur. The comparative maturation events in the egg and sperm may be summarized as follows:


Egg ( in Oogenesis)


Sperm (in Spermatogenesis)


1. Nuclear maturation

a. Homologous chromosomes synapse and undergo profound changes during which parts of homologous chromosomes may be interchanged; ultimately, the chromosome number is reduced to the haploid number

b. Nucleus enlarges, and contained nuclear fluid increases greatly; ultimately the nuclear fluid is contributed to cytoplasm upon germinal vesicle break down


c. Nuclear maturation occurs simultaneously with cytoplasmic differentiation


1. Nuclear maturation

a. (Similar to the female)


b. Nucleus remains relatively small and enlargement is slight; nuclear fluid small in amount; during spermiogenesis the nucleus may contract into a compact mass; considerable elongation of nucleus occurs in many species

c. Nuclear maturation occurs before spermiogenesis or cytoplasmic differentiation



SUMMARY OF EGG AND SPERM DEVELOPMENT


Egg ( in Oogenesis)

Sperm (in Spermatogenesis)

2. Cytoplasmic maturation

This involves:

2. Cytoplasmic maturation

This involves:

a. Polarization of cytoplasmic materials and the nucleus in relation to the future maturation phenomena; the nucleus becomes displaced toward one pole, the animal pole, and the yolk, and other cytoplasmic materials; in many eggs becomes displaced toward the opposite or vegetal pole

a. Polarization of nucleus and cytoplasmic materials along an elongated antero-posterior axis, with the head, neck, middle piece, and tail occupying specific regions along this axis. The nucleus occupies a considerable portion of the anterior region or head

b. Formation of deutoplasm or stored food material, varying greatly in amount in different animal species. The deutoplasm is composed of fats, carbohydrates, and protein substances

b. Little food substances stored within cytoplasm; food reserve in seminal fluid

c. Cytoplasm increased in amount; formation of basic organ-forming areas or cytoplasmic stuffs from which the future embryo arises

c. Discarding of a considerable amount of cytoplasm, some Golgi elements and mitochondria. Retention of some Golgi elements, centrioles, mitochondria, etc.

d. Formation of primary embryonic membranes

d. No specific membranes formed around sperm, although elaborate membranes for motile purposes are formed in some sperm

3. Physiological maturation or the development of a fertilizable stage

This involves:

a. Formation of an organization which when stimulated by external influences initiates and carries on the processes necessary for normal embryonic development

3. Physiological maturation or the development of the ability to contact and fertilize the egg

This involves:

a. Development of an organization which, when stimulated by proper external substances, responds by a directed movement resulting in locomotion; also capable of being attracted by egg substances

b. Acquisition of ability to enter into a developmental union with a sperm

b. Acquisition of ability to fertilize, i.e., to enter into a developmental union with an egg or oocyte

c. Development of ability to form and secrete gynogamic substances which aid in the fertilization process. (See Chap. 5)

c. Acquisition of ability to produce and secrete androgamic substances which aid in the fertilization process

d. Assumption of an inhibited or dormant condition during which metabolic processes proceed slowly in anticipation of the fertilization event

d. Assumption of an active metabolic state


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