Book - Embryology of the Pig 2
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.
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Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381
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Chapter 2. The Reproductive Organs; Gametogenesis
I. The Reproductive Organs
Any logical account of prenatal development in mammals must start with a consideration of the phenomena which initiate that development. It is necessary to know more than the mere structure of the conjugating sex cells. We must know something of how they are produced and of the extraordinary provisions which ensure their union in such a place and at such a time that each is capable of discharging its function. Of vital importance, also, arc the changes in the body of the mother that provide for the nutrition of the embryo during its intra-uterine existence, and for its feeding during the relatively long period after birth when it cannot subsist on food such as that eaten by its parents. Before it is possible to deal with any of these things intelligibly, or to appreciate the underlying factors in the periodic recurrence of the breeding impulse in animals, it is necessary to become familiar with the main structural features of the reproductive organs.
The Female Reproductive Organs
The location of the reproductive organs of the sow and their relations to other structures in the body are shown in figure 1. The paired gonads of the female, the ovaries^ lie in the pelvic portion of the abdominal cavity. Each ovary is nearly completely enwrapped by a funnel-like dilation of the end of the corresponding uterine tube. This relationship tends to ensure that the ova, when they become mature and are discharged from the ovary, will find their way into the uterine tubes and thence into the uterus. There, if they have been fertilized, they become attached and nourished during prenatal development (Fig. 52). The uterus of the sow is of the bicomate type, that is, it has two limbs or horns (Fig. 2). These right and left limbs, which are enlarged continuations of the oviducts, unite with each other mesially to form an unpaired portion known as the body (corpus) of the uterus. The body of the uterus is continuous caudally with the neck or cervix, a region characterized by an attenuated lumen and thick walls (Fig. 126, C). The cervix of the uterus projects into the anterior part of the vagina^ which serves the double function of an organ of copulation and a birth canal.
Fig. 1. Diagram showing the position and relations of the more important viscera of the pig. (Modified from Sisson.) Adult female viewed from the left.
Abbreviations: Bl., urinary bladder; Cae., cecum; Sp., spleen; St., stomach; Vag., vagina.
The Male Reproductive Organs
The general arrangement and relationships of the male reproductive system are shown in figure 3. The testes do not lie in the abdominal cavity as do the ovaries, but are suspended in a pouch-like sac called the scrotum. The sex cells produced in the testes must pass over an exceedingly long and elaborate series of ducts before reaching the outside (Fig. 3). From the convoluted or seminiferous tubules where the spermatozoa (spermia) ^ are formed, they find their way through short straight ducts (the tubuli recti) into an irregular network of slender anastomosing ducts known as the rete testis. From the rete testis the spermia are collected by the ductuli efferentes which in turn pass them on by way of the much coiled duct of the epididymis, into the ductus deferens. At the distal end of the ductus deferens is a glandular dilation known as the seminal vesicle. As their name implies, it has been believed that the seminal vesicles served as sort of reservoirs in which the spermatozoa were stored pending their ejaculation. Recently it has become known that the spermatozoa collect in the somewhat dilated distal ends (ampullae) of the ducti deferentes, and that the seminal vesicles are primarily glandular organs which produce a secretion serving as a vehicle for the spermatozoa.
Fig. 2. Diagram of uterus and adnexa of sow. (After Corner.) The right horn of the uterus and the right oviduct (uterine tube) have been cut away from the broad ligament (lig. latum) and pulled out straight to show theif dimensions.
Fig. 3. Diagram showing the arrangement of the reproductive organs of the adult boar. The testes and their ducts up to the entrance of the ejaculatory ducts into the urethra are paired structures, but for simplicity, only those on the left side have been represented.
- There are many instances in embryology where two or more synonymous words are in common usage. To facilitate collateral reading some of the most frequendy encountered synonyms have been inserted parenthetically.
When, during coitus, the spermatozoa are discharged they enter the urethra by way of the ejaculatory ducts (Fig. 3). Coincidently the contents of the seminal vesicles, the prostate gland, and the bulbourethral glands (Cowper's glands) are evacuated into the urethra providing a fluid medium in which the spermatozoa are actively motile. This mixture of secretions with spermatozoa suspended in it {semenY is swept out along the urethra by rhythmic muscular contractions.
II. Gametogenesis
The manner in which the sex cells or gametes are produced in the gonads demands attention less cursory than that accorded the sex organs as a whole. Nevertheless it is unnecessary to emphasize the peculiarities of gametogenesis in the pig. The processes can more profitably be traced in a manner sufficiently broad to be applicable to mammals generally.
Continuity of Germ Plasm
The cells which give rivSe to the gametes in any individual, collectively, are said to constitute its “germ plasm.†The cells which take no direct part in the production of gametes and which cease to exist with the death of the individual are called somatic cells (collectively, somatoplasm in antithesis to germ plasm).
The germ plasm is of paramount interest not only to the biologist but to all thinking persons, because some of its cells are all that are preserved from one generation to the next A single cell from the germ plasm of the male parent unites with a single cell from the germ plasm of the female parent. From the cell formed by their fusion a new individual develops. Some of the germ plasm qf the parents thus lives on in succeeding generations. All the other cells in their bodies die. The two conjugating gametes alone pass on the entire hereditary dowry of the species, not only from the immediate parents, but from all their ancestors,
2 In addition to being used as a convenient means of giving synonyms, parentheses have been used to introduce technical terms which may be unfamiliar. In such cases the term, as has been done here, is placed in parentheses following a characterizing phrase.
It is difficult to realize fully the implication carried in this simple statement of the continuity of the germ plasm. The entire future of any species depends on the germ plasm held in trust within the bodies of the individuals now living. Whatever changes for good or ill the germ plasm undergoes will inevitably be written into the history of the species. Fortunately, very early in the life of an individual, the germ plasm is segregated in the gonads and not subject to most of the diseases from which the somatic cells suffer. But the germ plasm, even though not directly affected, may nevertheless suffer indirectly through a poor environment forced on it by an unhealthy body.
Of greater importance still is the nature of the combination of germ plasm which occurs in each generation when the two gametes fuse. As surely as either gamete brings into the new combination defective germ plasm, so surely will both the body and the germ plasm of the new individual suffer therefrom.
Early History of the Primordial Sex Cells
It becomes a matter of fundamental interest, therefore, to go back of the production of gametes in the sexually mature individual. One naturally wants to know the whole story of how the germ plasm is handed on from one generation to the next, not just its last chapter. Obviously the germ plasm of the individual under consideration must have come to it from its parents by way of the ovum and spermium which united to initiate its development. But how and where is it cared for by the individual? When did it first become possible to recognize the germ plasm? When and how did it become segregated in the gonad, more or less protected from the accidents of injury or disease from which the body so commonly suffers? What is it doing during the long time before sexual maturity? All these questions naturally occur to us as leading up to the final maturation and liberation of the gametes in the adult.
The earliest part of the history of the germ plasm in an individual is as yet imperfectly known. The cells which are destined to give rise to the gametes are, however, definitely recognizable at a surprisingly early stage in development. Long before it is possible to tell whether an embryo is to become a male or a female, certain large cells become differentiated from their neighbors. By working backward from older stages where conditions are more clear cut, it has been possible to identify these cells as the progenitors of the gametes. In other words they constitute the germ plasm as it exists in an embryo of that age. These large cells are called the primordial sex cells. Since their early history is the same whichever sexual type of gamete, they will ultimately produce, no account needs to be taken at first of sex differences in the individual (Fig. 4).
See discussion page.
Fig. 4. Chart outlining, for one generation, the history of the gametes and the germ plasm from which they are derived.
The primordial sex cells are first easily identifiable in the mammalian embryo when they lie in the epithelial covering of the gonad. Until comparatively recently it was believed that they could not be recognized at all as germ cells any earlier in their history. Of late, however, much more detailed studies hav^e been made and there is cogent evidence being brought forth indicating that they can be identified prior to their appearance in the gonad. This recent work seems to show that the primordial germ cells become recognizably differentiated in most vertebrate embryos in the yolk-sac entoderm, and that they migrate thence to establish themselves in the gonads. How much farther back toward the fertilized ovum the lineage of the primordial sex cells may, in the future, be traced with definiteness it would be rash to predict. Even now, in some of the invertebrates, it is believed that the individual cell which gives rise to all the sex cells can be identified as far back as the early cleavage divisions.
Early Differentiation of the Testes
Shortly after the primordial sex cells become established in the gonads, sexual differentiation begins. It is then necessary to trace the course of events separately for the two sexes (Fig. 4). During the embryonic life of the young male individual the primordial sex cells grow from the epithelial covering of the testis into its substance and there become organized to form the seminiferous tubules. Many cell generations are of course occupied with the fabrication of a seminiferous tubule, but the cells which eventually constitute its wall can, nevertheless, be traced back to the primordial sex cells (Fig. 4). When they become established in the walls of a seminiferous tubule the cells are known as “sperm mother cells†or spermatogonia. At this stage of development the spermatogonia constitute the individual’s “germ plasm.†During early postnatal life and the growth period, these future gamete producers remain relatively quiescent and undeveloped. Their inactivity stands in sharp contrast to the rapid proliferation and differentiation of the remainder of the cells which go to make up the body of the growing individual. It is as if the spermatogonia, thus set apart, were hoarding their energy for the next generation. Only when the individual becomes sexually mature do they begin intensive activity.
Spermatogenesis
The mature testes contain a large number of much convoluted seminiferous tubules. Their positiem and relations are schematically indicated in figure 3. This figure, however, gives no conception of the astonishing total length of the gamete-producing tubules crowded within the testes. It has been estimated by Osterud and Bascom from a study of serial sections that the total length of the seminiferous tubules from one testis of a mature boar, pulled out straight and placed end to end, would reach 3200 meters. When one realizes the total length of these tubules it is not difficult to understand how each ejaculate of semen contains millions upon millions of fully formed, active spermatozoa.
Fig. 5. Semischcmatic figure showing small segment of the wall of an active seminiferous tubule. The sequence of events in the production of s{Xiniiia is indicated by the numbers. A spermatogonium (1) goes into mitosis (2) producing two daughter cells (2a and 2b). One daughter cell (2a) may remain peripherally located as a new spermatogonium eventually coming to occupy such a position as la. The other daughter cell (2b) may grow into a primary spermatocyte (3), being crowded meanwhile nearer the lumen of the tubule. When fully grown the primary spermatocyte will divide again (4) and produce two secondary spermatocytes (5, 5). Each secondary spermatocyte at once divides again (6, 6), producing spermatids (7). The spermatids become embedded in the tip of a Sertoli cell (7a), there undergoing their metamorphosis and becoming spennia (8), which when mature are detached into the lumen of the seminiferous tubule.
If we examine the spermatogonia lying at the periphery of an active adult seminiferous tubule we see many mitotic figures (Fig. 5, 2 ). A cell arising from such a spermatogonial division may do one of two things. It may cease dividing for a time and, by growing to a markedly larger size than its parent, become differentiated as a primary spermatocyte (Fig. 5, 3 ). It may remain like its parent and continue to produce other spermatogonia. The new cells thus formed take the place of the spermatogonia which have grown into spermatocytes and moved out of the spermatogonial layer toward the lumen of the tubule. Thus some of the cells always remain in the peripheral part of the tubule as spermatogonia and furnish a constant source of new cells ready for conversion into spermatocytes.
Once a cell has undergone the growth phase which differentiates it so that we call it a primary spermatocyte, its future history is very definitely determined. It first undergoes a division resulting in the formation of two smaller daughter cells called secondary spermatocytes (Fig. 5, 5 ). Each of these secondary spermatocytes, without any resting period which might allow the cells to grow to the size attained by their parents, promptly divides again and forms two spermatids (Fig. 5, 6, 7 ). Cell division then ceases and each spermatid is gradually transformed into a fully formed, potentially functional male gamete, the spermium. In the metamorphosis of a spermatid: the nuclear material becomes exceedingly compact to form the bulk of the head of the spermium; the centrosomal apparatus of the spermatid undergoes an elaborate modification to give rise to the motile axial filament of the tail of the spermium; and the cytoplasm is reduced greatly in bulk giving rise to an envelope with a tiny thickened cap (acrosome) about the head of the spermium, and a delicate investment of the axial filament of its middle piece and tail. During their transformation, the spermatids are embedded in the cytoplasm of nurse cells {supporting cells of Sertoli) which lie at intervals in the wall of the seminiferous tubule (Fig. 5, 7a, s). It is believed that the Sertoli cells in some way transfer food material to the metamorphosing spermatids from the small blood vessels in the connective tissue investing the seminiferous tubule. When they are fully mature the spermia free themselves from the Sertoli cells and are carried out along the lumen of the tubule toward the epididymis (Fig. 3).
Early Differentiation of the Ovaries
The origin, migration, and early segregation of the primordial sex cells in the gonads take place, as we have seen, before there is any sexual differentiation observable in the embryo. Consequently in tracing these phenomena we established a common starting point for following later developments in the female as well as in the male (Fig. 4). Even when the indifferent stage is passed and it is possible to say definitely that a given embryo is developing into a female, conditions in the ovary are at first similar in a general way to conditions in the testis at a corresponding stage of development. The sex cells, which have appeared in the epithelium investing the growing ovary, push centripetally into the ovarian connective tissue in a manner very suggestive of the way seminiferous tubules arise in the testis. The cords of sex cells thus invading the ovarian stroma are known as egg tubes (ovigerous cords, Pfliiger’s tubes) (Fig. 6).
From this point on, the structural resemblances of the growing gonads in the two sexes become less and less apparent. A striking homology, however, exists throughout the entire series of changes in the germ cells themselves, and should not be lost sight of even though its later phases occur in organs so divergently differentiated as the adult ovary and testis come to be. Although the inference will undoubtedly have been drawn from what has already been said of the early history of the primordial germ cells in the two sexes, it may be well to emphasize the fact that the cells of the ovigerous cords are homologous with the spermatogonia of the male. It is, of course, because of this homology that they are called obgonia (Fig. 4).
Oogenesis
Although the obgonia exhibit a growth period, and then undergo two rapidly succeeding maturation divisions just as do the spermatogonia, the details of these processes in the two sexes are quite unlike. Their differences are correlated with the antithetical nature of thje specializations in the gametes themselves. In the male, small, actively motile gametes with no stored food material are produced in enormous numbers. The energy which in the male goes into quantity production, in the female is expressed by more elaborate preparation of the gametes and the storing of food material in their cytoplasm. The ova thus become very large, non-motile cells and, compared with spermatozoa, relatively few of them are brought to maturity.
Very early in the history of the oogonia the tendency to specialize a few cells rather than many, becomes apparent. In the ovigerous cords, and in the egg nests which are formed by the breaking up of the cords, one or two of the cells will almost always be found to have grown largtjr than their neighbors (Fig. 6). All the cells of the cords or nests are potentially oogonia. The ones which show enlargement are already beginning their long slow growth to form primary oocytes. The cells which lie adjacent to one of these growing oogonia, figuratively speaking, forego their own chances of becoming oocytes and arrange themselves as a protecting and food-purveying investment about the future ovum. The entire group of cells thus formed is known as a primary ovarian {GraafianY follicle, (Fig. 6.) The cells surrounding the oocyte proliferate rapidly and form an increasingly thick covering about it. With continued growth there appears in the layer of follicle cells a cavity which fills with fluid and expands very rapidly. T his cavity is called the antrum^ and the fluid which fills it is known as the liquor fdliculi,^
During these changes the developing follicle has usually pushed its way deep into the connective tissue framework (stroma) of the ovary. When the follicle begins to fill with fluid it starts to work gradually toward the surface of the ovary. As its size is still further increas(‘d it cotnes to protrude from the ovary, appearing in gross, fresh material, much like a water-blister.
Such a follicle is nearly ready to rupture and release the contained ('gg cell (Fig. 6). In the course of its development through the stages which we have just sketched in brief outline, it has acquired a degree of differentiation which demands closer scrutiny. The egg cell at this stage is commonly called the ovum, but if we used the terminology which emphasizes the homologies of development in male and female gametes should call it by its more cumbersome name, primary oocyte, 1 1 has grown to a size many times that of the follicle cells which surround it, and its abundant cytoplasm is dotted with granules of stored food rnaterial (yolk, deutoplasm). The total yolk content in mammalian ova being relatively small and uniformly distributed, the nucleus is not crowded to one side but is centrally located within the cytoplasm (Fig. 7). The cell membrane has become considerably thickened. It still keeps its old name of ‘Vitelline membrane,†which along with such obsolescent terms as “germinal vesicle†for the nucleus, and “germinal spot†for the nucleolus, was given it before the true significance of “the egg†as a specialized cell was understood.
- Formerly it was customary to name structures after the first man describing them. For example the ovarian foJJkle in all the older literature will be found designated as the Graafian follicle, after the Dutch anatomist Reijnier de Graaf (16411673). While this old custom is interesting in that it preserves to us the names of pioneer workers, the present tendency is to make our nomenclature more logical and more easily remembered by replacing proper-name dipsignations with ones descriptive of the structure. Such a change can be accomplished only gradually, however, knd in many cases, as is tme in the present instance, a proper name has become so firmly established through long usage that it is necessary to know it as a synonymous term in order not to be confused by its constant appearance in reference reading.
Fig. 7. Drawing of an ovarian (Graafian) follicle approaching maturity, showing details of its structure and relations.
Surrounding the ovum is a transparent, non-cellular, secreted layer known as the zona peliucida.^ Outside the zona peliucida is an investment of radially elongated follicle cells (Fig. 7) some of which still cling to the ovum when it is discharged (Fig. 6). These cells constitute the corona radiata.
With the increase in size and fluid content which the antrum has by this time undergone, most of the follicle cells are crowded peripherally to constitute the so-called stratum granulosum of the follicular wall (Fig. 7). Outside the stratum granulosum, the immediately surrounding ovarian connective tissue has become condensed about the growing follicle. This secondary connective-tissue investment is known as the theca folliculi. It is differentiated into an outer, densely fibrous layer, and an inner layer less conspicuously fibrous and containing many cells and numerous small vessels (Fig. 7).
- In some forms this zone exhibits delicate radial striations. When these are conspicuous the term zona radiata instead of zona peliucida is applied to this same layar. The term, although descriptively appropriate, is unfortunate because it is so frequently confused with the totally different cellular investment outside it called the mom radiata.
At the point where the ovum lies among the follicle cells they form a hillock projecting from the stratum granulosum into the antrum. This hillock is known as the cumulus oophorus (Fig. 7). When first formed it is broad and low. As the follicle approaches maturity the cumulus becomes more elevated and somewhat undercut (Fig. 6). Finally the ovum is carried on a slender stalk of cells which readily releases it and allows it to escape in the follicular fluid set free when the follicle ruptures.
Ovulation
The precise mechanism which precipitates the rupture of ovarian follicles is not as yet known with certainty. In all probability there are several factors involved. We know from the way mature follicles protrude at the ovarian surface that the fluid pressure within them is considerable (Fig. 6). As the follicle bulges under this pressure its connective-tissue envelope {theca folliculi) is squeezed against the connective-tissue capsule {tunica albuginea) of the ovary. It seems not unlikely that this process would result in compressing the small blood vessels where the bulging is most pronounced. This would reduce the nutrition of the region affected and eventually the strength of the tissues. Such a mechanical effect of fluid pressure might well pave the way for the rupture of the follicle. Underlying the development of the follicle and the accumulation of its contained fluid, there is certainly tl)e stimulus of a hormone produced in the anterior lobe of the pituitary. According to the recent work of Joseph Smith, there appears to be a marked increase in the concentration of salts in the liquor folliculi as^ the time of rupture approaches. The endosmotic effect of such a concentration may well be a precipitating factor in bringing the internal fluid pressure to a point where rupture of the follicle occurs.
However future work may evaluate the importance of the several possible causative factors involved, we know that the rupture of the follicle when it does occur is an abrupt, almost explosive, process. Hill, Allen, and Kramer have succeeded in making a detailed micromoving-picture record of ovulation in the rabbit, and their film shows with great vividness the rapid terminal bulging of the follicle culminating in sudden rupture with a gush of follicular fluid which brings with it the ovum surrounded by its radiate corona of follicle cells. A slight hemorrhage can be seen to accompany the rupture of the follicle.
Atresia of Follicles
By no means all the follicles that start to enlarge as a given ovulatory period approaches go on to maturity and ovulation. A considerable number of follicles that have started to undergo marked enlargement and give every apparent indication that they are going to continue developing suddenly cease to grow and then begin to degenerate. The process is known as follicular atresia, and a follicle so involved is said to be atretic. The underlying regulatory factors causing the atresia of certain follicles while other neighboring follicles go on to maturity is not known.
Maturation of the Ovum
The maturation of the "ovum"(primary oocyte) takes place at just about the time of its liberation from the follicle. As in the male, two cell divisions occur in rapid succession, but instead of four functional gametes being formed as an end result, there is only one. At each maturation division two cells are formed. But one of these cells receives practically all the stored food material of the primary oocyte, while the other receives little or none and soon degenerates. The cell receiving no yolk maUTial was called a "polar body" before its significance was understood. It is of course an oocyte minus its proper share of cytoplasm.
The gross results of the two maturation divisions in the female are schematically summarized in the chart of figure 4. The primary oocyte divides to form two secondaiy oocytes. One of these receives little cytoplasm and is called the first polar body. The secondary oocyte which has preempted all the stored food material promptly undergoes another mitosis, the second maturation division. Again the bulk of the cytoplasm goes to one of the two resulting ootids. The ootid receiving it is commonly called the “matured ovum†and is now ready to be fertilized by a spermium. The other ootid is the second polar body. Occasionally the first polar body undergoes a second division, clearly indicating the homology of the maturation divisions in the two sexes (Fig. 4). Usually, however, it degenerates before such a division occurs. The second polar body likewise degenerates soon after it is formed, leaving, of the four potential ootids, only one which becomes functional.
Significance of Maturation
The events in the maturation of male and female gametes which have just been discussed are but the more evident phases of the process. There have been changes of profound significance going on at the same time in the nuclear material. It would carry us far afield into cytology and genetics, to discuss these changes in detail or to attempt an interpretation of their full meaning. We can, however, indicate briefly wherein their importance lies.
It has already been stated that the inheritance of an individual comes to it by way of the gametes arising from the germ plasm of its parents. We can be more definite. It comes by way of the chromosomes in the nuclei of the gametes. The chromosomal content of the nuclei in the cells which go to make up the body of an individual is definite and constant. The elaborate mechanism of mitosis splits the chromosomes lengthwise into qualitatively and quantitatively equivalent daughter chromosomes. Thus in each of the countless cell divisions involved in the growth of an individual the number of chromosomes remains in the daughter cells what it was in the parent cell. The chromosomal number so maintained is different in different species, but in the various cells of individuals of the same species it is fixed and definite, the species number of chromosomes in the parlance of cytologists and geneticists.
While the maintenance of the species number of chromosomes in an individual is dependent on the way each chromosome is split in the process of mitosis, it is preserved from generation to generation by the processes of maturation and fertilization. In the maturation divisions the number of chromosomes in the gametes is reduced to half the number characteristic of the species. When, in fertilization, a male and a female gamete each bearing half the species number of chromosomes unite with each other, the species number of chromosomes is reestablished in the individual of the new generation.
Cytologists have worked out the mechanism of the maturation divisions with great care in many forms. The process consists essentially of two specialized cell divisions which follow each other in rapid succession without the nucleus returning to a resting stage as happens between ordinary mitoses. To distinguish these maturation divisions they are called meiotic (in contrast to mitotic) divisions.
Another characteristic feature of maturation is the special behavior of the chromosomes in the prophase preceding the first of the two meiotic divisions. To understand this process, which is known as synapsis^ it is necessary to realize that the chromosomes making up the species number possessed by each somatic cell, and by all the germ cells before their maturation, are present in pairs. Thus the species numbei: of chromosomes is always an even number, for example, in man it is 48. A skilled cytologist working with favoraj^le material can see that these chromosomes have peculiarities of size and shape which make them individually identifiable. By careful comparison each chromosome can be matched to a similar one so they can all be arranged in pairs. The number of pairs will, of course, be half the total number of chromosomes characteristic for the species. Moreover, the hereditary implications of the fact that the chromosomes can thus be arranged in pairs are of the greatest importance, for we know that one member of each chromosomal pair came from the male parent and the other member from the female parent.
In an ordinary mitosis the members of the chromosomal pairs can be recognized but they appear to lie scattered in haphazard fashion. During the prophase, the chromosomes seem to have aggregated at once, and without any evident scheme of arrangement, at the equator of the spindle. In contrast, during the prolonged prophase of the first maturation division, the members of the chromosomal pairs come to lie close to each other and so remain for some time. It is this pairing off of the chromosomes that is called synapsis.
In one of the meiotic divisions (usually the first) the chromosomes which were brought together in synaptic pairs move apart without being split. Thus each daughter cell receives one member of each chromosomal pair, its chromosomal content thereby being reduced to half the species number. The maturation division which accomplishes this separation of the chromosomal pairs is appropriately enough called a reduction division. The daughter cells which contain but half the species number of chromosomes are said to contain the haploid number of chromosomes, in contrast to the diploid or species number contained before the reduction division. Following the reduction division, the second maturation division splits the chromosomes in much the manner of an ordinary mitosis. It, therefore, is called an equational division and produces daughter cells which still have the haploid number of chromosomes. When these matured male and female gametes, each containing half the species number of chromosomes, unite in fertilization, the diploid or species number is restored.
But there is more in maturation than a mechanism which maintains the species number of chromosomes. In the reduction divisions the chromosomes are not split but distributed, some to one ceil, some to the other. The resulting cells contain different hereditary potentialities because they contain different chromosomes, not halves of the same chromosomes as results in an ordinary mitosis. What hereditary possibilities are discarded into the polar bodies throvm off from the female gamete and what retained in the mature ovum is a matter of chance distribution. What potentialities find their way into the particular sperm which alone out of millions of its fellows fertilizes the ovum is likewise fortuitous.
Thus in the game of life, the maturation processes virtually shuffle the hereditary pack and deal out half a “hand†to each gamete. A full hand is obtained by drawing a partner from the "board", by combining with some other gamete of the opposite sex. Hence offspring resemble their parents because they play the game of life with the same kind of cards, but not, however, with the same hands. The minor differences in offspring, or the variations from the standard type that always go with these basic resemblances, are due to variations in the distribution of the genes during maturation, fertilization and cleavage.
Thus sufficient stability and variety is produced to insure continuity and progress. For the offspring will in the main resemble progenitors which have successfully lived in the prevailing conditions of the past, but will exhibit sufficient variability among themselves to insure that some of them shall successfully live in any conditions likely to arise in the future.[1]
Sex Determination. Probably no embryological question has been the subject of so much conjecture as sex determination. From time immemorial theory after theory purporting to explain why this embryo became a male and that one a female has been advanced only to be discredited. Under such circumstances one becomes exceedingly cautious in discussing even a tentative hypothesis. Of late, however, there has been so much discussion of the chromosome theory of sex determination that anyone who would consider himself well informed biologically must know what it is, regardless of its ultimate fate.
In discussing maturation, it was emphasized that the chromosomes present in the cells of a species could be arranged in pairs, the members of which were alike. In a male individual, however, one pair of chromosomes is an exception in that its members are strikingly unlike. The members of this pair are called the “X’’ and ‘‘Y†chromosomes. Although our knowledge is as yet fragmentary and unsatisfactory, there is sufficient evidence indicating that the X-Y pair of chromosomes is associated with the determination of sex, so that they are commonly referred to as the sex chromosomes. If the cells of a female individual are examined with reference to this peculiar pair of chromosomes, we find that instead of the large X and the small Y members characteristically appearing in the male, the female has two large X members.
Careful studies of maturation have yielded the clue as to how this sex diflference in chromosomal pattern comes about and is maintained. In the synapsis which occurs in the maturation divisions, the two members of the sex chromosomal pair, as is the case with any other chromosomal pair, will be found associated with each other. In the reduction division which separates the members of the chromosomal pairs in the spermatocytes of the male, the X chromosomes must inevitably go to one cell and the Y chromosomes to another. Since in all the cells of the female there is an X-X combination, in the reduction division which occurs in the maturation of the ova one of the X chromosomes must go to the polar body and one to the maturing ovum, so that all the ova will have an X chromosome.
When an ovum ready for fertilization is surrounded by swarms of spermatozoa, half of which have one chromosomal pattern and half another, it is obvious that there are equal chances as to which of the two types will be the one first to penetrate the ovum. If it is a sperm cell carrying an X chromosome, fertilization will establish in the zygote the X-X combination characteristic of the female. If, on the other hand, the successful sperm cell carried a Y chromosome, the X-Y combination characteristic of the male would result. If, for the sake of simplicity in diagraming, we use germ cells from an animal having a species number of only eight chromosomes, the essential happenings as postulated by the chromosome theory of sex determination may be schematically summarized as indicated in figure 8.
Perhaps the fairest way to assess this particular theory as to the determination of sex is to say that at the present time it accords with more of the known facts than any other theory. We must recognize, however, that as yet we know practically nothing as to the mechanism by which the characteristically different chromosomal pattern present in the two sexes may operate. There is some indication that the chromosomal combination established at the time of fertilization may provide merely the initial impetus toward sexual differentiation in one direction or the other, and that the action of certain internal environmental factors may be important in bringing about full differentiation.
The Corpus Luteum
When the ovarian follicle ruptures and liberates the ovum, its history is by no means closed. There remain in the ovary the great bulk of the follicle cells and the connective tissue theca which surrounded the follicle before its rupture. These structures do not degenerate at once but become involved in the development of the corpus luteum. The corpus luteum, so called because of the yellow color it exhibits in fresh material, grows very rapidly in bulk for a time and becomes an organ of internal secretion (endocrine organ, ductless gland). That is to say it produces a secretion which is not discharged by way of ducts as is the case with ordinary glandular secretions, but which is liberated into the blood stream. The secretion diffused from a ductless gland into blood vessels, an4 carried by the blood stream to some other place in the body where it exerts a definite physiological effect, is called a hormone. The probable action of the particular hormone produced by the corpus luteum is a subject to which we shall have occasion to return later in connection with the sexual cycle. For the moment we are concerned with the origin and structure of the corpus luteum itself.
Fig. 8 . Schematic diagram showing the separation of the members of the sex chromosorhe pair in maturation, and their recombination in fertilization. It is assumed that the species number of chromosomes is eight and that it is the male which produces gametes of different potentialities with regard to sex determination. The sex chromosomes are stippled; other chromosomes are drawn in solid black. (Patten: “Early Embryology of the Chick,†The Blakiston Company.)
The formation and histology of the corpus luteum of the sow have been described with great care by Corner to whose work reference should be made for a detailed account of the subject (see bibliography). Here but a brief sketch can be given.
When the ovarian follicle ruptures, escape of the contained fluid and contraction of the smooth muscle in the stroma of the ovary reduce the size of its lumen. Bleeding of the small vessels injured in the rupture of the follicle fills the partially collapsed antrum with blood which promptly becomes consolidated as a clot. A newly ruptured follicle thus filled with clotted blood is called a corpus haemorrhagicum (Fig. 6).
The blood clot is soon attacked peripherally by phagocytic white blood corpuscles and becomes progressively reduced in size. Concomitantly the follicular cells of the stratum granulosum increase greatly in size and are crowded into the area formerly occupied by the blood clot. At the same time small vessels from the connective-tissue theca penetrate the mass of enlarged follicle cells and ramify among them. These vessels bring in with them numerous small cells which become packed in among the more conspicuous cells which originated from the stratum granulosum. Thus both layers of the follicular wall contribute to the corpus luteum although the most conspicuous and characteristic cellular elements are derived from the follicle cells of the stratum granulosum.
Corpora lutea normally develop from all ruptured follicles, but if the liberated ova are not fertilized the corpora lutea soon degenerate. If, however, the ova are fertilized and implanted in the uterus, the corpora lutea undergo a greatly prolonged period of growth and persist much longer before degenerating. This difference in the history of the corpora lutea is recognized by designating the short-lived ones as the corpora lutea of ovulation and the ones which persist longer as the corpora lutea of pregnancy. Histologically they exhibit the same structural picture, their differences being apparently quantitative rather than qualitative.
When either type of corpus luteum begins to degenerate the retrogressive changes are in the nature of a fibrous involution. That is, the cellular part of the organ disintegrates and fibrous connective tissue takes its place. As this connective tissue grows older and more compact it gradually takes on the characteristic whitish appearance of scar tissue. Finally all that is left in the ovary to mark the site of what was ovarian follicle, and subsequently corpus luteum, is a shrunken patch of scar tissue called a corpus albicans (Fig. 6).
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Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Embryology of the Pig 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_2
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