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

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F. Characteristics of the male reproductive cycle and its relation to reproductive conditions in the female  
F. Characteristics of the male reproductive cycle and its relation to reproductive conditions in the female  


===D. Internal and External Factors Influencing Activities of the Testis===
Conditions which influence testicular activity are many. Many of the factors are unknown. Nevertheless, a few conditions which govern testis function
have been determined, especially in certain mammalian species. The general
results of experimental determination of some of the agents which affect
testicular function are briefly outlined below.
1. Internal Factors
a. Temperature and Anatomical Position of the Testis
It is well known that in those mammals which have a permanent scrotal
residence of the testes failure of the testis or testes to descend properly into the scrotum results in a corresponding failure of the seminiferous tubules
to produce sperm. In these instances the testis may appear shriveled and
shrunken (fig. 18). However, such cryptorchid (ectopic) conditions in most
cases retain the ability to produce the sex hormone at least to some degree.
A question therefore arises relative to the factors which inhibit seminiferous
tubule activity within the cryptorchid testis.
The failure of cryptorchid testes to produce viable sperm has been of
interest for a long time. Observations have demonstrated that the more hidden
Fig. 18. Experimental unilateral cryptorchidism in adult rat. The animal's left testis
was confined within the abdominal cavity for six months, whereas the right testis was
pernfitted to reside in the normal scrotal position. Observe the shrunken condition of the
cryptorchid member. (After Turner: General Endocrinology, Philadelphia, Saunders.)
the testis (i.e., the nearer the peritoneal cavity) the less likely are mature
sperm to be formed. A testis, in the lower inguinal canal or upper scrotal
area is more normal in sperm production than one located in the upper
inguinal canal or inside the inguinal ring. Studies made upon peritoneal and
scrotal temperatures of rats, rabbits, guinea pigs, etc., demonstrate a temperature in the scrotum several degrees lower than that which obtains in
the abdomen. These observations suggest that the higher temperature of the
non-scrotal areas is a definite factor in bringing about seminiferous tubule
injury and failure to produce sperm.
With this temperature factor in mind, Dr. Carl R. Moore (in Allen,
Danforth, and Doisy, ’39) and others performed experiments designed to
test its validity as a controlling influence. They found that confinement alone
of an adult guinea pig testicle in the abdomen led to marked disorganization
of all seminiferous tubules in seven days. After several months of such confinement the seminiferous tubules experience marked degenerative changes
and only Sertoli cells remain (fig. 19A, B). The interstitial tissue, however,
is not greatly impaired. If such a testis is kept not too long within the abnormal
position and once again is returned to the scrotum, spermatogenesis is rejuvenated (fig. 20A, B). In a second experiment, the scrotum of a ram was
encased loosely with insulating material; a rapid degeneration of the seminiferous tubules followed. Young (’27, ’29) in a third type of experiment found
that water 6 to 7° warmer than the body temperature applied to the external
aspect of the guinea-pig testis for a 15-minute period evoked degenerative
Fig. 19. Sections of experimental, cryptorchid, guinea-pig, seminiferous tubules and
interstitial tissue. (Modified from C. R. Moore in Sex & Internal Secretions, Williams &
Wilkins, Baltimore, 1939.) (A) Testis confined to abdomen for three months. (B)
Testis confined to abdomen for six months. Observe degenerate state of seminiferous
tubule after six months’ confinement. Interstitial tissue not greatly affected by confinement.
changes with temporary sterility (fig. 21). Recovery, however, is the rule in
the latter instance. Summarizing the effects of such experiments involving
temperature, Moore (in Allen, Danforth, and Doisy, ’39, p. 371) concludes:
“The injury developing from applied heat, although more rapidly effective,
is entirely similar to that induced by the normal body temperature when the
testicle is removed from the scrotum to the abdomen.”
The position of the scrotum and its anatomical structure is such as to
enhance its purpose as a regulator of testicular temperature (figs. 2, 6). When
the surrounding temperature is cold, the contraction of the dartos muscle
tissue of the scrotal skin contracts the scrotum as a whole, while the contraction of the cremaster muscle loops pulls the testes and the scrotum closer
to the body, thus conserving the contained heat. When the surrounding temperature is warm, these muscles relax, producing a more pendulous condition
to permit heat loss from the scrotal wall.
In accordance with the foregoing description of the scrotum as a necessary
thermoregulator for the testis, it has been further shown for those mammals
which possess a scrotum that testis grafts fare much better when transplanted
to the scrotal wall or into the anterior chamber of the eye (Turner, C. D., '48). The anterior chamber of the eyeball possesses a temperature much
cooler than the internal parts of the body.
Fig. 20. Sections of testis during and after abdominal confinement. (Modified from
C. R. Moore in Sex & Internal Secretions, Williams & Wilkins, Baltimore, 1939.) (A)
Section of left testis to show degenerate state of seminiferous tubules after 24 days of
abdominal confinement. (B) Section of right testis 74 days after replacement in scrotum.
Observe spermatogenic activity in tubules.
Fig. 21. Effect of higher temperature applied to external surface of guinea-pig testis.
Water, 47®, was applied to surface of scrotum for period of 10 minutes. Testis was
removed from animal 12 days after treatment. Seminiferous tubules are degenerate.
(Modified from Moore, ’39; see also Young, ’27, J. Exp. Zool., 49.)
Two types of seminiferous tubules are thus found in mammals. In a few
mammalian species (see p. 6) the temperature of the peritoneal cavity is
favorable to the well-being of the seminiferous tubule; in most mammalian
species, however, a lower temperature is required. On the other hand, the
activities of the interstitial tissue of the testis appear to be much less sensitive
to the surrounding temperature conditions, and the male sex hormone may
be produced when the testes are removed from the scrotum and placed within
the peritoneal cavity.
With regard to the functioning of the testis within the peritoneal cavity
of birds it has been suggested that the air sacs may function to lower the
temperature around the testis (Cowles and Nordstrom, ’46). In the sparrow,
Riley (’37) found that mitotic activity in the testis is greatest during the
early morning hours when the bird is resting and the body temperature is
lower, by 3 or 4° C.
b. Body Nourishment in Relation to Testicular Function
The testis is a part of, and therefore dependent upon, the well-being of
the body as a whole. However, as observed in the preceding pages the interstitial cells and their activities in the production of the male sex hormone
are less sensitive to the internal environment of the body than are the seminiferous tubules.
The separation of these two phases of testicular function is well demonstrated during starvation and general inanition of the body as a whole. A
falling off of sperm production is a definite result of starvation diets, although
the germinative cells do not readily lose their ability to proliferate even after
prolonged periods of starvation. But the interstitial cells and the cells of
Sertoli are not as readily affected by inadequate diets or moderate starvation
periods. Sex drive may be maintained in a starving animal, while his ability
to produce mature, healthy sperm is lost. On the other hand, long periods
of inanition also affect sex hormone production and the sexual interests of
the animal.
Aside from the abundance of food in a well-rounded dietary regime, adequate supplies of various vitamins have been shown to be essential. Vitamin
Bi is essential to the maintenance of the seminiferous tubules in pigeons.
Pronounced degenerative changes in the seminiferous tubules of rats and
other mammals occur in the absence of vitamins A and E (Mason, ’39).
Prolonged absence of vitamin E produces an irreparable injury to the testis
of rats; injury produced by vitamin A deficiency is reparable. The B-complex
of vitamins seems to be especially important for the maintenance of the
accessory reproductive structures, such as the prostate, seminal vesicles, etc.
The absence of vitamin C has a general body effect, but does not influence the testis directly. Spme of these effects may be mediated through the pituitary
gland. As vitamin D is intimately associated with the mineral metabolism of
the body, it is not easy to demonstrate its direct importance.
c. The Hypophysis and Its Relation to Testicular Function
The word “hypophysis” literally means a process extending out below.
The early anatomists regarded the hypophysis cerebri as a process of the
brain more or less vestigial in character. It was long regarded as a structure
through which waste materials from the brain filtered out through supposed
openings into the nasal cavity. These wastes were in the form of mucus or
phlegm, hence the name “pituitary,” derived from a Latin word meaning
“mucus.” The word pituitary is often used synonymously with the word
hypophysis.
The hypophysis is made up of the pars anterior or anterior lobe, pars
intermedia or intermediate lobe, and a processus infundibuli or posterior
lobe. The anterior lobe is a structure of great importance to the reproductive
system; its removal (ablation) results in profound atrophic changes throughout the entire reproductive tract.
The importance of the pituitary gland in controlling reproductive phenomena was aroused by the work of Crowe, Cushing, and Homans (TO)
and by Aschner (’12) who successfully removed the hypophysis of young
dogs. One of the first fruits of this work was a demonstration of the lack of
genital development when this organ was removed. Since that time many
the other cohabitants of man — rats, mice, cats, rabbits, etc. — have been
hypophysectomized, and in all cases a rapid involution and atrophy of the
genital structures results from pituitary removal. The testis undergoes profound shrinkage and regression following hypophysectomy, the degree of
change* varying with the species. In the rooster and monkey, for example,
regressive changes are more marked than in the rat. (Consult Smith, ’39, for
data and references.)
A striking demonstration of the influence of the hypophysis upon the
genital tract is the result of its removal from a seasonal-breeding species,
such as the ferret. Ablation of the pituitary in this species during the nonbreeding season causes slight if any change in the testis and accessory reproductive organs. However, when it is removed during the breeding season,
a marked regression to a condition similar to that present during the nonbreeding season occurs (Hill and Parkes, ’33).
The experimental result of hypophysectomy on many animal species thus
points directly to this structure as the site of hormonal secretion, particularly
to the anterior lobe (Smith, ’39). The initial work on the relation of pituitary
hormones and the gonad was done upon the female animal. The results of
these studies aroused the question whether one or two hormones were responsible. The latter alternative was suggested by the work of Aschheim and
Zondek (’27) and Zondek (’30) who concluded that two separate substances
appeared to be concerned with the control of ovarian changes.
Nevertheless, for a time the concept of only one gonad -con trolling (gonadotrophic) hormone was produced by the pituitary, continued to gain attention, and some workers suggested that the two ovarian elfects of follicular
growth and luteinization of the follicle were due to the length of time of
administration of one hormone and not to two separate substances. However, this position soon was made untenable by research upon the gonadotrophic substances derived from the pituitary gland. Studies along this line
by Fevold, Hisaw, and Leonard (’31) and Fevold and Hisaw (’34) reported
the fractionation, from pituitary gland sources, of two gonadotrophic substances, a follicle-stimulating factor or FSH and a luteinization factor or LH.
This work has been extensively confirmed. It should be observed in passing
that the male pituitary gland contains large amounts of FSH, although, as
mentioned below, the function of the testis and the male reproductive system
relies to a great extent upon the luteinizing factor. Some investigators refer
to the LH factor as the interstitial-cell-stimulating hormone, ICSH. (See Evans,
’47; and also Evans and Simpson in Pincus and Thimann, ’50.)
The action of these two hormones upon testicular tissue, according to
present information, is somewhat as follows: If pure follicle-stimulating hormone, FSH, which produces only FSH effects in the female, is injected in
low doses into hypophysectomized male rats, the seminiferous tubules are
stimulated and spermatogenesis occurs. Under these conditions, the interstitial
tissue remains unstimulated and the accessories continue in an atrophic state.
It has further been demonstrated that slight amounts of the luteinizing gonadotrophic hormone, LH (ICSH), added to the above injections of FSH,
effects a much better stimulation of the spermatogonial tissue, and the interstitial tissue also develops well.
On the other hand, when pure LH (ICSH) is given alone in small doses,
spermatogenesis is stimulated with slight or no effect upon the male accessory
structures. However, when larger doses of the LH (ICSH) factor alone are
injected, the interstitial tissue is greatly stimulated, and the testicular weight
increases much more than when FSH alone is given. Furthermore, the accessory reproductive structures are stimulated and become well developed, suggesting the elaboration of the male sex hormone. In agreement with these
results, the administration alone of testosterone, the male sex hormone, increases the weight and development of the accessory structures in hypophysectomized animals and it also maintains spermatogenesis. It appears, therefore, that the effects of the LH substance upon the seminiferous tubules and
the accessory organs occur by means of its ability to arouse the formation of
the male sex hormone.
A summary of the actions of the pituitary gonadotrophic hormones upon
testicular tissue may be stated as follows:
( 1 ) Pure FSH in small doses stimulates the seminiferous tubules and
spermatogenesis with little or no effect upon the interstitial tissue or
the accessory reproductive structures, such as the seminal vesicles or
prostate gland;
(2) Small doses of pure LH also stimulate spermatogenesis with little
or no stimulation of the accessory structures;
(3) Pure LH (ICSH) in larger doses stimulates the development of the
interstitial tissue with the subsequent secretion of the male sex hormone and hypertrophy of the accessory reproductive organs;
(4) The male sex hormone in some way aids or stimulates the process
of spermatogenesis, suggesting that the action of LH occurs through
the medium of the sex hormone (fig. 22).
(Consult Evans and Simpson in Pincus and Thimann, ’50, for data and
references; also Turner, C. D., ’48.)
The foregoing results of the action of the FSH and LH upon testicular
function might suggest that the LH substance alone is essential in the male
animal. However, it should be observed that without the presence of FSH,
LH is not able to maintain the tubules in a strictly normal manner, the
tubules showing a diminution of size. Also, in extreme atrophic conditions
of the tubules, pure FSH stimulates spermatogenesis better than similar quantities of LH. It is probable that FSH and LH (ICSH) work together to effect
complete normality in the male. This combined effect is known as a synergistic effect. It also is of interest that the injection of small doses of testosterone
propionate into the normal male, with the pituitary gland intact, results in
inhibition of the seminiferous tubules, probably due to the suppression of
pituitary secretion by the increased atnount of the male sex hormone in the
blood. However, high doses, while they likewise inhibit the pituitary, result
in a level of androgen which stimulates the seminiferous tubules directly
(Ludwig, ’50).
Aside from the above actions upon testicular tissue by the luteinizing hormone (LH;ICSH) certain other functions of this substance should be mentioned (see fig. 22). One of these is the apparent dependence of the Sertoli
cells upon the presence of the interstitial cells (Williams, ’50). Interstitial
tissue behavior and development in turn relies mainly upon LH (ICSH)
(Fevold, ’39; Evans and Simpson in Pincus and Thimann, ’50). As the sperm
are intimately associated with the Sertoli elements during the latter phases
of spermatogenesis in which they transform from the spermatid into the form
of the adult sperm, a very close association and reliance upon the presence
of the luteinizing hormone thus appears to be established in sperm development.
A further study of the LH factor is associated with the maintenance of
the seminiferous tubules themselves. In aged males, the interstitial tissue and
the seminiferous tubules normally involute and regress with accumulation
of large amounts of connective tissue material. In testicular grafts made into
the rabbit’s ear, Williams (’50) found, when interstitial tissue was present
in the grafts, the seminiferous tubules were more nearly normal; when absent,
the tubules underwent fibrosis.
Another function of the LH substance apparently is concerned with release
of the sperm from the Sertoli cells. De Robertis, et al. (’46), showed that
anterior pituitary hormones possibly cause release of sperm from the Sertoli
cells in the toad by the production of vacuoles and apical destruction of
the cytoplasm of the Sertoli elements. In testicular grafts Williams (’50) accumulated evidence which suggests that vacuoles and secretion droplets in
the Sertoli cells occurred as a result of LH administration. The combined
results of these investigators suggest that sperm release from the Sertoli cell
is dependent, in some way, upon LH (ICSH) activity.
A final function is concerned with the physiological maturing of sperm
in the reproductive duct, at least in many vertebrate species. The well-being
of the epididymis and vas deferens is dependent upon the presence of the
male sex hormone (Creep, Fevold, and Hisaw, ’36). As the male sex hormone results from stimulation of the interstitial cells by the interstitial-cellstimulating substance, LH (ICSH), the connection between this substance
and the physiological maturation of the sperm cell is obvious.
2. External Environmental Factors and Testis Function
As we have seen above, the anterior lobe of the hypophysis acts as the
main internal environmental factor controlling the testes and, through them,
the reproductive ducts. It has been observed also that food, vitamins, and
anatomical position of the testis are important influences in regulating testicular function. Furthermore, general physiological conditions such as health
or disease have an important bearing upon the gonads (Mills, ’19). All of the above conditions are contained within the body of the organism, and as
such represent organismal conditions.
Fio. 22. Chart showing the effects of the hypophyseal anterior lobe upon the developing gametes. It also suggests the various factors influencing pituitary secretion of the
gonadotrophic hormones, FSH and LH. Observe that the primitive gamete in the cortex
of the ovary is subjected to the cortical environment and develops into an oocyte, whereas
in the medullary or testicular environment it develops into a spermatocyte. Experiments
upon sex reversal have demonstrated that the medullary and cortical portions of the
gonad determine the fate of the germ cell. In the male area or medulla, the germ cell
differentiates in the male direction, while in the cortex, the differentiation is in the
direction of the female gamete or oocyte, regardless of the innate sex-chromosome constitution of the primitive germ cell. The fate of the germ cell thus is influenced by four
main sets of factors: (1) Internal and external environmental factors, controlling the
secretions of the pituitary body, (2) Fnvironment of the testicular tissue (medulla) and
possible humoral substances produced in this tissue, (3) Environment of the ovarian
tissue (cortex) and possible humoral substances elaborated there, and (4) Secretions of
the anterior lobe of the pituitary body.
The following question naturally arises: Do factors or conditions external
to the body impinge themselves in such a way as to control pituitary and
gonadal function?
a. Light as a Factor
Aside from the supply of nutritive substances or the collision of the many
nervous stimuli with the individual which may arouse or depress the sexual
activities, two of the most important obvious external factors are temperature
and light. Research on the reproductive behavior of many animal species,
during the past twenty years, has shown that both of these factors have great
significance on the reproductive activities of many vertebrate species. Bissonnette (’30, ’32, ’35, a and b) has accumulated evidence which demonstrates
that light is a potent factor in controlling the reproductive behavior of the
European starling (Sturnus vulgaris) and also of the ferret (Putorius vulgaris).
In the starling, for example, the evidence shows that green wave lengths of
the spectrum inhibit testicular activity, while red rays and white light arouse
the reproductive function (fig. 23). The addition of electric lighting to each
day’s duration produced a total testis size in midwinter which surpassed the
normal condition in the spring. In the ferret artificially increased day length
beginning at the first part of October brings the testis to maximum size and
activity coupled with a normal mating impulse as early as November and
December (fig. 24). Under normal conditions the male ferret is able to breed
only during February and early March,
These findings relative to the influence of light on the reproductive periodicity of animals confirm a fact which has been known for a long time,
namely, that seasonal breeders brought from the northern hemisphere to the
southern hemisphere reverse their breeding season. For example, ferrets which
normally breed from spring to summer in the northern hemisphere shift their
breeding habits to the September-February period when moved to the southern
hemisphere. Inasmuch as the hypophysis is instrumental in bringing about
secretion of the gonadotrophic hormones responsible for the testicular activity,
it is highly probable that light coming through the eyes (see Hill and Parkes,
’33) influences the nervous system in some way arousing the hypophysis and
stimulating it to secrete these substances in greater quantity. However, one
must keep in mind the caution given by Bissonnette, that light is not the only
factor conditioning the sexual cycles of ferrets and starlings.
While numerous animals, such as the migratory birds, ferret, mare, many
fish, frogs, etc., normally are brought into a breeding condition during the
period of light ascendency, a large number of animals experience a sexual
resurgence only during the time of year when the light of day is regressing
in span. This condition is found in some sheep, goats, buffalo in nature.
Fig. 23. Sections of testis of the starling (Sturnus vulgaris), showing the effect of
electric lighting added to the bird’s normal daily duration of light during the autumn.
(After Bissonnette, Physiol. Zool., 4.) (A) Inside young control bird — no light added
— kept inside as control for (B) from November 9 to December 13. (B) Inside young
experimental bird, receiving additional light from “25 watt” bulb from November 9 to
December 13. Total treatment, 34 davs.
Fig. 24. Sections of testis and epididymis, showing modification of sexual cycle in the
ferret, Putorius vulgaris, by exposure to increasing periods of light. (After Bissonnette,
’35b.) (A) Seminiferous tubules from normal male over 1 year old, made on October
3, no lighting. (B) Epididymis of normal male on October 3, no lighting. (C) Seminiferous tubules of experimental male on November 7, 36 days of added lighting. (D)
Epididymis of experimental males on Nov Tiber 7, 36 days of added lighting.
deer, some fish, etc. Bissonnette (’41) working with goats found that: “Increasing daily light periods from January 25 to April 5 — followed by diminishing periods until July 5, while temperatures remained normal for the seasons,
with four Toggenburg female goats and one male Toggenburg and one Nubian
female — led to cessation of breeding cycles in February instead of March,
followed by initiation of breeding cycles in May and June instead of September.” In the ewe, Yeates (’47) also found that a change from increasing
daylight to decreasing length of day induced reproductive activity. In a similar
manner. Hoover and Hubbard (’37) were able to modify the sexual cycle
in a variety of brook trout which normally breeds in December to a breeding
season in August.
b. Temperature Influences
In the case of the animals mentioned above, temperature does not appear
to be a major factor in inducing reproductive activity. However, in many
animals temperature is vitally influential in this respect. For example, in the
thirteen-lined spermophile (ground squirrel) Wells (’35) observed that breeding males kept at 40° F. continued in a breeding condition throughout the
year. Under normal conditions this rodent hibernates during the winter months
and comes forth in the spring ready to breed; sperm proliferation and general
reproductive development take place during the period of hibernation. As
the temperature rises during the spring and summer, testicular atrophy ensues,
followed by a period of spermatogenesis and reproductive activity when the
lowered temperatures of autumn and winter come again. Light, seemingly, is
not a factor in this sexual cycle. Another instance of temperature control
occurs in the sexual phase of the common red newt, Triturus viriciescens. Here
it is the rising temperature of the summer which acts as the inducing agent,
and sperm thus produced are discharged into the accessory ducts during the
fall and winter to be used when copulation occurs in early spring. However,
if this species is kept at a relatively low temperature of 8 to 12° C. during
the summer months, spermatogenesis is inhibited and the testis regresses. In
the stickleback, Gasterosteus aculeatus, as reported by Craig-Bennett (’31),
spermatogenesis occurs during July to early September and appears to be
conditioned by a rising temperature, whereas the interstitial tissue and the
appearance of secondary sexual features reach their greatest development
under increased light conditions and slowly rising temperatures (fig. 15).
Bissonnette, in his work on ferrets, also observed a difference in the behavior
of these two testicular components; the interstitial tissue responds to large
increases of daily light periods, whereas the seminiferous tubules are stimulated by small, gradually increasing periods of light.
The above examples emphasize the importance of a single environmental
factor on the pituitary-gonadal relationship. However, in the hedgehog,
Allanson and Deansley (’34) emphasize temperature, lighting, and hormone
injections as factors modifying the sexual cycles, while Baker and Ransom
(’32, ’33, a and b) show that light, food, temperature, and locality affect
the sexual cycles and breeding habits of the field mouse. In some vertebrates,
therefore, a single factor may be the dominant one, whereas in others, numerous factors control the action of the pituitary and reproductive system.
E. Internal Factors Which May Control Seasonal and Continuous Types
of Testicular Function
In endeavoring to explain the differences in response to external environmental factors on the part of seasonal and continuous breeders, one must
keep in mind the following possibilities:
(1) The anterior lobe of the hypophysis in some forms (e.g., ferret)
cannot be maintained in a secretory condition after it has reached its
climax; that is, it apparently becomes insensitive to the light factor. As a
result, regression of the pituitary and testis occurs (Bissonnette, ’35b).
(2) In the starling, the anterior hypophysis may be maintained by the
lighting, but the testis itself does not respond to the presence of the
hypophyseal hormones in the blood (Bissonnette, ’35b). The possibility in this instance may be that testicular function wanes because
the body rapidly eliminates the hormone in some way (see Bachman,
Collip, and Selye, ’34).
(3) Consideration also must be given to the suggestion that the activities
of the sex gland by the secretion of the sex hormone may suppress
anterior lobe activity (Moore and Price, ’32).
We may consider two further possibilities relative to continuous testicular
function :
(4) If the “brake actions” mentioned above are not present or present
only in a slight degree, a degree not sufficient to interrupt the activities
of the anterior lobe or of the sex gland, a more or less continuous
function of the testis may be maintained.
(5) When several or many environmental factors are concerned in producing testicular activity, a slight altering of one factor, such as light,
may prove insufficient to interrupt the pituitary-germ-gland relationship, and a continuous breeding state is effected in spite of seasonal
changes.
Underlying the above possibilities which may control testicular function is
the inherent tendency or hereditary constitution of the animal. In the final
analysis, it is this constitution which responds to environmental stimuli, and
moreover, controls the entire metabolism of the body. In other words, the
above-mentioned possibilities tend to oversimplify the problem. The organism as a whole must be considered; reproduction is not merely an environmentalpituitary-sex gland relationship.
F. Characteristics of the Male Reproductive Cycle and Its Relation to
Reproductive Conditions in the Female
As indicated above, reproduction in the male vertebrate is either a continuous process throughout the reproductive life of the individual or it is a
discontinuous, periodic affair. In the continuous form of reproduction the
activities of the seminiferous tubules and the interstitial or hormone-producing
tissues of the testis function side by side in a continuous fashion. In the
discontinuous, periodic type of testicular function, the activities of the seminiferous tubules and of the interstitial tissue do not always coincide. The
activities of the seminiferous tubules, resulting in the production of sperm
for a particular reproductive cycle, tend to precede, in some species by many
months, the activities of the sex-hormone-producing tissue. Evidently, the
output of the FSH and LH substances from the pituitary gland are spread
out over different periods of the year to harmonize with this activity of the
testicular components.
It will be seen in the next chapter that a continuous breeding faculty is
not present in the female comparable to that of the male. All females are
discontinuous breeders. In some species, the cycles follow each other with
little rest between each cycle unless the female becomes pregnant or “broody.”
Some have a series of cycles over one part of the year but experience sexual
quiescence over the remaining portion of the year. However, in most female
vertebrates there is but one reproductive cycle per year.
In harmony with the above conditions, the continuous variety of testicular
function is always associated with the condition in the female where more
than one reproductive cycle occurs per year. Continuous reproductive conditions in the male, therefore, are adapted to serve one female two or more
times per year or several different females at intervals through the year.
Furthermore, the complicated, highly glandular, greatly extended type of
male-reproductive-duct system is adapted to conditions of (1) continuous
breeding, or (2) service to more than one female during one breeding season
of the year, whereas the simple type of reproductive duct is adapted to the
type of service where all or most of the genital products are discharged during
one brief period. In other words, the entire male reproductive system and reproductive habits are adapted to the behavior of female reproductive activities.


===Bibliography===
===Bibliography===

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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 Testis and Its Relation to Reproduction

A. Introduction

1. General description of the male reproductive system

2. Importance of the testis

B. Anatomical features of the male reproductive system

1. Anatomical location of the testis

2. Possible factors involved in testis descent

3. General structure of the scrotum and the testis in mammals

a. Structure of the scrotum

b. General structure of the testis

4. Specific structures of the mammalian testis which produce the reproductive cells and the male sex hormone

a. Seminiferous tubules

b. Interstitial tissue

5. The testis of vertebrates in general

6. Accessory reproductive structures of the male

a. The reproductive duct in forms utilizing external fertilization

b. The reproductive duct in species practicing internal fertilization

C. Specific activities of the various parts of the male reproductive system

1. Introduction

a. Three general functions of the male reproductive system

b. Some definitions

2. Activities of the testis

a. Seasonal and non-seasonal types of testicular activity

b. Testicular tissue concerned with male sex-hormone production

c. Testicular control of body structure and function by the male sex hormone

1) Sources of the male sex hormone

2) Biological effects of the male sex hormone

a) Effects upon the accessory reproductive structures

b) Effects upon secondary sex characteristics and behavior of the individual

c) Effects upon the seminiferous tubules

d. Seminiferous-tubule activity and formation of sperm

e. The seminiferous tubule as a sperm-storing structure

3. Role of the reproductive duct in sperm formation

a. Vertebrates without a highly tortuous epididymal portion of the reproductive duct

b. The epididymis as a sperm-ripening structure

c. The epididymis and vas deferens as sperm-storage organs

d. Two types of vertebrate testes relative to sperm formation

4. Function of the seminal vesicles (vesicular glands)

5. Function of the prostate gland

6. Bulbourethral (Cowper’s) glands

7. Functions of seminal fluid

a. Amount of seminal fluid discharged and its general functions

b. Coagulation of the semen

c. Hyaluronidasc

d. Accessory sperm

e. Fructose

f. Enzyme-protecting substances

D. Internal and external factors influencing activities of the testis

1. Internal factors

a. Temperature and anatomical position of the testis

b. Body nourishment in relation to testicular function

c. The hypophysis and its relation to testicular function

2. External environmental factors and testis function

a. Light as a factor

b. Temperature influences

E. Internal factors which may control seasonal and continuous types of testicular function

F. Characteristics of the male reproductive cycle and its relation to reproductive conditions in the female


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