Book - Embryology of the Pig 3

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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.

Online Editor

This historic 1951 embryology of the pig textbook by Patten was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available. By the same author: Patten BM. The Early Embryology of the Chick. (1920) Philadelphia: P. Blakiston's Son and Co. Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381 Modern Notes pig

Historic Disclaimer - information about historic embryology pages

Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 3

Tke Sexual Cycle; Fertilization

I. The Sexual Cycle

llie periodic recurrence of times when the mating impulse becomes the dominating factor in the reactions of an animal has long excited comment and conjecture. Why does the urge to sexual union suddenly manifest itself in an animal but recently indifferent to the opposite sex? Why should it become latent for considerable stretches of time even when pregnancy has not ensued? What changes are going on inside the body correlated with, or causing, the animal’s abrupt changes in reactions? To such questions no answers other than surmises have until recently been forthcoming. Many phases of these questions cannot yet be answered with particularity and there arc still problems involved on which we have virtually no tested evidence. But the main points of the story have been fitted together and checked experimentally and new details are constantly being added.

As is inevitable in any field where our knowledge is growing rapidly, there is much difference of opinion as to the significance of certain of the observed facts. It is through the discussion and evaluation of just such divergent interpretations that further progress comes. But it is neither possible nor advisable in such a brief account as this to become involved in the points of controversy. We must content ourselves with a mere outline of the facts which seem best established and realize that even some of these facts may be subjected to different interpretations.

Sexual periodicity is much less strongly developed as a rule in the male than in the female. In some species the male is sexually potent without interruption throughout adult life. In many species the sexual instinct in the male seems to be aroused from latency periodically merely by the presence of females manifesting the mating impulse. In either of these cases there are no apparent structural changes in the body or in the reproductive organs of the male at different seasons. In contrast to such conditions there are manifested by the males of many

28

THE SEXUAL CYCLE

29

forms, short, well marked periods of intense sexual activity alternating with long periods of sexual impotence. Such a cycle is accompanied by actual structural fluctuations in the state of the reproductive organs themselves. Sometimes the changes in the gonads are accompanied by the development of the very striking secondary sexual characteristics such as the antlers of the buck, which appear only at the breeding season to be shed afterwards and grown again at the next breeding season. A brief period of pronounced sexual activity, when it occurs in males, is known to animal breeders as their “rutting season.” It always corresponds in time with the females' period of strong mating impulse which breeders call the “period of heat” and biologists speak of as the estrus,

I’he term estrus originally referred merely to the existence of a period of strong sexual desire made evident through behavior. As more information has been acquired about the concomitant changes going on within the body, it has become evident that this period of desire is but an external indication that all the complicated internal mechanism of reproduction is ready to become functional. If pregnancy does not occur at this time regressive changes follow and another period of preparation must ensue before conditions are again favorable for reproduction. This repeated series of changes is known as the estrous cycle. Its phases in the absence of pregnancy are: (1) a short time of complete preparedness for reproduction accompanied by sexual desire (estrus) ; (2) a short period of regressive changes in which evidences of a fruitless preparation for pregnancy disappear (post- or metestrum) ; (3) a relatively long period of rest (diestrum) ; followed by (4) a period of active preparatory changes (proestrum) leading up to the next estrus when everything is again in readiness for reproduction.

There is wide variation in the length of time occupied by this cycle in different animals. In some it occupies an entire year, the estrus recurring at the same time each year and being so placed seasonally that when in due time the young are born, conditions are favorable for their rearing. The deer family exemplifies this condition. Their mating season comes in the autumn and the fawns are born in the spring. The young then have an entire summer of plentiful food supply before they are subjected to the rigors of winter conditions. Whether or not pregnancy occurs, mating does not take place again until the next autumn. Forms having thus but one breeding season in the year are said to be monestrous.

Many forms, in contrast, exhibit a regularly recurring series of

30

THE SEXUAL CYCLE; FERTILIZATION

mating periods in a year. Such forms are said to be polyestrous. The pig is an example of this condition. Sows failing to become pregnant will ‘^come in heat” again after an interval of about 21 days, and maintain such a cycle until it is interrupted by pregnancy.

On the basis of our present knowledge it would appear that a polyestrous rhythm such as that occurring in the sow is the underlying condition in mammals generally. Many factors mask or modify it in different cases, but in the forms which have been most fully studied it is unmistakably present. Accepting this proposition tentatively, it is not unreasonable to suppose that an annual cstrus such as that exhibited by the deer family has become established through the suppression of other periods, primarily because of the regular recurrence of pregnancies of long duration following what was originally merely the most favorable of several estrous periods. It is well known, furthermore, that the estrous cycle may be interrupted by many things other than pregnancy. Thus starvation, extreme exposure, or severe sickness may cause the suppression of an estrus. A contributing cause in reducing a polyestrous to a monestrous rhythm, operative in the case of females failing to become pregnant, might well be the severity of the conditions under which they live during the winter.

It is known with certainty that many animals, as for example the sheep, which have but one breeding season in the year when living in their wild state, develop a polyestrous rhythm when living in domestication. An underlying polyestrous condition is necessarily obscured when a pregnancy follows each estrus, as occurs normally among wild animals. It becomes apparent, however, when such an animal under conditions of domestication is not permitted to become pregnant. Then there is a brief period of rest and preparation followed shortly by another estrus. Living conditions under domestication being relatively uniform, suppression of an estrus through starvation or exposure does not occur and the estrous periods recur at fairly regular intervals until one of them is consummated by pregnancy.

Before seeking the factors which underlie the recurrent estrous manifestations it will be well to have clearly in mind the changes which are going on in the internal reproductive organs in correlation with the alternate conspicuousness and abeyance of the mating instinct.

In the ovaries a group of Graafian follicles begins rapid enlargement just before the onset of estrus (Fig. 9). These follicles usually rupture during or toward the close of the estrus, in other words at the

THE SEXUAL CYCLE

31

time when active spermatozoa are most likely to be awaiting them in the oviducts where fertilization usually occurs.

The oviducts in the sow have been shown by Seckinger to exhibit a gradually increasing rate of muscular activity during estrus which reaches its height toward the close of estrus at about the time the discharged ova are in the oviduct (Fig. 9). It is probable that such a condition will be found to exist in other mammals also, when observations are extended to them. It seems clearly correlated with the transportation of the ovum to the uterus. The lining epithelium of the oviduct is also thicker and apparently more active in secretion at this time. It would be interesting to know whether or not there is also an increase in the power of its ciliary action.

Toward the close of estrus, the uterus begins to exhibit a marked congestion which reaches its height during the postestrous period (Fig. 9). Accompanying the excessive congestion there is hypertrophy of the uterine mucous membrane and increased activity of the glands. In the uterus of primates, especially in man, this stage of congestion

Proestrum Estrus Postestrum Diestrum Proestrum Estrus Postestrum |

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Fig. 9. Graph showing correlation of changes which occur during the estrous cycle in the sow. (Compiled from the work of Corner, Seckinger, and Keye.) Note the coincidence of the important events leading toward pregnancy (coitus, ovulation, fertilization of the ovum and its migration through the oviduct to the uterus, and finally its attachment to the uterine mucosa) with the height of local activity as indicated by the curves.

32

THE SEXUAL CYCLE; FERTILIZATION

and hypertrophy is very pronounced and is terminated rather abruptly by hemorrhage and by sloughing of the uterine epithelium (menstruation). In most mammals the analogous changes are more gradual and less extensive, and are accomplished without hemorrhage.

By the time the ovum has traversed the oviduct and reached the uterus, the uterine mucous membrane is at the height of its constructive phase. The experiments of Leo Loeb in transplanting embryonic placental tissue indicate that the uterine mucosa is at this time in its most favorable condition for the attachment of embryos.

One would scarcely expect the mechanism by which such an elaborate series of changes are induced and synchronized to be a simple one. It is indeed exceedingly complex. Recent investigations have, however, thrown much light on it. Although there remains much to be cleared up we are at least finding the nature of the factors involved if not their precise action.

With the reservation that much of the information is recently acquired and therefore not thoroughly tested and subject to revision as new facts come to light, the main thread of the story seems to be unfolding in this manner. The development of the reproductive system as a whole appears to be accelerated at the appropriate phase of bodily maturity through the operation of hormones, the most active of which are derived from the anterior lobe of the pituitary gland.

The factors immediately responsible for the maintenance of the estrous cycle during maturity have long been known to be resident in the gonads. When the ovaries are removed from an immature female the estrous phenomena never develop. When the ovaries are removed from an adult female the estrous cycle ceases. The experimental studies of Edgar Allen and his co-workers gave the first conclusive evidence that the maturing ovarian follicles were the source of a potent estrogenic hormone. They were able by injections of follicular extracts to induce typical estrous reactions in females which had previously ceased to show estrous changes as a result of complete ovariectomy. Such a source for the hormone would account for the height of sexual desire occurring at about the time of the liberation of the ova, when the follicles are turgid with hormonecontaining fluid. It would account, also, for the initiation of congestion in the uterus, and at the same time for the subsidence of these phenomena after the escape of the follicular contents (see Fig. 9). The ripening, after a period of rest, of another crop of follicles would account in the same way for the succeeding estrus.

THE SEXUAL CYCLE

33

Extensive experimental work by many investigators, following the pioneering studies by P. E. Smith, has shown that the ovaries cannot function effectively unless they are activated by hormones produced in the anterior lobe of the pituitary gland. These hormones are essential for initiating the growth of the gonads which occurs with the onset of puberty, and for maintaining the effective function of the gonads during sexual maturity. They are designated as gonadotropic hormones.

From ovarian follicles after their rupture, corpora lutea are formed. C'onstantly increasing evidence has been brought out by a large number of workers that the corpus luteum also plays an important part in the reproductive cycle. The injection of corpus luteum extract has repeatedly been shown to inhibit ovulation. This effect is entirely consistent with the time relations between the maximal development of the corpora lutea of ovulation and the other events of the estrous cycle (Fig. 9). Corpora lutea attain a marked degree of development and give every histological indication of active secretion a few days after the rupture of the follicles from which they are formed. They begin to exhibit retrogressive changes on microscopical examination, and to decrease visibly in size at about the time a new group of follicles first begins to show rapid growth. Such findings point to the corpora lutea of ovulation as the source of a second hormone which retards the development of the next group of follicles with their activating hormone, until a period of rest has been enforced.

During pregnancy the corpora lutea attain a greater growth and persist longer than they do in the absence of pregnancy (Fig. 10). Their persistence throughout pregnancy and for a variable time thereafter would seem to account for the inhibition of ovulation which occurs during pregnancy and the early part of the period of lactation.

In addition to their inhibitory effect on the development of the follicles, the corpora lutea of ovulation produce a hormone {progesterone) which acts on the uterine mucosa in such a way that the implantation of the fertilized ovum is facilitated. Moreover in their extended development during pregnancy they have, in conjunction with the anterior lobe of the pituitary gland, an activating influence on the mammary glands.

Thus the sexual desire manifested at estrus is merely one event in a closely coordinated series of changes involving all the reproductive organs. All these changes are synchronized with one another by the regulatory action of hormones. All of them are preparatory for pregnancy. The recurrence of the estrus when pregnancy does not

26 28 30 32 34 36 38 40 42

ncy

Days 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 tfc

B. In Pregnancy

Fig. 10. Graphs showing the difference in history of the corpus luteum of ovulation and the corpus luteum of pregnancy in the sow. (After Corner.)

ensue is but the outward sign that the entire female mechanism is again, after a period of rest, in a state of preparedness to carry through the rearing of oflFspring. If pregnancy occurs the estrous rhythm is suppressed until the young have been bom and suckled.

II. Fertilization

Direct observations of the processes involved in fertilization in mammals are exceedingly fragmentary. Nevertheless, interpreting these observations in the light of the much more detailed information available from the study of water-living forms where fertilization normally occurs outside the body of the mother, it is possible to piece out a fairly circumstantial story of the main events.

Even though coitus in most mammals occurs only at the phase of the estrous cycle when mature follicles are ready to rupture and release ova, actual fertilization of the ova does not inevitably follow. The immediate result of copulation is merely the deposition of semen

FERTILIZATION

35

in the vagina {insemination). Thence the spermia must make their way by their own motility through the uterus and into the oviducts where fertilization ordinarily takes place. In comparison with the size of the spermia the distance they must travel is exceedingly great. The enormous numbers of spermia contained in an ejaculate of semen give great probability, but not positive assurance, that some of them will reach the oviduct while they are still capable of penetrating and fertilizing the ovum.

Ordinarily within a few hours of the coitus of healthy individuals great numbers of spermia will have made their way to the oviducts and surround the ova as they are carried through the oviducts toward the uterus. However great may be the numbers present, normally only a single spermium enters the ovum (Fig. 1 1, A). As soon as it has been entered by a spermium an ovum appears to undergo at once changes which result in the cessation of penetrating activity on the part of other spermia in its neighborhood. This phenomenon may readily be observed in manv marine forms where fertilization can be

Fig. 11. Diagrams illustrating schematically tne process of fertilization and the formation of the first cleavage spindle. (After William Patten.) As in figure 8 the species number of chromosomes is assumed to be eight. Abbreviations: sp., spermium; p.c., polar cell (polar body). The male and female symbols designate the male and female pronuclei respectively.

36

THE SEXUAL CYCLE; FERTILIZATION

carried out in a dish of sea-water under the microscope. When the spermia are first introduced into a dish containing ova, one secs swarms of them surrounding each ovum. Even the relatively enormous bulk of the egg cell may actually be set in rotation by their combined activity. Abruptly, when one spermium has penetrated the ovum, its surface membrane becomes thickened and less readily penetrated; coincidently the remaining spermia appear to lose their directive activity, and soon only scattered ones remain in the neighborhood of the fertilized ova. That this change is due to the fertilization of the ovum and not to loss of activity on the part of the spermia may be demonstrated readily by adding unfertilized ova to the dish and watching the process repeat itself.

Only the head of the spermium, which is almost entirely condensed nuclear material, and the neck containing the centrosomal apparatus enter the ovum. The tail is dropped off (Fig. 11, B). Once within the ovum, the nuclear material contained in the head of the spermium loses its condensed form and begins to show its chromosomal content. It is now known as the male pronucleiis (Fig. 11, C). In most mammals the second maturation division of the ovum is not completed until after ovulation. Not uncommonly it is delayed until a spermium has penetrated the egg cell. Always, however, by the time the male pronucleus has been formed the second maturation division has occurred. The reduced nucleus of the ovum is then known as the female pronucleiis. Fertilization is said to have occurred only when the male and female pronuclei have merged with each other (Fig. 11, E). As each pronucleus contributes half the species number of chromosomes, the full species number of chromosomes is reestablished in the fertilized ovum.

During the period between the penetration of the ovum by the spermium and the actual fusion of the pronuclei, a mitotic spindle has been forming from the centrosomal apparatus brought in by the spermium (Fig. 11, C, D). On this spindle the chromosomes from both the male and female pronuclei become arranged preparatory to the first mitotic division in the development of the new individual.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

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Cite this page: Hill, M.A. (2024, April 27) Embryology Book - Embryology of the Pig 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_3

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