Book - Embryology of the Pig 10
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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 10. The Development of the Urogenital System
The excretory and reproductive systems are so closely related both anatomically and embryologically that they must inevitably be considered together. Neither system is particularly simple in organization and the two of them together present quite a formidable array of structures. Naturally the development of such a composite group involves much of special interest to the embryologist. We shall see organs formed by the association of parts which arise independently at different places. Certain organs appear and then disappear completely without ever having become functional. Other organs fall into disuse in their original capacity and begin to degenerate only to have some part seized upon and salvaged by a new organ for a new function. We have, as it were, in the story of the development of these systems many characters. Each character, individually, is doing things of interest. Sooner or later their activities cross. The method of the novelist in dealing with such a situation would be to switch from one character to another to keep us in confusion and suspense as to what is going to happen next. Our method in dealing with this embryological story must be exactly the reverse. To prevent the various threads of the story from becoming entangled we must, as far as possible, follow one group of structures from their origin to their completion before becoming involved with another. Because the , excretory system appears earlier than the reproductive system, we shall take it up first and follow it through. Then we must return to young embryos and pick up the story of the internal reproductive system, watching constantly its relations to that of the excretory system with which we have already become familiar. Yet again we must go back and follow the differentiation of the external genitalia. Any attempt to develop all the threads of the story synchronously would lead only to confusion.
I. The Urinary System
The General Relationships of Pronephros, Mesonephros, and Metanephros
As a preface to the account of the development of the excretory organs in the pig, it is desirable to review certain facts about the structure and development of the excretory organs in the vertebrates generally. Without such information as a background the story of the early stages of the formation of these organs in a mammal seems utterly without logical sequence. With it, the progress of events encountered in mammalian development seems but natural, because it is so clearly an abbreviated recapitulation of conditions which existed in the adult stages of ancestral forms.
There occur in adult vertebrates three distinct excretory organs. The most primitive of these is the pronephros which exists as a functional excretory organ only in some of the lowest fishes. As its name implies, the pronephros is located far cephalically in the body. In all the higher fishes and in the Amphibia the pronephros has degenerated and its functional role has been assumed by the mesonephros, a new organ located farther caudally in the body. In birds and mammals a third excretory organ develops caudal to the mesonephros. This is the metanephros or permanent kidney. All three of these organs are paired structures located retroperitoneally in the dorsolateral body-wall. Each consists essentially of a group of tubules which discharge by way of a common excretory duct. In the different nephroi the tubules vary in structural detail but their functional significance is, in all cases, much the same. They are concerned in collecting waste material from the capillary plexuses associated with them and excreting it from the body.
In the development of the urinary system of birds and mammals, pronephros, mesonephros, and metanephros appear in succession, furnishing an excellent epitome of the same evolutionary history which may be learned in more detail from comparative anatomy. In embryos sufficiently young we find only the pronephros established. It consists of a group of tubules emptying into ducts, called the pronephric ducts, which discharge into the cloaca (Fig. 114, A).
A little later in development there arises in close proximity to each pronephric duct a second group of tubules more caudal in position than the pronephros. These are the mesonephric tubules. In their growth they extend toward the pronephric ducts and soon open into them (Fig. 114, B). Meanwhile the pronephric tubules begin to degenerate and the ducts which originally arose in connection with the pronephros are appropriated by the developing mesonephros. After the degeneration of the pronephric tubules these ducts lose their original name and are called mesonephric ducts because of their new associations (Fig. 114, C).
At a considerably later stage outgrowths develop from the mesonephric ducts near their cloacal ends (Fig. 114, C). These outgrowths form the ducts of the metanephroi. They grow cephalo-laterad and eventually connect with a third group of tubules which constitute the metanephros (Fig. 114, D). With the establishment of the metanephroi or permanent kidneys, the mesonephroi begin to degenerate. The only parts of the mesonephric system to persist, except in vestigial form, are some of the ducts and tubules which, in the male, are appropriated by the testis as a duct system (Fig. 114, D, right).
The Pronephros
In the embryos of birds and mammals the pronephros is an exceedingly transitory structure. Very young embryos show rudimentary proncphric tubules arising from the intermediate mesoderm opposite a few of the somites lying well cephalically in the body. (Birds, usually 5th to 16th somites; mammals, usually 6th to 14th somites.) The significance of these vestigial tubules can readily be understood by comparing them with a plan of the fully developed and functional pronephric tubules of which they are a sketchy recapitulation (Fig. 115, A, B).
Fig. 114 . Schematic diagrams to show the relations of pronephros, mesonephros, and metanephros. (Patten: ‘‘Early Embryology of the Chick,†The Blakiston Company.)
The pronephric duct arises at the level of the pronephric tubules by the extension caudad of the distal end of each tubule till it meets and fuses with the tubule behind it to form a continuous channel. The duct thus established continues to grow caudad beyond the level of the tubules until it eventually opens into the cloaca (Fig. 114). Since the pronephric tubules never become functional in mammalian embryos, we need give them no further consideration. The pronephric duct, however, becomes of importance through its subsequent relations to the mesonephros.
Fig. 115. Drawings to show structure of nephric tubules.
A, Pronephric tubule from section through 12th somite of a 16-somite chick embryo. (After Lillie.)
B, Diagram of functional pronephric tubule. (After Wiedersheim.)
C, Primitive mesonephric tubule with rudimentary nephrostome, from section through 17th somite of 30-somite chick embryo.
D, Schematic diagram of functional mesonephric tubule of the primitive type which retains the nephrostome. (After Wiedersheim.)
Fig. 116. Drawings showing the development of the mesonephric tubules in the pig. (Based on figures by McCallum and Lewis.) Abbreviation: T, mesonephric tubule.
The Mesonephros
The mesonephros in young mammalian embryos attains a high degree of development. In the pig it is especially large, being one of the most conspicuous organs in the embryo (Figs. 59 and 60). Its tubules become highly differentiated and, pending the development of the metanephros, are believed to play an active part in the embryo's elimination of nitrogenous waste.
As was the case with the pronephric tubules, the mesonephric tubules are derived from the intermediate mesoderm. At the time the tubules arise from it the intermediate mesoderm shows no trace of segmentation. When viewed in reconstructions or dissections showing its longitudinal extent, it appears as a continuous band connecting the somites with the lateral mesoderm. For this reason it is sometimes spoken of as the nephrogenic cord. When the mesonephric tubules are first budded off from the intermediate mesoderm they appear as cell clusters very close to, but not in contact with, the mesonephric (old pronephric) duct (Fig. 116, A). Once the process of tubule formation starts, the nephrogenic tissue is soon completely converted into young tubules, three or four tubules being formed opposite each somite from about the 14th to the 32nd.
The newly formed tubules grow rapidly, extending toward the mesonephric duct with which they soon attain connection (Fig. 116, B). In birds a few of the more cephalic tubules show a rudimentary nephrostome opening to the coelom, a condition comparable to that in some of the lower forms in which the mesonephros is the adult functional kidney (Fig. 115, C, D). Most if not all of the tubules in the mammalian mesonephros slur over this phase in recapitulation and develop without a nephrostome (Figs. 116 and 117). Having no ciliated nephrostome capable of drawing in coelomic fluid, such tubules obtain their liquid content from the glomerular capillaries (Fig. 117). This fluid serves to carry off by way of the mesonephric duct waste materials from the blood stream. The discarding of the nephrostome by the more specialized of the mesonephric tubules is an interesting step toward the still more highly differentiated tubule we shall encounter in the metanephros.
Fig. 117. Diagrams showing the relations of the blood vessels to a mesonephric tubule. (Based on figures by McCallum.)
After they have attained connection with the duct, the mesonephric tubules elongate rapidly. Starting from a simple S-shaped configuration, their pattern is complicated by a series of secondary bendings (Fig. 116, C, F). This growth in length greatly increases their surface exposure, thereby enhancing their capacity for interchanging materials with the blood in the adjacent capillaries.
The relations of the mesonephric tubules to the vascular system are indicated schematically in figure 117. The mesonephros is fed by many small arteries arising ventro-laterally from the aorta. Each of these arterial twigs pushes into the dilated free end of a developing tubule, forming from it a double-walled cup called a glomerular (Bowman's) capsule (Fig. 117, B). Within the capsule the artery breaks up into a knot of capillaries known as the glomerulus. Blood from the glomerulus leaves the capsule over one or more vessels (efferent with reference to the glomerulus) which again break up into capillaries. This time the capillaries form a plexus in close relation to the body of the tubule in its tortuous course from glomerulus to duct. From these capillaries the blood passes to collecting veins which are for the most part peripherally located in the mesonephros and more or less circularly disposed about it (Fig. 1 17, A). These collecting veins form a freely anastomosing system connecting both with the posterior cardinals and the subcardinals through which the blood is eventually returned to the general circulation.
Although it is relatively more conspicuous earlier in development, the mesonephros does not attain its greatest actual bulk until the embryo has reached a size of about 60 mm. When the metanephros becomes well developed, the mesonephros undergoes rapid involution and ceases to be of importance in its original capacity. In dealing with the reproductive system, however, we shall see that its ducts and some of its tubules still persist and give rise to structures of vital functional importance.
The Metanephros. The metanephros has a dual origin. It arises in part from the mesonephric duct, and in part from the intermediate mesoderm. Of these separate primordia the diverticulum arising from the mesonephric duct is the first to appear. In embryos as small as 5 or 6 mm. this metanephric diverticulum can usually be identified as a tiny bud-like outgrowth just cephalic to the point where the mesonephric duct opens into the cloaca (Fig. 118, A). Almost from its first appearance the blind end of the metanephric diverticulum is dilated, foreshadowing its subsequent enlargement to form the lining of the pelvis of the kidney. The portion of the diverticulum near the mesonephric duct remains slender, presaging its eventual fate as the duct draining the kidney {ureter).
As the metanephric diverticulum pushes out, it collects about its distal end mesoderm which has arisen from the nephrogenic cord of intermediate mesoderm caudal to the mesonephros. The original relations of this mass of mesoderm are soon entirely lost because it becomes closely massed about the pelvic end of the metanephric diverticulum and pushed farther and farther away from its point of origin as the diverticulum continues to grow ccphalad (Figs. 118, 119, and 120). This mesoderm gives rise to the secretory tubules of the metanephros or permanent kidney, and is, therefore, often designated as metanephrogenous tissue.
While the metanephric primordium is being pushed cephalad, it is increasing rapidly in size and encroaching on the space occupied by the mesonephros. Coincidently, rapid internal differentiation is progressing. The pelvic end of the diverticulum expands within its investing mass of mesoderm and takes on a shape suggestive of the pelvic cavity of the adult kidney (Figs. 119 and 120). From this primitive pelvic dilation arise numerous outgrowths which push radially into the surrounding mass of nephrogenic mesoderm (Fig. 138). These outgrowths become hollow, forming ducts which branch and rebranch as they extend toward the periphery. These are the collecting ducts of the kidney {straight collecting tubules) (Fig. 119, E).
The first changes in the mesoderm which presage the formation of the uriniferous tubules, occur near the distal ends of terminal branches of the collecting ducts. The mesodermal cells become arranged in small vesicular masses which lie in close proximity to the blind end of the collecting duct (Fig. 121, A). Each of these vesicular cell masses is destined to become a uriniferous tubule draining into the duct near which it arises. (In figure 121, A, this condition is represented schematically, only two uriniferous tubules being shown in relation to the end of the collecting ducts whereas there are actually several. The tubule on the right is represented as slightly further differentiated than that on the left.) As the developing tubules extend toward the end of the collecting duct, the bud-like tips of the duct itself grow out to meet them (Fig. 121, B). Soon the two become confluent (Fig. 121, C). In this stage the metanephric tubules are very similar to young mesonephric tubules (cf. Fig. 116). In their later development the metanephric tubules become much more elaborately convoluted than the mesonephric but their functional significance is the same.
Fig. 119, Diagrams showing a series of stages in the growth and differentiation of the metanephric diverticulum. (Patten. "Human Embryology," The Blakiston Company.)
As the kidney grows in mass additional generations of tubules are formed in its peripheral zone. New orders of straight collecting tubules arise from buds, called ampullae, which appear at about the point where the excretory tubules become confluent with the straight collecting tubules of the previous order (Fig. 121, D). At the tips of the new straight collecting tubules a new order of excretory tubules is formed from the metanephrogenous tissue in the same manner that the previous group was formed. This process is repeated many times during the growth of the kidney, about 12 to 14 generations of tubules usually having made their appearance by the time of birth. Some additional generations of tubules may be formed in the period of rapid growth immediately following birth but most of the postnatal growth of the kidneys, by which they keep pace with increased body mass, is due to the growth of the tubules rather than to further increase in their number.
Fig. 120. Drawings (X 15) of transverse sections through the pelvic region of a 15 mm. pig embryo. The level of each section is indicated on the inset lateral plan.
Fio. 121. Diagrams showing the development of the mctanephric tubules of mammalian embryos. (After Huber, from Kelly and Burnarn: “Diseases of Kidneys, Ureters, and Bladder,'’ courtesy, D. Appleton-Uentury Co.)
The blood supply to the metanephros, instead of coming directly from the aorta by numerous small branches as is the case in the mesonephros, is brought in from the aorta through the renal artery and thence distributed by a very elaborate system of smaller vessels. Nevertheless the relations of the smaller vessels to the tubules are essentially alike in the two organs. An arterial twig breaks up into a glomerulus within a capsule at the distal end of each tubule. An efferent vessel leaves the glomerulus to break up again in a meshwork of capillaries in close relation to the tortuous tubule. Collecting veins return the blood to the general circulation, freed of the nitrogenous waste matter which is a constant by-product of metabolism.
Formation of the Bladder and Early Changes in the Cloacal Region. In dealing with the development of the extra-embryonic membranes we have already taken up the formation of the allantois as an evagination from the caudal end of the primitive gut (Fig. 37). Shortly after this occurs, the gut caudal to the point of origin of the allantois becomes enlarged to form the cloaca (Fig. 118, A). When the cloacal dilation is first formed, the hind-gut still ends blindly, but there is an ectodermal depression under the root of the tail which has sunk in toward the gut until the tissue separating the gut from the outside is very thin (Fig. 37, D). This ectodermal depression is known as the proctodaeum and the thin plate of tissue still closing the hindgut is called the cloacal membrane. Eventually the cloacal membrane ruptures, establishing a caudal outlet for the gut. This rupture is similar to the rupture of the oral plate which has previously established communication between the stomodaeum and the cephalic end of the primitive gut.
Before this occurs important changes take place internally. The cloaca begins to be divided into two parts, a dorsal part which forms the rectum and a ventral part, the urogenital sinus (Fig. 122). This division is effected by the growth of the urorectal Jold., a crescentic fold which cuts into the cephalic part of the cloaca where the allantois and the gut meet (Fig. 122). The two limbs of the fold bulge into the lumen of the cloaca from either side, eventually meeting and fusing with each other! The progress of this partitioning fold toward the proctodaeal end of the cloaca makes it difficult to keep track of the original limits of the allantois, since as the urogenital sinus is lengthened, it is, in effect, added onto the allantois (cf. Fig. 118, A~D). The point of entrance of the mesonephric ducts, however, affords a landmark which is sufficiently accurate for all practical purposes. Before the urorectal fold has changed the relations, the mesonephric ducts open from either side into the cephalic part of the cloaca. After the urorectal fold has divided the cloaca, the mesonephric ducts appear to empty into the allantois (Fig. 122). This gives us our bearings, for the mesonephric ducts are actually opening into the newly established urogenital sinus which is continuous with the allantois.
Before the cloacal membrane ruptures, separation of the cloaca is
Fig. 122. Schematic ventro-latcral view of the urogenital organs of a young mammalian embryo. (Redrawn from Kelly and Burnam, ‘'Diseases of Kidneys, Ureters and Bladder,†courtesy, D. Appleton-Century Co.) The figure as originally drawn was based on human embryos of 12 to 14 mm. In all essentials the relations shown are applicable to 14 to 15 mm. pig embryos.
complete and its two parts open independently. The opening of the rectum is the anus and that of the urogenital sinus is the ostium urogenitale (Fig. 118, D).
Meanwhile the proximal part of the allantois has become greatly dilated and may now quite properly be called the urinary bladder. We should remember, however, that the neck of the bladder has been formed largely from tissue which was originally part of the cloaca.
In the growth of the bladder the caudal portion of the mesonephric duct is absorbed into the bladder wall. This absorption progresses until the part of the mesonephric duct caudal to the point of origin of the metanephric diverticulum has disappeared. The end result of this process is that the mesonephric and metanephric ducts open independently into the urogenital sinus. The metanephric duct, possibly due to traction exerted by the kidney in its migration headwards, acquires its definitive opening somewhat laterally and cephalically to that of the mesonephric duct. It then discharges into the part of the urogenital sinus which was incorporated in the bladder. The mesonephric ducts open into the part of the urogenital sinus which remains narrower and gives rise to the urethra (Fig. 118, D-F). The urethra acquires quite diflferent relations in the two sexes. It is, therefore, desirable to defer consideration of it and take it up in connection with the external genitalia.
II. The Development of the Internal Reproductive Organs
The Indifferent Stage. One of the striking things in the development of the reproductive system is the condition which at first exists as to sexual differentiation. One might expect that reproductive mechanisms as totally unlike as those of adult males and females would be sharply differentiated from one another from their earliest appearance. Such is not the case. Young embryos exhibit gonads which at first give no evidence as to whether they are destined to develop into testes or ovaries. Along with these neuter or indifferent gonads there are present in an undeveloped state two different duct systems. If the individual develops into a female, one of these duct systems forms the oviducts, uterus, and vagina, and the other remains rudimehtary. If the individual is destined to become a male, the potentially female ducts remain rudimentary and the other set gives rise to the duct system of the testes. In dealing with the embryology of the reproductive organs, therefore, conditions as they exist in the indifferent stage (Fig. 123) form a common starting point for the consideration of the later developmental changes in either sex.
Origin of the Gonads. From their earliest appearance the gonads are intimately associated with the nephric system. While the mesonephros is still the dominant excretory organ, the gonads arise as ridge-like thickenings (gonadial ridges, germinal ridges) on its ventromesial face (Fig. 108, F, and 138). Histologically the gonadial ridge consists essentially of a mesenchymal thickening covered by mesothelium. The mesothelial coat of the developing gonad is directly continuous with the mesothelium covering the mesonephros — is in fact merely a part of it stretched over the mesenchymal thickening. It soon, however, begins to show characteristics which differentiate it from the adjacent mesothelium. It grows markedly thicker and its cells round out and increase in size. Some of the cells in the germinal
Fig. 123. Schematic diagram showing plan of urogenital system at an early stage when it is still sexually undifferentiated. (Modified from Hertwig.)
epithelium, as this modified layer of mesothelium is now termed, are conspicuously larger than their neighbors. These large cells are the primordial germ cells of the gonad. Considerable evidence has been adduced of late that these germ cells are not formed in situ by the differentiation of mesothelial cells. It is maintained that they can be identified elsewhere in the body before they appear in the germinal epithelium, and that they migrate from their place of origin (yolk-sac entoderm) to settle down in the germinal epithelium and there rear their families. Whatever their previous history may be, they are clearly recognizable in the germinal epithelium and it is not difficult to follow their differentiation from then on, through succeeding generations, to give rise, finally, to the gametes.
If the gonad is to develop into a testis the cells of the germinal epithelium grow into the underlying mesenchyme and form cord-like masses. These cords eventually become differentiated into the seminiferous tubules in which the spermatozoa are formed. In case the gonad develops into an ovary the primordial germ cells grow into the mesenchyme and there become differentiated into ovarian follicles containing the ova. (See Chap. 2.)
The Sexual Duct System in the Male. The ducts which in the male convey the spermatozoa away from the testis are, with the exception of the urethra, appropriated from the mesonephros - a developmental opportunism facilitated by the proximity of the growing testes to the degenerating mesonephros (Fig. 128). The mesonephric structures which are taken over by the testes are shown schematically in figure 124.
Epididymis. Some of the mesonephric tubules which lie especially close to the testes arc retained as the efferent ductules (Figs. 3 and 124). They, together with that part of the mesonephric duct into which they empty, become the epididymis. Cephalic to the tubules which are converted into efferent ductules a few mesonephric tubules sometimes persist in vestigial form as the appendix of the epididymis. Caudal to the efferent ductules a cluster of mesonephric tubules almost invariably persists in rudimentary form as the paradidymis.
Ductus Deferens, Seminal Vesicle, and Ejaculatory Duct. Caudal to th^ epididymis the mesonephric duct receives a thick investment of smooth muscle and becomes the ductus (vas) deferens. A short distance before the vasa deferentia enter the urethral part of the urogenital sinus, local dilations appear in them which become elaborately sacculated and form the seminal vesicles (Fig. 3). The short part of the mesonephric duct between the seminal vesicles and the urethra constitutes the ejaculatory duct. From this point on, the spermatozoa traverse the urethra which thus serves as a common passageway to the exterior for both the sexual cells and the renal excretion.
Prostate and Cowper's Glands. From the urethral epithelium the prostate and bulbo-urethral glands develop. The prostate sur* rounds the urethra near the neck of the bladder; the bulbo-urethral (Cowper’s) glands lie adjacent (Fig. 124). Their secretions, discharged into the urethra with that of the seminal vesicles, serve as a conveying fluid for the spermatozoa.
Fig. 124. Diagram of the male sexual duct system in mammalian embryos. (Modified from Hertwig.)
The Female Duct System. The Mullerian ducts first appear close beside and parallel to the mesonephric ducts. They are the primordial structures from which the uterine tubes (oviducts), uterus, atid vagina arise in the female. It is possible that phylogenetically the Mullerian ducts arose directly from the mesonephric ducts. Ontogenetically in the mammals any such process of splitting has been slurred over and they seem to arise side by side from the same parent tissue. The mesonephric ducts become well developed earlier than the Mullerian ducts and it is very easy to overlook the Mullerian ducts altogether in young specimens. By the time embryos have reached 30 or 40 mm. in length, however, it should be possible to locate them readily in sections or, with care, by dissection.
Fig. 125. Diagram of the female sexual duct system in mammalian embryos.
(Modified from Hertwig.)
Vagina. When the Mullerian ducts first appear they are paired throughout their entire length. At their cloacal ends the right and left ducts lie close to the mid-line. In this region they soon approach and fuse with each other in the mid-line to form the vagina (Fig. 125).
Uterus. In some of the primitive mammals fusion of the Mullerian ducts does not progress cephalad beyond the vagina. Such animals have paired uteri formed by enlargement of the Mullerian ducts cephalic to their entrance into the vagina (Fig. 126, A). In all the higher mammals, fusion of the Mullerian ducts involves the caudal end of the uterus so that it opens into the vagina in the form of an unpaired neck or cervix. Toward the ovary from the cervix there is great variation in the degree of fusion encountered in the different groups (Fig. 126, B, C, D). In the sow the fusion is carried only a short way beyond the cervix to form a typical bicornate uterus (Fig. 126, C).
Fig. 126. Four types of uteri occurring in different gioups of mammals. (After Wiedersheim.)
A, Duplex, the type found in marsupials.
B, Bipartite, the type found in certain rodents.
C, Bicornate, the type found in most ungulatse, and carnivores.
D, Simplex, the type characteristic of the primates.
Uterine Tubes. The part of the Mullerian duct between the uterus and the ovary remains slender and forms the uterine tube (oviduct). Near its cephalic end; but not usually at the extreme tip, a more or less funnel-shaped opening develops {ostium tubae abdominale). In different forms the detailed configuration of the ostium and its relation to the ovary are quite variable. Conditions range all the way between a pouch-like dilation which almost completely invests the ovary (sow), and an elaborately fringed, funnel-shaped ostium which opens in the general direction of the ovary (man). Whatever the morphological eccentricities of the ostium may be, they apparently make less difference in its efficiency in picking up the discharged ovum than one might suppose. Even in forms where the relation of the ostium to the ovary is least intimate, abdominal pregnancies resulting from the fertilization of an ovum which the ostium failed to catch and start on its way to the uterus are comparatively uncommon.
Vestigial Structures in the Genital Duct System. In the conversion of the primordial duct systems to their definitive conditions, some of the parts which are not utilized in the formation of functional structures persist in vestigial form even in the adult. Mention has already been made of the rudimentary mesonephric tubules which persist in the male as the paradidymis and the appendix of the epididymis. Traces of the old Mullerian duct system also can usually be found in the male. Attached to the connective tissue investing the testis there is sometimes a well-marked vesicular structure called the appendix of the testis {hydatid) which represents the cephalic end of the Mullerian duct. These ducts also leave a vestige at their opposite ends in the form of a minute diverticulum {prosiaiic simis^ vagina mascidina) which persists where the fused Mullerian ducts opened into the urogenital sinus (Fig. 124).
In the female the ostium of the oviduct does not ordinarily develop at the extreme cephalic end of the Mullerian duct. The tip of the duct is likely to persist in rudimentary form as a stalked vesicle (hydatid) attached to the oviduct (Fig. 125).
The mesonephric tubules and ducts may remain recognizable to a variable extent. Usually there is embedded in the mesovarium a cluster of blind tubules and traces of a duct, corresponding to the part of the mesonephric duct and tubules which in the male form the epididymis. Ifhese vestiges are called the epodphoron. Less frequently the more distal portion of the mesonephric duct (the part which in the male forms the vas deferens) leaves traces known as the canals of Gartner in the broad ligament close to the uterus and vagina.
Changes in Position of the Gonads. Neither the testes nor the ovaries remain located in the body at their place of origin. The excursion of the testes is particularly extensive. Many factors are involved in their descent from the mesonephric region, where they first appear, to their definitive position in the scrotal sac. We can only sketch very briefly the course of events.
The urogenital organs arise in the dorsal body-wall, covered by the mesothelial lining of the coelom. Later when the coelomic mesothelium of the abdominal region is reinforced by connective tissue, the two layers together constitute the peritoneum. As to position of origin with reference to the body cavity, the urogenital organs may, therefore, be briefly characterized as retroperitoneal. This primary positional relationship is already familiar but it is emphasized again here because it is involved in many phases of the change in position and relations undergone by the reproductive organs.
Fig. 127. Dissection showing the relative position of mesonephros, metanephros, and testis in a 33 mm. pig embryo. (Modified from Hill.)
Descent of the Testes. When the mesonephros begins to grow rapidly in bulk, it bulges out into the coelom, pushing ahead of itself a covering of peritoneum. At either end of the mesonephros the peritoneum is, in this process, thrown into folds. One of them extends cephalad to the diaphragm and is known as the diaphragmatic ligament of the mesonephros 124 and 127). The other, which extends to the extreme caudal end of the coelom, becomes fibrous and is then known as the inguinal ligament of the mesonephros (Fig. 124). The inguinal ligament is destined to play an important part in the descent of the testes.
We have already seen that when the testis develops it causes a local expansion of the peritoneal covering of the mesonephros to accommodate its increasing mass. As the testis grows, the mesonephros decreases in size and the testis takes to itself more and more of the peritoneal coat of the mesonephros (Figs. 127 and 128). In this process it becomes closely related to the inguinal ligament of the mesonephros. In effect the inguinal ligament extends its attachment to include the growing testis as well as the shrinking mesonephros. With this change the ligament is spoken of as the gubernaculum (Figs. 124, 129, and 131, A).
In the meantime a pair of coclomic evaginations are formed, one in the inguinal region of each side of the pelvis where the caudal end of the gubernaculum is attached. These are the scrotal pouches. Due perhaps in part to traction exerted by the gubernaculum, the testes and the mesonephric structures which give rise to the epididymis begin to shift their relative position progressively farther caudad (cf. Figs. 127-130). Eventually they come to lie in the scrotal pouches. It would be more direct and vivid to say that the gubernaculum “pulls the testis down.†Although the end results of their association very nearly justify such a phrase, it would not be strictly correct. We would be overlooking the more important factor of differential growth. Failure of the gubernaculum to elongate in proportion to the growth of surrounding pelvic structures is more responsible for the traction exerted on the testis than actual shortening on its part.
Fig. 128. Dissection showing the relative size and position of mesonephros, metanephros, and testis in an 87 mm. pig embryo. (Modified from Hill.)
Fig. 129. Dissection of 128 mm. embryo showing an early stage in the descent of the testis. (After Hill.) Abbreviations: A., aorta; B., urinary bladder; E., epididymis (the retrogressing 'mesonephros); G., gubernaculum; K., kidney (metanephros) ; R., rectum; Ring, ‘‘inguinal ring,†the fibrous tissue surrounding the opening (inguinal canal) into the scrotal sac; T., testis; U., ureter; U. A., umbilical artery; W. M., mesonephric (Wolffian) duct and Mullerian duct. The common investment of connective tissue largely conceals the smaller Mullerian duct. In this figure the line of demarcation between the two is most clearly shown just to the left of the gubernaculum.
Fig. 130. Dissection of 210 mm. pig embryo showing the testis just entering the inguinal canal. (After Hill.) Abbreviations: A., aorta; B., bladder; E., epididymis; K., kidney; M. D.; Mullerian duct; R., rectum; Ring, inguinal ring; Sp. Art., spermatic artery; T., testis; U. A., umbilical artery; U., left ureter; Ur., right ureter (cut); W. D., mesonephric (Wolffian) duct.
In its entire descent, the testis moves caudad beneath the peritoneum. It does not, therefore, enter the lumen of the scrotal pouch directly but slips down under the peritoneal lining and protrudes into the lumen, reflecting a peritoneal layer over itself (Fig. 131). This layer of reflected peritoneum is known anatomically as the visceral tunica vaginalis. In most mammals when the testis has come to rest in the scrotal sac, the canal connecting the sac with the abdominal cavity becomes closed. In some of the rodents, however, it remains patent and the testes descend into the scrotum only during the breeding season, to be retracted again into the abdominal cavity until the next period of sexual activity. Even in those forms normally exhibiting complete closure of the inguinal canal the obliteration of the opening is not uncommonly incomplete or structurally weak as evidenced by the not infrequent occurrence of inguinal hernias.
Fig. 131. Schematic diagrams illustrating the descent of the testis as seen from the side. Abbreviations: d. def., ductus deferens; Proc. Vag., processus vaginalis (the diverticulum of the peritoneum pushed into the scrotal sac).
Descent of the Ovaries. Although the ovaries move through far less distance than the testes their change in position is quite characteristic and definite. As they increase in size, both the gonads and the Mullerian ducts sag progressively farther into the body cavity. In so doing they pull with them peritoneal folds comparable to the mesenteries of the intestinal tract. As these folds are stretched out they allow the ovaries, uterine tubes, and uterus to move caudally, laterally, and somewhat ventrally (Fig. 125). The peritoneal folds remain attached to the dorsal and lateral body-walls, become reinforced by fibrous tissue, and constitute the broad ligaments. The inguinal ligament of the mesonephros which in the male forms the gubemaculum, in the female is caught in the peritoneal folds which form the broad ligaments. When the ovaries move caudad and laterad the inguinal ligament is bent into angular form. Cephalic to the bend it becomes the round ligament of the ovary, caudal to it, the round ligament of the uterus (Fig. 125). Thus the changes in position of the female reproductive organs are carried out in a manner quite different from those in the male. In both sexes the organs arise retroperitoneally, but in the male the testes slide along close to the body-wall beneath the peritoneum, while in the female the ovaries, oviducts, and uterus stretch the peritoneum into a mesentery-like structure which permits a certain latitude of positional change and at the same time serves as a supporting ligament and a path of ingress for blood vessels and nerves.
The Adrenal Glands. The adrenal glands and the accessory chromaffin bodies are endocrine organs which are in no way part of the urogenital system. But the close proximity of the adrenal glands to the kidneys (Figs. 127 and 128) makes it convenient to give them a word of comment at this point.
Certain cells which migrate ventrally from the neural crest at the time the sympathetic ganglia are formed become, not nerve cells, but gland cells active in the production of a specific hormone. Due presumably to the presence of this internal secretion in their cytoplasm they exhibit a characteristic reaction with chromic acid salts which has led to their designation as chromaffin cells. Clusters of these chromaffin cells become located in close proximity to each sympathetic ganglion. These clusters are called the paraganglionic chromaffin bodies. Otheij masses of chromaffin tissue from the same source appear in various places beneath the mesoderm lining the coelom. There is usually a considerable amount of chromaffin tissue present in the region of the abdominal sympathetic plexus. This mass constitutes the aortic chromaffin body (organ of Zuckerkandl). The largest mass of extra-sympathetic chromaffin tissue appears just cephalic to the kidney and becomes converted into the medulla of the adrenal.
The cortical portion of the adrenal gland appears very early in development. Even in embryos of 9~12 mm. there is accelerated local proliferation of cells from the splanchnic mesoderm around the notch on either side of the base of the primary dorsal mesentery adjacent to the cephalic pole of the mesonephros. These cells push into the underlying mesenchyme and begin to show a tendency to become arranged in cords. By the 15-17 mm. stage the aggregation
Fig. 132. Photographs (X 5) of the external genitalia of a series of pig embryos. Abbreviations: A, anus; C, clitoris; F, genital fold; GC, glans clitoridis; GP, glans penis; L, labia majora; N, labia minora (nymphae); O, urogenital orifice; P, penis; Pr, prepuce; Proc, proctodaeum; R, raphe; S, genital swelling; Sc, scrotum; T, genital tubercle.
of cell cords which constitutes the primordium of the adrenal cortex is quite conspicuous (Fig. 138). Later in development the migrating cells which give rise to the medulla of the adrenal invade the cortical primordium and become encapsulated within it.
III. The External Genitalia
Indifferent Stages. Still another thread in the story which has to be picked up separately is the development of the external genitals of either sex by divergent differentiation from a common starting point.
In very young embryos there is formed in the mid-line just cephalic to the proctodaeal depression, a vaguely outlined elevation known as the genital eminence. This is soon differentiated into a central prominence {genital tubercle) closely flanked by a pair of folds {genital Jolds) extending toward the proctodaeum. Somewhat farther to either side are rounded elevations known as the genital swellings (Figs. 132, A, B, C). Between the genital folds is a longitudinal depression which attains communication with the urogenital sinus to establish the urogenital orifice (ostium urogenitale). This opening is separated from the anal opening by the urorectal fold (Figs. 118, 122, and 132 A-C).
The Male Genitalia. If the individual develops into a male the genital tubercle becomes greatly elongated to form the penis, the genital folds ensheath the penis as the prepuce, and the genital swellings become enlarged to form the scrotal pouches (Fig. 132, C, D, E, F). During the growth of the penis there develops on its caudal face a groove extending throughout its entire length. Posteriorly the groove is continuous with the slit-like opening of the urogenital sinus. This groove in the penis later becomes closed over by a ventral fusion of its margins, establishing the penile portion of the male urethra. That portion of the urogenital sinus between the neck of the bladder and the original opening of the urogenital sinus becomes the prostatic urethra (Fig. 118, F). Since the margin of the slit-like urogenital orifice closes coincidently with the closure of the urethral groove in the penis, the prostatic urethra and the penile urethra become continuous and the urogenital orifice is projected to the tip of the penis. The line of fusion in the urogenital sinus region and along the caudal surface of the penis is clearly marked by the persistence of a ridge-like thickening known as the raphe (Fig. 132, D, E, F).
Female Genitalia. In the female the genital tubercle becomes the clitoris, the genital folds become the labia minora and the genital swellings the labia raajora (Fig. 132, C, G, H, I). The original opening of the urogenital sinus undergoes no such changes ds occur in the male but persists nearly in its original position. Its orifice, enlarged and flanked by the labia, becomes the vestibule into which open the vagina and the urethra (Fig. 118, E). The urethra in the female is derived entirely from the urogenital sinus, being homologous with the prostatic portion of the male urethra.
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Cite this page: Hill, M.A. (2024, April 19) Embryology Book - Embryology of the Pig 10. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_10
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