McMurrich1914 Chapter 13
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter XIII. The Development of the Urinogenital System
The excretory and reproductive systems of organs are so closely related in^their development that they must be considered together. They both owe their origin to the mesoderm which constitutes, the intermediate cell-mass (p. 77), this, at an early period of development, becoming thickened so as to form a ridge projecting into the dorsal portion of the ccelom and forming what is known as the Wolffian ridge (Fig. 207, wr). The greater portion of the substance of this ridge is concerned in the development of the primary and secondary excretory organs, but on its mesial surface a second ridge appears which is destined to give rise to the ovary or testis, and hence is termed the genital ridge (gr).
Fig. 207. - Transverse Section through the Abdominal Region of a Rabbit Embryo of 12 mm.
a, Aorta; gl., glomerulus; gr, genital ridge; m, mesentery; nc, notochord; t, tubule of mesonephros; wd, Wolffian duct; wr, Wolffian ridge. - (Mihalkovicz.)
The development of the excretory organs is remarkable in that three sets of organs appear in succession. The first of these, the pronephros, exists only in a rudimentary condition in the human embryo, although its duct, the pronephric or Wolffian duct, undergoes complete development and plays an important part in the development of the succeeding organs of excretion and also in that of the reproductive organs. The second set, the mesonephros or Wolffian body, reaches a considerable development during embryonic life, but later, on the development of the final set, the definite kidney or metanephros, undergoes degeneration, portions only persisting as rudimentary structures associated for the most part with the reproductive organs.
The Development of the Pronephros and the Pronephric Duct
The first portions of thppjrrejl? r y system to make their appearance are the pronephric orWolffian ducts, which develop as outgrowths of the dorsal wallsof_t he intermedi atecejljnasses ; At first ThT outgrowths are solid cords of cells (Fig. 208, wd), but" later a lumen appears in the center of each and the canal so formed from each intermediate cell mass, bending backward at its free end, comes into contact and fuses with the canal from the next succeeding segment. Two longitudinal canals, the pronephric or Wolffian ducts, are thus formed, with which the cavities of the intermediate cell masses communicate. The formation of the ducts begins in the anterior segments before the segmentation of the posterior portions of the mesoderm has taken place, and the further backward extension of the ducts takes place independently of the formation of excretory tubules, apparently by a process of terminal growth. The free end of each duct comes into intimate relation with the ectoderm above it, so much so that its posterior portion has been held^by some observers to be formed from that layer, but it seems more probable that the relation to the ectoderm is a secondary process and that the ducts are entirely of mesodermal origin. They reach the cloaca in embryos of a little over 4 mm., and later they unite with that organ, so that their lumina open into its cavity.
Fig. 208. - Transverse Section through Chick Embryo of about Thirty-six Hours.
en, Endoderm; im, intermediate cell mass; ms, mesodermic somite; nc, notochord; so, somatic, and sp, splanchnic mesoderm; wd, Wolffian duct. - (Waldeyer.)
The pronephric tubules make their appearance in embryos of about 1.7 mm., while as yet there are only nine or ten mesodermic somites, and they are formed from the intermediate cell masses of the seventh to the fourteenth segment, and perhaps from those situated still more anteriorly. The entire series, however, is never in existence at any one time, for before the more posterior tubules are formed, those of the anterior segments have undergone degeneration. Each pronephric tubule, when fully formed, consists of a portion which unites it to the Wolffian duct, and opens at its other end into an enlargement, the pronephric chamber, (Fig. 209, pc), which, on its part opens mto the ccelomic cavity by means of a nephrostome canal. In the neighborhood of the ccelomic opening, or nephrostome, an outgrowth of the ccelomic epithelium is formed, and a branch from the aorta penetrates into this to form a stalked external glomerulus lying free in the coelomic cavity (Fig. 209, e.g.). Internal glomeruli, such as occur in connection with the mesonephric tubules do not occur in the pronephros of the human embryo, and this fact, together with the presence of external glomeruli and the participation of the tubules in the formation of the Wolffian duct, serve to distinguish the pronephros from the mesonephros.
Fig. 209. - Diagram showing the Structure of a Fully Developed Pronephric tubule. Ao, Aorta; Coe, ccelom; ec, Ectoderm; eg, external glomerulus; en, endoderm; Ms, mesodermic somite; iV, nervous system; n, nephrostome; nc, notochord; pc, pronephric chamber; Wd, Wolffian duct. - (Modified from Felix.)
The pronephric tubules, are, as has been stated, transitory structures and by the time the embryo has reached a length of about 5 mm. they have all disappeared. Before their disappearance is complete, however, a second series of tubules has commenced to develop, forming what is termed the mesonephros or Wolffian body.
The Development of the Mesonephros
The pronephric duct does not disappear with the degeneration of the pronephric tubules, but persists to serve as the duct for the mesonephros and to play an important part in the development of the metanephros also. In the Wolffian ridge there appear in embryos of between 3 and 4 mm. a number of coiled tubules, which arise by some of the cells of the ridge aggregating to form solid cords, at first entirely unconnected with either the coelomic epithelium or the Wolffian duct. Later the cords become connected with the ccelomic epithelium and acquire a lumen, and near the coelomic end of the tubule, at a region corresponding to the chamber of a pronephric tubule, a condensation of the mesenchyme of the Wolffian ridge occurs to form a glomerulus into which a branch extends from the neighboring aorta. The tubules finally acquire connection with the Wolffian duct and at the same time lose their connections with the coelomic epithelium, their nephrostomes being accordingly but transitory structures. The tubules rapidly increase in length and become coiled, and the glomeruli project into their cavities, pushing in front of them the wall of the tubule so that it has the appearance represented in Fig. 210.
Fig. 210. - Transverse Section of the Wolffian Ridge of a Chick Embryo of Three Days.
ao, Aorta; gl, glomerulus; gr, genital ridge; mes, mesentery; ml, mesonephric tubule; vc, cardinal vein; Wd, Wolffian duct. - (Mihalkovicz.)
In its anterior portion the Wolffian ridge is formed by distinct intermediate cell masses, but posterior to the tenth segment it becomes distinguishable from the rest of the mesoderm before this has become segmented, and, failing to undergo transverse division into segments, it forms a continuous column of cells, known as the nephrogenic cord. The anterior tubules of the mesonephros make their appearance in the intermediate cell masses belonging to the sixth cervical segment, its tubules thus overlapping those of the pronephros, and from this level they appear in all succeeding segments and in the nephrogenic cord as far back as the region of the third or fourth lumbar segment, where the cord is partially interrupted. This interruption marks the dividing line between the mesonephric and metanephric portions of the cord, the portions posterior to it being destined to give rise to the metanephros. But, as is the case with the pronephros, the entire series of mesonephric tubules is never in existence at any one time, a degeneration of the anterior ones supervening even before the posterior ones have differentiated, and the degeneration proceeds to such an extent that in an embryo of about 21 mm. all the tubules of the cervical and thoracic segments have disappeared, only those of the lumbar segments persisting.
This does not mean, however, that the number of persisting tubules corresponds with that of the segments in which they occur, for the tubules are not segmental in their arrangement, but are much more numerous than such an arrangement would allow. Two, three, or even as many as nine may correspond with the extent of a mesodermic somite and when the reduction is complete in an embryo of 21 mm., where only the tubules corresponding with four or five segments remain, they may number twenty-six in each mesonephros (Felix). This arrangement of the tubules together with the size which they assume when fully developed brings it about that the Wolffian ridges become somewhat voluminous structures in their mesonephric portions, projecting markedly into the ccelomic cavity (Fig. 211). Each is attached to the dorsal wall of the body by a distinct mesentery and has in its lateral portion, embedded in its substance, the Wolffian duct, while on its mesial surface anteriorly is the but slightly developed genital ridge (/). This condition is reached in the human embryo at about the sixth or seventh week of development, and after that period the mesonephros again begins to undergo rapid degeneration, so that at about the sixteenth week nothing remains of it except the duct and a few small rudiments whose history will be given later.
Fig. 211. - Urinogenital Apparatus of a Male Pig Embryo of 6 cm.
ao, Aorta; b, bladder; gh, gubernaculum testis; k, kidney; md, Mullerian duct; sr, suprarenal body; t, testis; w, Wolffian body; wd, Wolffian duct. - (Mihalkovicz.)
The Development of the Metanephros
The first indication of the metanephros or permanent kidney is a tubular outgrowth from the dorsal surface of the Wolffian duct shortly before its entrance into the cloaca (Fig. 170). When first formed this outgrowth lies lateral to the posterior portion of the Wolffian ridge, which, as has already been noted (p. 342), is separated from the portion that gives rise to the mesonephros. This terminal portion of the ridge forms what is termed the metanephric blastema and in embryos of 7 mm. it has come into relation with the outgrowth from the Wolffian duct and covers its free extremity as a cap. Since both the blastema and the outgrowth from the Wolffian duct take part in the formation of the uriniferous tubules, these have a double origin.
The outgrowth from the Wolffian duct as it continues to elongate comes to lie dorsal to the mesonephros, carrying the cap of blastema with it, and it soon assumes a somewhat club-shaped form, its terminal enlargement or ampulla forming what may be termed the primary renal pelvis, while the remainder represents theureter. The primary renal pelvis then gives rise to from three to six, usually four, tubular outgrowths, which may be termed primary collecting tubules, and with their formation the original cap of metanephric blastema undergoes a division into as many portions as there are tubules, so that each of the latter has its own cap of blastema. As soon as each tubule has reached a certain length it begins to enlarge at its free extremity to form an ampulla, just as did the primary renal pelvis, and from this ampulla there grow out from two to four secondary collecting tubules, a further corresponding division of the metanephric blastema taking place. In their turn these secondary tubules similarly enlarge at their extremities to form ampullae (Fig. 212, A) from which tertiary collecting tubules are budded out, accompanied by a third fragmentation of the blastema and so the process goes on until about the fifth fetal month, the number of generations of collecting tubules formed being between eleven and thirteen, each tubule of the final generation having its cap of blastema.
Fig. 212. - Diagrams of Early Stages in the Development of the Metanephric Tubules. t, Urinary tubule; Ur, ureter; v, renal ampulla. - (Haycrqft.)
In this way there is formed a complicated branching system of tubules all of which ultimately communicate with the primary renal pelvis, and all of which have, in the last analysis, had their origin from the Wolffian duct. They represent, however, only the collecting portions of the uriniferous tubules, their excreting portions having yet to form, and these take their origin from the metanephric blastema.
Fig. 213. - Four Stages in the Development of a Uriniferous Tubule of a Cat. A, Arched collecting tubule, C, distal convoluted tubule; C, proximal convoluted tubule; H, loop of Henle; M, glomerulus; T, renal vesicle; V, ampulla (drawn from reconstructions prepared by G. C. Huber).
When the terminal collecting tubules have been formed the blastemic cap in connection with each one condenses to form a renal vesicle (Fig. 213, A, T), which is at first solid, but later becomes hollow and proceeds to elongate to an S-shaped tubule, one end of which becomes continuous with the neighboring ampulla (Figs. 212, B, and 213, B), and in the space enclosed by_what may be termed the lower loop of the S a collection of mesenchyme cells appears, into which branches penetrate at an early stage from the renal artery to form a glomerulus, the neighboring walls of the tubule becoming exceedingly thin and being transformed into a capsule of Bowman. The upper loop of the S now begins to elongate (Fig. 213, C), growing toward the hilus of the kidney, parallel to the branch of the outgrowth from the Wolffian duct to which it is attached and between this and the glomerulus, and forms a loop of Henle. From the portion of the horizontal limb of the S which lies between the glomerulus and the descending limb of the loop of Henle the proximal convoluted tubule (C) arises, while the distal convoluted and the arched collecting tubules (C and A) are formed from the uppermost portion of the upper loop (Fig. 213, D). The entire length of each uriniferous tubule from Bowman's capsule to the arched collecting tubule inclusive is thus derived from a renal vesicle, that is to say, from the metanephric blastema.
Since the tubules of the kidney are formed by the union of two originally distinct structures it is conceivable that in the cases of certain tubules there may be a failure of the union. The blastemic portion of the tubules would, nevertheless, continue their development and become functional and, since there would be not means of escape for the secretion, the result would be a cystic kidney. Occasionally the two blastemata of opposite sides fuse across the middle line, the result being the formation of a single transverse or horse-shoe shaped kidney, or, what is much rarer, the blastema of one side may cross the middle line to fuse with that of the other, the result being an apparently single kidney with two ureters which open normally into the bladder.
The primary renal pelvis is the first formed ampulla and does not exactly represent the definitive pelvis. This is produced partly by the enlargement of the primary pelvis and partly by the enlargement of the collecting tubules of the first four generations, those of the third and fourth generations later being taken up or absorbed into those of the second generation, so that the tubules of the fifth generation appear to open directly into those of the second, which form the calices minores, while those of the first constitute the calices majores. In some kidneys the process of reduction of the earlier formed collecting tubules proceeds a step further, those of the first generation being taken up into the primary renal pelvis, the secondaries then forming a series of short calices arising from a single pelvic cavity.
At about the tenth week of development the surface of the human kidney becomes marked by shallow depressions into lobes, of which there are about eighteen, one corresponding to each of the groups of tubules which arise from the same renal vesicle. This lobation persists until after birth and then disappears completely, the surface of the kidney becoming smooth.
The Development of the Mullerian Duct and of the Genital Ridge
At the time when the Wolffian body has almost reached its greatest development the Wolffian ridge is distinctly divided into three portions (Fig. 214), a median or mesonephric portion attached to the body wall, a lateral or tubal portion containing the Wolffian duct and attached to the mesonephric portion, and a genital portion, formed by the genital ridge and also attached to the mesonephric portion, but to its medial surface. In the tubal portion a second longitudinal duct, known as the Mullerian duct (Fig. 214, Md), makes its appearance. Near the anterior end of each Wolffian ridge there is formed on the free edge of the tubal portion an invagination of the peritoneal covering, and by the proliferation of the cells at its tip this invagination gradually extends backward in the substance of the tubal portion and reaches the cloaca in embryos of about 22 mm. The primary peritoneal invagination becomes the abdominal ostium of the Mullerian duct, the backward prolongation forming the duct itself.
In Fig. 214 it will be seen that the tubal portion of the left Wolffian ridge is somewhat bent inward toward the median line and in the lower parts of their extent this becomes more pronounced in both tubal portions until finally their free edges come in contact and fuse in the median line, while at the same time their lower edges fuse with the floor of the ccelomic cavity. In this way a transverse partition is formed across what will eventually be the pelvis of the adult, this cavity being thus divided into two compartments, a posterior one containing the lower portion of the intestine and an anterior one containing the bladder. With the formation of this transverse fold, which is represented by the broad ligament in the female, the Miillerian ducts of opposite sides are brought into contact and finally fuse in the lower portions of their course to form an unpaired utero-vaginal canal.
Fig. 214. - Transverse Section through the Abdominal Region of an Embryo of 25 MM. _ Ao, Aorta; B, bladder; I, intestine; L, liver; M, muscle; Md, Miillerian duct; N, spinal cord; Ov, ovary; RA, rectus abdominis; Sg, spinal ganglion; UA, umbilical artery; Ur, ureter; V, vertebra; W, Wolffian body; Wd, Wolffian duct. - (Keibel.)
Upon the lateral surface of the mesonephric portion of the Wolffian ridge a longitudinal elevation is formed at about this time. It is the inguinal fold and on the union of the transverse fold with the floor of the ccelomic cavity it comes into contact and fuses with the lower part of the anterior abdominal wall, just lateral to the lateral border of the rectus abdominis muscle. In the substance of the fold the mesenchyme condenses to form a ligament-like cord, the inguinal ligament, whose further history will be considered later on.
The genital ridge makes its apearance as a band-like thickening of the epithelium covering the mesial surface of the Wolffian ridge (Fig. 207, gr). Later columns of cells grow down from the thickening into the substance of the Wolffian ridge, displacing the mesonephric tubules to a greater or less extent. These columns are composed of two kinds of cells: (i) smaller epithelial cells with a relatively small amount of cytoplasm and (2) large, spherical cells with more abundant and clear cytoplasm known as sex-cells. The growth of the cell-columns down into the substance of the Wolffian body does not take place, however, to an equal extent in all portions of the length of the genital ridge. Indeed, three regions may be recognized in the ridge; an anterior one in which a relatively small number of cell-columns, extending deeply into the stroma, is formed; a middle one in which numerous columns are formed; and a posterior one in which practically none are formed. The first region has been termed the rete region and its cell-columns the retecords, the second region the sex-gland region and its columns the sex-cords, and the posterior region is the mesenteric region and plays no part in the actual formation of the ovary or testis.
In the human embryo all the sex-cells seem to have their origin from the epithelium of the genital ridge, but in the lower vertebrates and also in mammals (Allen, Rubaschkin) they have been found to make their appearance in the endoderm of the digestive tract. Thence they wander into the mesentery and some of them eventually into the peritoneum covering the mesial surface of the Wolffian ridge, where they give rise to the sex-cells found in the epithelium of the genital ridge. This origin of the sex-cells has not yet been observed in the human embryo.
The various steps in the differentiation of the reproductive organs so far described occur in all embryos, no matter what their future sex may be. The later stages, however, differ according to sex, and consequently it will be necessary to follow the further development first of the testis and then of the ovary, the changes that take place in the ducts and other accessory structures being reserved for a special section.
Fig. 215. - Section through the Testis and the Broad Ligament of the Testis of an Embryo of 5.5 mm.
ep, Epithelium; md, Miillerian duct; mo, mesorchium; re, rete-cords; sc, sex-cords; wd, Wolffian duct. - (Mihalkovicz.)
The Development of the Testis
At about the fourth or fifth week there appears in the sex-gland region of the genital ridge a structure which serves to characterize the region as a testis. This is a layer of somewhat dense connective tissue which grows in between the epithelial and stroma layers of the sex-gland region and gradually extends around almost the entire sex-gland to form the tunica albuginea. By its development the sex-cords are separated from the epithelium, which later becomes much flattened and eventually almost disappears. Shortly after the appearance of the albuginea the sex-cords unite to from a complicated network and the rete-cords grow backward along the line of attachment of the testis to the mesonephric portion of the Wolffian ridge, coming to lie in the hilus of the testis (Fig. 215). They then develop a lumen and send off branches which connect with the sex-cord reticulum and they also make connection with the glomerular portions of the tubules belonging to the anterior part of the mesonephros. Since like the sexcords, they have by this time separated from the epithelium that gave rise to them, they now extend between the sex-cord reticulum and the anterior mesonephric tubules. Certain portions of the sex-cords now begin to break down leaving other portions to form convoluted stems which eventually become the seminiferous tubules, while from the rete-cords are formed the tubuli recti and rete testis, by which the spermatozoa are transmitted to the mesonephric tubules and so to the Wolffian duct (see p. 355).
Fig. 216. - Longitudinal Section of the Ovary of an Embryo Cat of 9.4 cm. cor, Cortical layer; ep, epoophoron; Mc, medullary cords; Mn, mesonephros; pf, peritoneal fold containing Fallopian tube; R, rete; T, Fallopian tube. - (Coert, from Bilhler.)
The development of the seminiferous tubules is not, however, completed until puberty. The stems derived from the sex-cords form cylindrical cords, between which lie stroma cells and interstitial cells derived from the stroma; but until puberty these cords remain solid, a lumen developing only at that period. The cords contain the same forms of cells as were described as occurring in the epithelium of the germinal ridge, and while in the early stages transitional forms seem to occur, in later periods the two varieties of cells are quite distinct, the sex-cells becoming spermatogonia (see p. 14) and being the mother cells of the spermatozoa, while the remaining epithelial cells perhaps become transformed into the connective-tissue walls of the tubules.
The Development of the Ovary
In the case of the ovary, after the formation of the sex-cords, connective tissue grows in between these and the epithelium, forming a layer equivalent to the tunica albuginea of the testis. It is, however, a much looser tissue than its homologue in the male, and, indeed, does not completely isolate the sex-cords from the epithelium, although the majority of the cords are separated and sink into the deeper portions of the ovary where they form what have been termed the medullary cords. In the meantime the germinal epithelium has continued to bud off cords which unite to form a cortical layer of cells lying below the epithelium and separated from the medullary cords by the tunica albuginea (Fig. 216).
Later the cortical layer becomes broken up by the ingrowth of stroma tissue into spherical or cord-like masses, consisting of sexcells and epithelial cells (Fig. 217). The invasion of the stroma continuing, these spheres or cords (Pfluger's cords) become divided nto smaller masses, the primary ovarian follicles, each of which consists as a rule of a single sex-cell surrounded by a number of epithelial cells, the whole being enclosed by a- zone of condensed stroma tissue, which eventually becomes richly vascularized and forms a theca folliculi (Fig. 10). The epithelial cells in each follicle are at first comparatively few in number and closely surround the sex-cell (Fig. 217,/), which is destined to become an ovum, but in certain of the follicles they undergo an increase by mitosis, becoming extremely numerous, and later secrete a fluid, the liquor folliculi, which collects at one side of the follicle and eventually forms a considerable portion of its contents. The follicular cells are differentiated by its appearance into the stratum granulosum, which surrounds the wall of the follicle, and the discus froligerus, in which the ovum is embedded (Fig. 10, dp), and the cells which immediately surround the ovum, becoming cylindrical in shape, give rise to the corona radiata (Fig. 11, cr).
A somewhat similar fate is shared by the medullary cords, these also breaking up into a number of follicles, but sooner or later these follicles undergo degeneration so that shortly after birth practically no traces of the cords remain. It must be noted that degeneration of the follicles formed from the cortical layer also takes place even during fetal life and continues to occur throughout the entire periods of growth and functional activity, numerous atretic follicles being found in the ovary at all times. Indeed it would seem that degeneration is the fate of the great majority of the follicles and sex-cells of the ovary, but few ova coming to maturity during the life-time of any individual. 23
Fig. 217. - Section of the Ovary of a New-born Child.
a, Ovarial epithelium; b, proximal part of a Pfl tiger's cord; c, sex-cell in epithelium; d and e, spherical masses; /, primary follicle; g, blood-vessel. - (From Gegenbaur, after Waldeyer.)
Rete-cords developed from the rete portion of the germinal ridge occur in connection with the ovary as well as with the. testis and form a rete ovarii (Fig. 216, R). They do not, however, extend so deeply into the ovary, remaining in the neighborhood of the mesovarium, and they do not become tubular, but resemble closely the medullary cords with which they are serially homologous. They separate from the epithelium and make connections with the glomeruli of the anterior portion of the mesonephros, on the one hand, and on the other with medullary cords, and in later stages show a tendency to break up into primary follicles, which early degenerate and disappear like those of the medullary cords.
The Transformation of the Mesonephros and the Ducts
At one period of development there are present, as representatives of the urinogenital apparatus, the Wolffian body (mesonephros) and duct, the Miillerian duct, and the developing ovary or testis. Such a condition forms an indifferent stage from which the development proceeds in one of two directions according as the genital ridge becomes a testis or an ovary, the Wolffian body in part undergoing degeneration and in part persisting to form organs which for the most part are rudimentary, while in the female the Wolffian duct also degenerates except for certain rudiments and in the male the Miillerian duct behaves similarly.
In the Male
It has been seen that the Wolffian body, through the rete cords, enters into very intimate relations with the testis, and it may be regarded as divided into two portions, an upper genital and a lower excretory. In the male the genital portion persists in its entirety, serving as the efferent ducts of the testis, which, beginning in the spaces of the rete testis, already shown to be connected with the capsules of Bowman, open into the upper part of the Wolffian duct and form the globus major of the epididymis. The excretory portion undergoes extensive degeneration, a portion of it persisting as a mass of coiled tubules ending blindly at both ends, situated near the head of the epididymis and known as the paradidymis or organ of Giraldes, while a single elongated tubule, arising from the portion of the Wolffian duct which forms the globus minor of the epididymis, represents another portion of it and is known as the vas aberrans.
The Wolffian duct is retained complete, the portion of it nearest the testis becoming greatly elongated and thrown into numerous coils, forming the body and globus minor of the epididymis, while the remainder of it is converted into the vas deferens and the ductus ejaculatorius. A lateral outpouching of the wall of the duct to form a longitudinal fold appears at about the third month and gives rise to the vesicula seminalis, the lateral position of the outgrowth explaining the adult position of the vesiculse lateral to the vasa deferentia.
With the Mullerian ducts the case is very different, since they disappear completely throughout the greater part of their course, only their upper and lower ends persisting, the former giving rise to a small sac-like body, the sessile hydatid of Morgagni, attached to the upper end of each testis near the epididymis. It has been seen (p. 349) that the lower ends of the Mullerian ducts, in the male as well as the female, fuse to form the utero-vaginal canal, and the lower portion of this also persists to form what is termed the uterus masculinus, although it corresponds to the vagina of the female rather than to the uterus. It is a short cylindrical pouch of varying length, that opens into the urethra at the bottom of a depression known as the utriculus prostaticus (sinus pocularis).
The transverse pelvic partition, produced by the union of the two tubal portions of the Wolffian body, is formed in the male embryo, but at an early stage its anterior surface fuses with the posterior surface of the bladder and consequently there is in the male no pelvic compartment equivalent to the vesico-uterine pouch of the female. The male recto-vesical pouch is, however, the homologue of the rectouterine pouch of the female.
The formation of the inguinal ligament on the surface of the mesonephros has been described on p. 349. On the degeneration of the mesonephros the layer of peritoneum that covered it persists to form a mesorchium extending from the body wall to the hilus of the testis and the inguinal ligament now comes to have its origin from the lower pole of that organ, whence it extends to the anterior abdominal wall. Owing to the rudimentary nature of the uterus masculinus and the slight development of its walls the inguinal ligament does not become involved with it, but remains independent and forms the gubemaculum testis of the adult, whose adult position is brought about by the descent of the testis into the scrotum (see p. 366).
In the Female
In the female the transverse partition of the pelvis does not fuse w'th the bladder but remains distinct as the broad ligament. Consequently there is in the female both a vesicouterine and a recto-uterine pouch. Since the genital ridges form upon the mesial surfaces of the Wolffian ridges and the tubal portions are their lateral portions, when these latter unite to form the broad ligament the ovary will come to lie upon the posterior surface of that structure, projecting into the recto-vesical pouch. On the degeneration of the mesonephros the peritoneum that covered it becomes a part of the broad ligament, forming that part of it which contains the Fallopian tubes and hence is known as the mesosalpinx, while the lower part of the ligament, on account of its relation to the uterus,. is termed the mesometrium.
The genital portion of the mesonephros, though never functional as ducts in the female, persists as a group of ten to fifteen tubules, situated between the two layers of the broad ligament and in close proximity to the ovary; these constitute what is known as the epoophoron (parovarium or organ of Rosenmuller) . The tubules terminate blindly at the ends nearest the ovary, but at the other extremity, where they are somewhat coiled, they open into a collecting duct which represents the upper end of the Wolffian duct. Near this rudimentary body is another, also composed of tubules, representing the remains of the excretory portion of the mesonephros and termed the paroophoron which, however, degenerates during the early years of extra-uterine life. So far as the mesonephros is concerned, therefore, the persisting rudiments in the female are comparable to those occurring in the male.
As regards the ducts, however, the case is different, for in the female it is the Mtillerian ducts which persist, while the Wolmans undergo degeneration, a small portion of their upper ends persisting in connection with the epoophora, while their lower ends persist as straight tubules lying at the sides of the vagina and forming what are known as the canals of Gartner. The Mtillerian ducts, on the other hand, become converted into the Fallopian tubes (tubas uterince), and in their lower portions into the uterus and vagina. From the margins of the openings by which the Mullerian ducts communicate with the ccelom projections develop at an early period and give rise to the fimbria, with the exception of the one connected with the ovary, the fimbria ovarica, which is the persisting upper portion of the original genital ridge. From the utero-vaginal canal the two structures which give it its name are formed, the entire canal being transformed into the mucous membrane of the uterus and vagina. Indeed, the lower ends of the Fallopian tubes are also taken up into the uterus, for the condensation of mesenchyme that takes place around the mucosa to form the muscular wall of the uterus is so voluminous that it includes not only the utero-vaginal canal but also the adjacent portions of the Mullerian ducts. The histological differentiation of the uterus from the vagina begins to manifest itself at about the third month, and during the fourth month the vaginal portion of the duct becomes flattened and the epithelium lining its lumen fuses so as to completely occlude it and, a little later, there appears at its lower opening a distinct semicircular fold. This is the hymen, a structure which seems to be represented in the male by the colliculus seminalis. The obliteration of the lumen of the vagina persists until about the sixth month, when the cavity is re-established by the breaking down of the central epithelial cells.
The extent of the mesenchymal condensation to form the muscularis uteri also produces a modification of the relations of the inguinal ligament in the female. For the ligament becomes for a short portion of its length included in the condensation and thus attached to the upper portion of the uterus. It is consequently divided into two portions, one extending from the lower pole of the ovary to the uterus and forming the ligamentum ovarii proprium and the other extending from the uterus to the anterior abdominal wall and forming what is known in the adult as the round ligament of the uterus.
The diagram, Fig. 218, illustrates the transformation from the indifferent condition which occurs in the two sexes, and that the homologies of the various parts may be clearly understood they may also be stated in tabular form as on the next page.
Fig. 218. - Diagrams Illustrating the Transformation of the Mullerian and Wolffian Ducts. B, Bladder; C, clitoris; CG, canal of Gaertner; CI, cloaca; Eo, epoophoron; Ep, epididymis; F, Fallopian tube; G, genital gland; HE, hydatid of epididymis; HM, hydatid of Morgagni; K, kidney; MD, Mullerian duct; O, ovary; P, penis; Po, paroophron; Pr, prostate gland; R, rectum; T, testis; U, urethra; UM, uterus masculinus; Ur, ureter; US, urogenital sinus; Ut, uterus; V, vagina; Va, vas aberrans; VD, vas deferens; VS, vesicula seminalis; WB, Wolffian body; WD, Wolffian duct. - (Modified from Huxley.)
Genital ridge J
Fimbria ovarica. Ovary.
Ovarian ligament. Round ligament.
Globus major of epididymis.
Body and globus minor of epididymis. Vasa deferentia. Seminal vesicles.' Ejaculatory ducts.
Collecting tubules of epoophoron.
Canal of Gartner.
Sessile hyatid. Uterus masculinus.
In addition to the sessile hydatid, a stalked hydatid also occurs in connection with the testis, and a similar structure is attached to the fimbriated opening of each Fallopian tube. The significance of these structures is uncertain, though it has been suggested that they are persisting rudiments of the pronephros.
A failure of the development of the various parts just described to be completed in the normal manner leads to various abnormalities in connection with the reproductive organs. Thus there may occur a failure in the fusion of the lower portions of the Miillerian ducts, a bihorned or bipartite uterus resulting, or the two ducts may come into contact and their adjacent walls fail to disappear, the result being a median partition separating the vagina or both the vagina and uterus into two compartments. The excessive development of the fold which gives rise to the hymen may lead to a complete closure of the lower opening of the vagina, while, on the other hand, a failure of the Miillerian ducts to fuse may produce a biperforate hymen.
The Development of the Urinary Bladder and the Urogenital Sinus
So far the relations of the lower ends of the urinogenital ducts have not been considered in detail, although it has been seen that in the early stages of development the Wolffian and Miillerian ducts open into the sides of the ventral portion of the cloaca; that the ureters communicate with the lower portions of the Wolffian ducts; that from the ventral anterior portion of the cloaca the allantoic duct extends outward into the belly-stalk; and, finally (p. 281), that the cloaca becomes divided into a dorsal portion, which forms the lower part of the rectum, and a ventral portion, which is continuous with the allantois and receives the urinogenital ducts (Fig. 219). It is the history of this ventral portion of the cloaca which is now to be considered.
Fig. 219. - Reconstruction of the Cloacal Region op an Embryo of 14 mm.
al, Allantois; b, bladder; gt, genital tubercle; i, intestine; n, spinal cord; nc, notochord ; r, rectum; sg, urogenital sinus; ur, ureter; w, Wolffian duct. - (Keibel.)
It may be regarded as consisting of two portions, an anterior and a posterior, the line of insertion of the urinogenital ducts marking the junction of the two. The anterior or upper portion is destined to give rise to the urinary bladder (Fig. 219, b), while the lower one forms what is known for a time as the urogenital ^inus (sg). The bladder, when first differentiated, is a tubular structure, whose lumen is continuous with that of the allantois, but after the second month it enlarges to become more sac-like, while the intra-embryonic portion of the allantois degenerates to a solid cord extending from the apex of the bladder to the umbilicus and is known as the urachus. During the enlargement of the bladder the terminal portions of the urinogenital ducts are taken up into its walls, a process which continues until finally the ureters and Wolffian ducts open into it separately, the ureters opening to the sides of and a little anterior to the ducts. This condition is reached in embryos of about 14 mm.
Fig. 220. - Reconstruction of the Cloacal Structures of an Embryo of 25 mm. bl, Bladder; m, Mullerian duct; r, rectum; sg, urogenital sinus; sy, symphysis pubis; u, ureter; ur, urethra; w. Wolffian duct. - (Adapted from Keibel.)
(Fig. 219), and in later stages the interval between the two pairs of ducts is increased (Fig. 220), resulting in the formation of a short canal connecting the lower end of the bladder which receives the ureters with the upper end of the urogenital sinus, into which the Wolffian and Mullerian ducts open. This connecting canal represents the urethra (Fig. 220, ur), or rather the entire urethra of the female and the proximal part of that of the male, since a considerable portion of the latter canal is still undeveloped (see p. 364). From this urethra there is developed, at about the third month, a series of solid longitudinal folds which project upon the outer surface and separate from the urethra from above downward. These represent the tubules of the prostate gland and are developed in both sexes, although they remain in a somewhat rudimentary condition in the female. The muscular tissue, so characteristic of the gland in the adult male, is developed from the surrounding mesenchyme at a later stage.
The bladder is, accordingly, essentially a derivative of the cloaca and its mucous membrane is therefore largely of endodermal origin. Portions of the Wolffian ducts which are of mesodermal origin are, however, taken up into the wall of the bladder and form a portion of it. The extent of the portion so formed is indicated by the position of the orifices of the ureters above and of the ejaculatory ducts below, and it corresponds therefore with what is termed the trigonum vesica together with the floor of the urethra as far as the openings of the ejaculatory ducts. Throughout this region the mucous membrane is of mesodermal origin.
The urogenital sinus is in the early stages also tubular in its upper part, though it expands considerably below, where it is closed by the cloacal membrane. This, by the separation of the cloaca into rectum and sinus, has become divided into two portions, the more ventral of which closes the sinus and the dorsal the rectum, the interval between them having become considerably thickened to form the perineal body. In embryos of about 17 mm. the urogenital portion of the membrane has broken through, and in later stages the tubular portion of the sinus is gradually taken up into the more expanded lower portion, until finally the entire sinus forms a shallow depression, termed the vestibule, into the upper part of which the urethra opens, while below are the openings of the Wolffian (ejaculatory) ducts in the male or the orifice of the vagina in the female. From the sides of the lower part of the sinus a pair of evaginations arise toward the end of the fourth month and give rise to the bulbo-vestibular glands (Bartholin's) of the female or the corresponding bulbo-urethral glands (Cowper's) in the male.
The Development of the External Genitalia
At about the fifth week, before the urogenital sinus has opened to the exterior, the mesenchyme on its ventral wall begins to thicken, producing a slight projection to the exterior. This eminence, which is known as the genital tubercle (Fig. 219, gt), rapidly increases in size, its extremity becomes somewhat bulbously enlarged (Fig. 221, gl) and a groove, extending to the base of the terminal enlargement, appears upon its vestibular surface, the lips of the groove forming two wellmarked genital folds (Fig. 221, gf). At about the tenth week there appears on either side of the tubercle an enlargement termed the genital swelling (Fig. 221, gs), which is due to a thickening of the mesenchyme of the lower part of the ventral^abdominal wall in the
Fig. 221. - The External Genitalia of an Embryo of 25 mm. a, Anus; gf, genital fold; gl, glans; gs, genital swelling; p, perineal body. - (Keibel.) region where the inguinal ligament is attached, and with the appearance of these structures the indifferent stage of the external genitals is completed.
In the female the growth of the genital tubercle proceeds rather slowly and it becomes transformed into the clitoris, the genital folds becoming prolonged to form the labia minora. The genital swellings increase in size, their mesenchyme becomes transformed into a mass of adipose and fibrous tissue and they become converted into the labia majora, the interval between them constituting the vulva.
In the male the early stages of development are closely similar to those of the female; indeed, it has been well said that the external genitals of the adult female resemble those of the fetal male. In early stages the genital tubercle elongates to form the penis and the integument which covers the proximal part of it grows forward as a fold which encloses the bulbous enlargement or glans and forms the prepuce, whose epithelium fuses with that covering the glans and only separates from it later by a cornification of the cells along the plane of fusion. The genital folds meet together and fuse, converting the vestibule and the groove upon the vestibular surface of the penis into the terminal portion of the male urethra and bringing it about that the vasa deferentia and the uterus masculinus open upon the floor of that passage. The two genital swellings are at the same time brought closer together, so as to lie between the base of the penis and the perineal body and, eventually, they form the scrotum. The mesenchyme of which they were primarily composed differentiates into the same layers as are found in the wall of the abdomen and a peritoneal pouch is prolonged into them from the abdomen, so that they form sacs into which the testes descend toward the close of fetal life (p. 366).
The homologies of the portions of the reproductive apparatus derived from the cloaca and of the external genitalia in the two sexes may be perceived from the following table.
Proximal portion of urethra.
The rest of the urethra.
Prepuce and integument of penis.
It is stated above that the layers which compose the walls of the scrotum are identical with those of the abdominal wall. This may be seen in detail from the following scheme:
Abdominal Walls. Scrotum.
Superficial fascia. Dartos.
External oblique muscle. Intercolumnar fascia.
Internal oblique muscle. Cremasteric fascia.
Transverse muscle. Infundibuliform fascia.
Peritoneum. Tunica vaginalis.
Numerous anomalies, depending upon an inhibition or excess of the development of the parts, may occur in connection with the external genitalia. Should, for instance, the lips of the groove on the vestibular surface of the penis fail to fuse, the penial portion of the urethra remains incomplete, constituting a condition known as hypospadias, a condition whic,h offers a serious bar to the fulfilment of the sexual act. If the hypospadias is complete and there be at the same time an imperfect development of the penis, as frequently occurs in such cases, the male genitalia closely resemble those of the female and a condition is produced which is usually known as hermaphroditism. It is noteworthy that in such cases there is frequently a somewhat excessive development of the uterus masculinus, and a similar condition may be produced in the female by an excessive development of the clitoris. Such cases, however, which concern only the accessory organs of reproduction, are instances of what is more properly termed spurious hermaphroditism, true hermaphroditism being a term which should be reserved for possible cases in which the genital ridges give rise in the same individual to both ova and spermatozoa. Such cases are of exceeding rarity in the human species, although occasionally observed in the lower vertebrates, and the great majority of the examples of hermaphroditism hitherto observed are cases of the spurious variety.
The Descent of the Ovaries and Testes
The positions finally occupied by the ovaries and testes are very different from those which they possess in the earlier stages of development, and this is especially true in the case of the testes. The change of position is partly due to the rate of growth of the inguinal ligaments being less than that of the abdominal walls, the reproductive organs being thereby drawn downward toward the inguinal regions where the ligaments are attached. The point of attachment is beneath the bottom of a slight pouch of peritoneum which projects a short distance into the substance of the genital swellings and is known as the canal of Nuck in the female, and in the male as the vaginal process.
In the female a second factor combines with that just mentioned.
The relative shortening of the inguinal ligaments acting alone would draw the ovaries toward the inguinal regions, but since they are united to the uterus by the ovarian ligaments movement in that direction is prevented and the ovaries come to lie in the recto-uterine compartment of the pelvic cavity.
With the testes the case is more complicated, since in addition to the relative shortening of the inguinal ligaments there is an elongation of the vaginal processes into the substance of the genital swellings, and it must be remembered that the testes, like the ovaries, are primarily connected with the peritoneum. Three stages may be recognized in the descent of the testes. The first of these depends on the slow rate of elongation of the inguinal ligaments or gubernacula. It lasts until about the fifth month of development, when the testes lie in the inguinal region of the abdomen, but during this month the elongation of the gubernaculum becomes more rapid and brings about the second stage, during which there is a slight ascent of the testes, so that they come to lie a little higher in the abdomen. This stage is, however, of short duration, and is succeeded by the stage of the final descent, which is characterized by the elongation of the vaginal processes of the peritoneum into the substance of the scrotum (Fig. 222, A). Since the gubernaculum is attached to the abdominal wall beneath this process, and since its growth has again diminished, the testes gradually assume again their inguinal position, and are finally drawn down into the scrotum with the vaginal processes.
Fig. 222. - Diagrams Illustrating the Descent of the Testis. il, Inguinal ligament; m, muscular layer; s, skin and dartos of the scrotum; t, testis; tv, tunica vaginalis ; vd, vas deferens ; vp, vaginal process of peritoneum. - (After Hertwig.)
The condition which is thus acquired persists for some time after birth, the testicles being readily pushed upward into the abdominal cavity along the cavity by which they descended. Later, however, the size of the openings of the vaginal processes into the general peritoneal cavity becomes greatly reduced, so that each process becomes converted into an upper narrow neck and a lower sac-like cavity (Fig. 222, B), and, still later, the walls of the neck portion fuse and become converted into a solid cord, while the lower portion, wrapping itself around the testis, becomes the tunica vaginalis (tv). By these changes the testes become permanently located in the scrotum. During the descent of the testes the remains of each Wolffian body, the epididymis, and the upper part of each vas deferens together with the spermatic vessels and nerves, are drawn down into the scrotum, and the mesenterial fold in which they were originally contained also practically disappears, becoming converted into a sheath of connective tissue which encloses the vas deferens and the vessels and nerves, binding them together into what is termed the spermatic cord. The mesorchium, which united the testis to the peritoneum enclosing the Wolffian body, does not share in the degeneration of the latter, but persists as a fold extending between the epididymis and the testis and forming the sinus epididymis.
In the text-books of anatomy the spermatic cord is usually described as lying in an inguinal canal which traverses the abdominal walls obliquely immediately above Poupart's ligament. So long as the lumen of the neck portion of the vaginal process of peritoneum remains patent there is such a canal, placing the cavity of the tunica vaginalis in communication with the general peritoneal cavity, but the cord does not traverse this canal, but lies outside it in the retroperitoneal connective tissue. When, however, the neck of the vaginal process disappears, a canal no longer exists, although the connective tissue which surrounds the spermatic cord and unites it with the tissues of the abdominal walls is less dense than the neighboring tissues, so that the cord may readily be separated from these and thus appear to He in a canal.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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