Book - Human Embryology and Morphology 9
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Chapter IX. The Uro-Genital System
The Wolffian Body or Meso-nephros
In lower vertebrates (Fishes and Amphibians) the Wolffian body is the functional kidney ; in higher vertebrates (Reptile, Birds, and Mammals) it is merely a temporary or embryonic structure, the renal function being taken over by the permanent kidney. Nothing is known of how or when the permanent kidney arose and supplanted the Wolffian Body in the evolution of the vertebrates. Its presence in the human embryo and in the embryonic stages of the three great classes of higher vertebrates, with the presence of many curious stages in the development of their genito-urinary system, can be explained only by the fact that these higher forms are descended from ancestors of the lower.
In Fig. 79 the Wolffian body, such as occurs in the frog, is represented diagrammatically and it corresponds in structure to the Wolffian body which appears in the human embryo. Each body is made up of a main duct and a series of tubules. In the frog, as in the human embryo, the hind gut ends in a dilatation, the cloaca. In the cloaca open the rectum, allantois, and the two Wolffian ducts — right and left. In the frog, the Wolffian bodies lie on each side of the spine, their anterior ends reaching forwards to the region of the heart. Each duct is joined by numerous convoluted tubules — the Wolffian tubules. Each tubule is furnished with a glomerulus at its blind extremity and in most features agrees with a secretory tubule — such as are seen in the permanent kidney. These tubules secrete the urine ; the Wolffian duct conveys the urine from the tubules to the cloaca. The anterior tubules, however, loose their secretory function and become associated with the genital gland. In the male frog they convey the spermatozoa to the Wolffian duct, which thus carries both urine, and spermatozoa. In the female, the genital Wolffian tubules are connected with the ovary but are quite functionless (Fig. 79).
The Wolffian Body in the Human Embryo
At the beginning of the second month of foetal life, the Wolffian body is well developed : by the end of that month it has become vestigial, the only parts remaining being those connected with the genital organs. It projects as a ridge from the lumbar and dorsal regions on each side of the mesentery, extending, on each side of the spine, from the posterior cervical region, where the diaphragm is developed, to the pelvis behind, where the ridges become approximated (Fig. 80). To its inner side, in the lower dorsal region, lies the genital ridge. The genital and the Wolffian bodies have each its own mesentery but these two mesenteries have a common attachment — the common uro-genital mesentery (Fig. 80). On section the Wolffian ridge is seen to be made up of convoluted tubules terminating at their blind extremities in glomeruli. The tubules open into the Wolffian duct just as in the frog; the duct is situated in the basal or attached part of the ridge. It runs backwards in this ridge and turns into the pelvis to end with the Müllerian duct (also situated in the Wolffian ridge) in the cloaca of the hind gut. The whole arrangement is similar to that seen in the frog. Further, as in the frog (Fig. 79), the anterior or genital tubules are connected with the genital glands, and are not, as the posterior are, secretory in nature. If the testis were functional at this time — which it is not — the spermatozoa and urine of the Wolffian body would pass to the cloaca by the Wolffian duct.
Origin of the Wolffian Duct and Tubules
The tubules which compose the Wolffian body are developed in the intermediate cell mass (Fig. 85). At first they are minute transverse vesicles formed by mesoblastic cells in the intermediate mass ; these vesicles become tubular ; one end opens into the Wolffian duct ; at the other a glomerulus is developed.
The origin of the duct has been traced by Kollmann from the epiblast. It arises on the lateral surface of the body by an invagination of the epiblast and comes to lie subsequently in the intermediate cell mass (Fig. 85). Thus it will be seen that the lining of the Wolffian duct and of the structures formed from it, are epiblastic in nature and liable to all the diseases to which epiblastic structures are subject.
The Pro-nephros (Fig. 79).— Even before the meso-nephros (Wolffian body) there appears to have been another kidney — the pro-nephros. While only permanently functional in some of the lowest fishes and even in them it is partly replaced by the mesonephros, it still appears transiently in the embryos of vertebrates and is said to occur also in the human embryo. It is developed in the cervical region at the anterior end of the Wolffian ridge. Like the Wolffian body it is composed of a longitudinal duct and tubules ; the duct appears to be an anterior prolongation of the Wolffian duct but its tubules are different. They open into the coelom (peritoneal cavity) by trumpet-shaped ciliated ends, and are derived from invaginations of the mesothelial lining of the coelom. They are coiled and terminate in the pro-nephric duct, an anterior continuation of the Wolffian. A glomerulus is developed in the course of each pro-nephric tubule (Fig. 79). The pro-nephric tubules are probably representatives of the segmental (nephridial) tubules of the Vermes.
The Fate of the Wolffian Body (meso-nephros) and Pronephros
In the Female
In Fig. 81 are shown the various remnants of the embryonic renal formations which may persist in the adult female. The Müllerian duct, the upper part of which becomes the Fallopian tube, is situated in the Wolffian ridge (Fig. 80). Hence when the ovary and tube migrate to the pelvis, the Wolffian mesentery, which comes to form the meso-salpinx, is also drawn within the pelvis and with it all the Wolffian remnants in the female. A hydatid attached to the meso-salpinx (part of the broad ligament) at the fimbriated extremity of the Fallopian tube (Fig. 81) is situated at the anterior end of the Wolffian duct and represents the most anterior (cephalic) of the Wolffian tubules or perhaps the cephalic end of the Wolffian duct, or even the pro-nephros, although it is improbable that this transient embryonic structure should persist (J. H. Watson). It certainly corresponds to the pro-nephric remnant found in the frog. It may become enlarged or cystic but never to a great extent. The Wolffian duct (Fig. 81) runs towards the body of the uterus in the meso-salpinx ; it reaches the side of the uterus and passing down in the superficial tissue of the cervix and vagina, terminates in the vulval cleft at the outer side of the opening of the vagina near the duct of Bartholin. Only the upper part of the duct (meso-salpingeal part) persists in women. The uterine and vaginal segments disappear. Parts of these may remain ; they constantly do so in the sow. The uterine and vaginal segments, if they persist, get the name of duct of Gartner. The genital tubules, those attached to or connected with the ovary, persist and form the epoophoron, Organ of Eosenmuller, or parovarium (Fig. 81). They frequently become cystic and give rise to large tumours. The renal Wolffian tubules — those which acted as renal structures in the embryo, also persist, sometimes unconnected with the duct. They lie between the ovary and uterus and form the paroophoron (Fig. 81). They too may form cysts. 2. In the Male.
Fig. 81. Remnants of the Wolffian Body in the Female.
In the Male
In the male (Fig. 82) the Wolffian duct forms :
- The tube of the epididymis, which is coiled up in the globus major, body and globus minor of the epididymis
- The vas deferens and common ejaculatory duct. The duct opens, as in the female, at each side of the uterus masculinus in the prostatic urethra
- The vesiculae seminales arise from the Wolffian ducts as acino-tubular diverticula.
Fig. 82. Remnant of the Wolffian Body in the Male.
The stalked hydatid frequently seen on the upper extremity of the testicle corresponds to the hydatid at the fimbriated extremity of the Fallopian tube in the female, and is of similar origin (Figs. 81 and 82).
The genital tubules of the Wolffian body become the vasa efferentia and coni vasculosi.
The renal tubules of the Wolffian body form :
- The vasa aberrantia found in the globus minor ;
- The paradidymis or organ of Geralde's occasionally situated in the cord above the globus major. All these tubules, both genital and renal of the Wolffian body, are situated originally in the mesentery of the Wolffian body (Fig. 80).
Thus it will be seen that while in the male the Wolffian tubules and duct become part of the genital system, in the female they become functionless ' and only of pathological importance Their presence in the female is due to their being inherited from the male.
Origin of the Permanent Kidney
In Fishes and Amphibians the Wolffian body alone acts as a kidney. In Reptiles, Birds and Mammals, the permanent or hind kidney appears and supplants the Wolffian kidney. The kidney appears in the human embryo at the beginning of the second month. It arises (see Fig. 83) as a bud from the dorsal side of the Wolffian duct near the termination of that duct in the cloaca. At first it is a stalked bud with a narrow lumen ; it rapidly extends forwards to the lumbar region behind the Wolffian body and behind the peritoneum. The stalk of the bud forms the ureter. The connection of the stalk with the Wolffian duct is lost ; the termination of the ureter migrates along the duct until it reachei that part of the cloaca which afterwards forms the bladder (Fig 84). The dilated cephalic end of the bud divides into severa secondary buds. The dilated terminal part forms tb pelvis of the kidney, its infundibula and calyces. The tubule of the kidney are formed from groups of epithelial cells on the convex margin of the dilated end of the renal bud. They arise as tubular buds which grow out into the intermediate mesoblast glomeruli, etc. cell mass, subdividing as they grow spring out in groups, each group out forming inter at its mass, the .
|Fig. 83. The Origin of the Renal Bud (diagrammatic).||Fig. 84. The Termination of the Ureter in the Bladder and Sub-division of the Renal Bud.|
The tubular buds pyramid. The tubules at first are straight, but as they grow into the mediate cell mass, each tubule becomes convoluted, and blind end a glomerulus, formed in the intermediate cell is developed and comes to project within the tubule, glomeruli, and perhaps also the convoluted tubules, are produced from the renal mesoblast beneath the capsule, batch after batch being formed as the kidney grows. The loop of Henle is late in appearing. Thus it will be seen that the ureter, the pelvis of the kidney and at least the collecting tubules of the kidney are derived from the epithelial bud which springs from the Wolffian duct ; the capsule of the kidney, the intra-renal connective tissue, the vessels and glomeruli, and possibly the convoluted tubules, ar derived from the mesoblast of the intermediate cell mass. It wil be remembered that the epithelial lining of the Wolffian duct i epiblastic in origin, and the renal bud which springs from it mus also be of a similar nature.
The kidney grows forward until it touches the supra-rena bodies. At the same time it undergoes a rotation so that the hilum, instead of pointing backwards to the pelvis, is directed inwards and forwards. The secretory tubules are grouped in lobules or pyramids. Up to the time of birth and for somi time afterwards the lobules remain distinct, and this is the per manent condition in mammals, such as the ox, bear, seal, etc In man, with the formation of new cortex beneath the capsule all marks of separation between the surface parts of the rena lobules disappear, only their apices or pyramids remaining distinct. As the renal buds of opposite sides grow forwardE they may come in contact and fuse partially together at thei: caudal extremities. In this way the condition known as horse shoe kidney is produced. The renal bud may subdivide at iti commencement, thus giving rise to two or even three ureters Although the ureters remain distinct, the renal parts commonlj fuse again to form one kidney.
The Müllerian Ducts
The Müllerian Ducts or Oviducts are present in almost all vertebrates, and convey the ova to the surface of the body. In fishes Amphibians, reptiles, birds and lower mammals (Marsupials) the ducts terminate in the cloaca. This is also the case in thi embryonic stages of man and all higher mammals. The develop ment of the duct in man is very simple (Fig. 85). It is developed on the outer surface of the Wolffian ridge, below (ventral to) tin Wolffian duct, by a tubular invagination of the mesothelium of the coelom. The anterior (cephalic) end remains connected with tin coelom and forms the ostium abdominale. As it passes back wards in the Wolffian ridge it lies below and internal to the Wolffian duct and comes in contact with the Müllerian duct on the opposite side in the pelvis (Fig. 87). The Müllerian duct is formed in the embryo later than the Wolffian duct. In fishes the Müllerian is derived from the Wolffian duct. They open in that part of the cloaca which forms the wall of the bladder, between the openings of the Wolffian ducts (Fig. 86). They are developed in the male as well as the female embryo.
Fig. 85. A transverse section to show the manner in which the Wolffian and Müllerian Ducts arise, and their position in the Wolffian Ridge. After Julius Kollmann (1834 - 1918)
The Genital Cord - During the 3rd month the Müllerian ducts show two distinct stages in their course :
- Lumbar, which lies in the Wolffian ridge and is suspended from the posterior abdominal wall by the Wolffian mesentery. This stage afterwards forms the Fallopian tube (Fig. 87).
- A pelvic stage, where it lies in the genital cord. The posterior ends of the Wolffian ridges, with their contents, the Wolffian and Müllerian ducts fuse in the pelvis, and thus form the genital cord. The parts of the Müllerian ducts within the cord form the uterus and vagina.
The genital cord of the foetus at the second month shows the two Müllerian and two Wolffian ducts — in the male as well as in the female (Fig. 86).
Fig. 85. Diagram of the Genital Ducts at the commencement of the 3rd month of foetal life (lateral view).
The Round Ligament of the Uterus is attached to the Müllerian duct on each side (Fig. 87). The point of attachment marks the junction of the uterine and Fallopian segments of the Müllerian ducts. The round ligament corresponds to the gubernaculum testis in the male and its development is similar. Both are developed in the following manner : Part of the Wolffian ridge is continued backwards as a peritoneal fold to the groin, this part forming the inguinal fold (Fig. 87). Into this peritoneal fold muscle cells grow from the rudiments of the transversalis and internal oblique in the abdominal wall and form the round ligament of the uterus. The point from which the ingrowth springs becomes the internal abdominal ring. Others grow out into the labium majus, carrying with them a process of peritoneum — the canal of Nuck. The inguinal canal and external abdominal ring and extra-abdominal part of the round ligament are thus formed.
|Fig. 86. Diagram of the Genital Ducts at the commencement of the 3rd month of foetal life (lateral view).||Fig. 87. Diagram of the Müllerian Ducts at the commencement of the 3rd month (ventral view).|
Formation of Uterus and Vagina
Within the genital cord the Müllerian ducts fuse and form the uterus and vagina (Fig. 87). In all the members of the vertebrate series below and including the Monotremes, the Müllerian ducts remain separate and open in the cloaca (Fig. 88 A). The process of fusion begins in the 3rd month. The septum formed by the fused mesial walls (Fig. 89) disappears first in the region of the uterine cervix ; the process may be arrested at this stage — a stage shown by some adult marsupials. Then the lower or vaginal part of the septum disappears; the human uterus then (3 \ months) resembles that of higher mammals (carnivora, etc., Fig. 88. G). It may be arrested at this stage (uterus bicornis). Lastly the upper part of the septum disappears (4| months, Fig. 89). The fundus, which is the last part to be developed and is only found in the highest primates, is quite well marked in the child at birth.
Fig. 88. Evolution of the Human Form of Uterua. A Form seen in lowest mammals, reptiles, amphibians, fishes, and in the 2nd month human foetus. B. Form of Müllerian Ducts in rodents. C. Form in Camivora, etc., and in the 4th month human foetus. D. Form found in man and higher primates.
Fig. 89. Showing the manner in which the Mulleriau Ducts fuse to form the Uterus and Vagina.
The lining epithelium of the lower third of the vagina is derived, according to the researches of Berry Hart, from the lower ends of the Wolffian ducts. A solid epithelial bud (epiblast) grows from the end of each duct, and fills the lower part of the united Müllerian ducts. The central cells of the Wolffian buds disappear, while the peripheral form the lining of the lower third of the vagina.
The Müllerian Ducts in the Male
All that remains of the Müllerian ducts in the adult male are their fused terminal segments forming the sinus pocularis or uterus masculinis in the prostate (Figs. 82 and 90). Its depth is commonly about 3 or 4 mm., but occasionally such a form as is represented in Fig. 91 occurs and shows the real nature of the sinus pocularis. The vagina, uterus, and part of the Fallopian tubes can be recognised (Primrose).
Fig. 90. A section of the Prostate showing the remnants of the lower ends of the Mttllerian Ducts in the male.
Fig. 91. A section of a Prostate showing an unusually developed Uterus Masculinus. (After Primrose.) The fimbriated ends of the Müllerian ducts persist as the sessile hydatids on the testicle (Fig. 82). The intermediate part of the tube becomes greatly stretched during the descent of the testicle and disappears, but a remnant of its upper end can be found in the sharp anterior border of the epididymis until quite a late period in foetal life (J. H. Watson). The mesosalpinx shrinks and completely disappears in the anterior border of the epididymis.
The Uro-genital Sinus or Canal. — The Müllerian ducts open into that part of the cloaca which becomes the neck of the bladder (Kg. 86). That part of the cloaca which serves as a common channel for bladder, Müllerian, and Wolffian ducts is the urogenital sinus (Figs. 95 A and B). In the female foetus at the 4th month it is still well marked (Fig. 92 A). In all mammals except man it retains this form. By the beginning of the 6 th month in the female foetus (Fig. 92 B) it will be seen that the vesicovaginal septum (a in Fig. 92) has grown down towards the pudenal cleft, and all that remains of the uro-genital sinus is the small space of the female pudendal cleft from the fossa navicularis behind to the vestibule of the vulva in front. The vagina (Müllerian ducts) thus comes to open in the pudendal cleft in the female. In the male (Fig. 93) the early foetal form is retained, and the uro-genital sinus becomes that part of the male urethra between the sinus pocularis and the anterior layer of the triangular ligament. The female urethra corresponds to the prostatic part of the male urethra above the opening of the sinus pocularis (Figs. 92 and 93).
Fig. 92. Section showing the Uro-genital Sinus. A. In the 4th month female human foetus. B. In the 5th month female human foetus.
Fig. 93. Section showing the Uro-genital Sinus in the male foetus. a indicates the part corresponding to the vesico-vaginal septum of the female. It is occupied by the 3rd lobe of the prostate.
The Hymen is formed at the junction of the vagina with the uro-genital sinus. The terminal parts of the Müllerian ducts, which form the vagina, are at first solid epithelial cords, the epithelial mass being derived from the bulbous terminations of the Wolffian ducts (Berry Hart). After the Müllerian ducts fuse to form the vagina they become hollowed out, all but the terminal piece, which forms the hymen and it, as a rule, is perforated (Fig. 94).
Fig. 94. A section to show the condition of the Vagina and Uterus at the 7th month of foetal life.
Cases occur in which the hymen is a complete or even thick septum, or occasionally the whole vagina may retain the foetal ■ solid form (atresia vaginae).
The Vagina is at first not only solid, but extremely short ; the downgrowth of the vesico-vaginal septum adds greatly to its length (Fig. 92 A and B). It is separated off from the uterine segment of the Müllerian duct in the fifth month (Fig. 92 B) by (1) the growth of the external os and formation of the labia; (2) by its epithelium becoming stratified, while that of the uterus and upper half of the cervix remains columnar.
The Uterus is formed by the fusion of the Müllerian ducts ; its muscular walls and thickened mucous lining appear in the 5th month. By the seventh month (Fig. 94) it is divided into two parts, the cervix or lower segment and body or upper segment. The lower segment or cervix forms then two-thirds of the uterus ; its walls are thick and its upper part is lined by columnar nonciliated epithelium, containing mucous racemose glands. Its mucous membrane is arranged in palmate folds. The upper or uterine segment proper composes only a third of the uterus. It is lined by columnar ciliated epithelium. Uterine glands are developed in it after birth. At puberty the body of the uterus, instead of being half the size of the foetal cervix, becomes larger than it. The cervix takes no part in menstruation nor in containing the foetus ; its true function is unknown. It is probably glandular.
The Perineal Body (Figs. 94 and 95) is the triangular septum of tissue which is developed in the perineum so as to separate the rectum from the vagina. It contains the origin of the sphincters of the anus and vagina, fat and fibrous tissue, and all the structures between the recto-uterine fold of the peritoneum above and the perineum below.
In the male the corresponding part lies between the urethra and the rectum. In man the perineal body is extremely large. By its development the rectal part of the cloaca is separated from that part of it which forms the uro-genital sinus (Fig. 95). The perineal body is formed at the end of the 1st month of foetal life in the following manner : The allantois grows out from the hind gut ; a prolongation of the enteric cavity forms the canal of the allantois (Fig. 15, p. 96). Hence in the 3rd week the hindermost part of the gut is common to allantois and intestine, the common part forming the cloaca. There is then no perineal body (Fig. 95 A). This is the permanent condition in all vertebrates save the higher mammals — those higher than the Monotremes. The Wolffian ducts open in the cloaca near the neck of the allantois. A cleft appears on each side of the cloaca, and the dorsal or rectal part is separated from the ventral or uro-genital part. The ventral or uro-genital part of the cloaca forms the bladder and uro-genital sinus, which are thus separated from the rectum by the ingrowth of two lateral cloacal septa.
Fig. 95. The Division of the Cloaca into Rectal and Uro-genital Parts.
A. First appearance of the separation of tne Cloaca into Rectal and Uro-genital Parts (3rd week).
B. Separation of the Cloaca into Rectal and Uro-genital Parts (5th week).
The Proctodaeum or Primitive Perineal Depression
Although the main part of the perineal body is formed in the manner just described, a second element, which forms the surface part of the perineal body is formed thus : Between the caudal protuberance behind, and the genital tubercle in front (Fig. 95i?), the epiblast dips in as a median depression to form the proctodaeum or primitive perineal depression. The depressed area comes in contact with the ventral surface of the cloaca (Fig. 95^4). By the ingrowth of a perineal septum from each lateral margin of the depression, the proctodaeum is separated into a posterior or anal part, and an anterior or uro-genital part.
The cloacal membrane or plate (Fig. 95 A), formed of the applied layers of epiblast and hypoblast between the perineal depression and the cloaca, breaks down before the end of the 1st month of foetal life. The uro-genital sinus then comes to open into the uro-genital part of the depression, while at a rather later date the rectum opens into the anal part. The lateral perineal ingrowths, which divide the proctodaeum into uro-genital and anal parts, complete the perineal body (see Fig. 98).
This condition occurs not unfrequently, and in many forms. It is due to a mal- development of the cloacal and perineal septa. In some cases it is merely the anal part of the cloacal plate that fails to break down (Fig. 96). The white line, situated at the lower ends of the columns of Morgagni, marks the situation of the anal plate and the junction of the epiblast of the anal depression with the hypoblast of the rectum.
Fig. 96. A case of Imperforate Anus due to a persistence of the Anal Plate.
In over 50% of cases of imperforate anus the rectum ends blindly two inches or more above the anus. In such cases the lateral septa which divide the cloaca appear to have been malplaced and included the whole of the cloacal part of the hind gut in the uro-genital canal. On the other hand the cloacal septa may be incomplete, and the uro-genital sinus only partially separated from the rectal. In such cases there is a communication between the rectum and termination of the vagina in the female (Fig. 97), and the part of the urethra formed from the uro-genital sinus in the male (Fig. 93).
Post-anal Gut. — It will be observed that the anus is formed on the ventral aspect of the hind gut (Fig. 108, p. 136). There is a projection of the gut behind the proctodaeal depression. This is the post-anal gut. It commonly disappears, no trace of it being found after the 1st month, but remnants of it are said to give rise to ano-coccygeal tumours and cysts.
Fig. 97. A case in which the Rectal part of the Anal Plate has persisted and the Cloacal Septum has failed to fuse with the Perineal Septum.
The Neurenteric Canal — Ano-coccygeal tumours are also believed to arise from remnants of the neurenteric canal. The neurenteric canal, or blastopore, it will be remembered (p. 8 9, Figs. 68, 75 and 158) is a communication of the cavity of the hypoblast with the surface of the epiblast. It probably represents the point at which an area of cells was invaginated within the epiblast to form the hypoblast. As it opens at the anterior end of the primitive streak, which afterwards is included in the posterior end of the neural groove, such a canal, if it persisted, might be expected to perforate the coccyx and communicate between the canal of the notochord and filum terminale. No remnant of this canal has been found after the 1st month of foetal life.
The Uro-genital Cleft or Depression is the anterior part of the proctodaeal or primitive perineal depression, cut off from the posterior or anal part by the development of the perineal septa. With the absorption of the cloacal membrane, the uro-genital sinus opens in the cleft (Fig. 95.5). The cleft remains alike in both sexes until the third month. It forms the greater part of the pudendal cleft of the female. In its anterior angle is developed a prominent tubercle — the genital tubercle, the apex of which becomes the glans penis or glans clitoris, according to sex. It is bounded by lateral folds, the genital folds (Fig. 98). The genital folds divide in front ; the inner division of each runs to the glans and forms the fraenum ; the lateral divisions meet over the glans and form the prepuce.
External Genitals of the Female
In the female the parts retain closely the early fdetal form just described. The genital tubercle becomes the glans clitoris. In the genital eminence — of which the tubercle is merely the summit, the corpora cavernosa develop. The lateral genital folds form the labia minora, the prepuce and fraenum. By the junction of the genital folds within the urogenital depression behind, the fourchette is formed. Within the lateral folds or labia minora, are developed the bulbs of the vestibule. The uro-genital depression becomes the pudendal cleft. After the third month external genital folds arise and form the labia majora. By their anterior union they give rise to the mons Veneris. Their posterior extremities unite to form the posterior commissure.
Fig. 98. The Uro-genital Cleft or Depression and the Genital Tubercle and Folds towards the end of the 2nd month.
External Genitals of the Male
In the male, at the end of the third month, the lateral genital folds begin to unite from behind forwards, thus closing the uro-genital cleft and forming the floor of the penile urethra. While the floor of the penile urethra is formed thus, its roof, corresponding to the vestibule of the female, is derived from an angular forward prolongation of the uro-genital sinus. (See Fig. 95 B.) When the genital folds unite to form the urethral floor, the erectile tissue contained in them, corresponding to the bulbs of the vestibule of the female, also fuses and thus the corpus spongiosum is formed. The part of the urethra within the glans is the last part to be formed, and its development is peculiar. It is formed by a solid rodlike ingrowth of epiblast within the glans which burrows backwards until it reaches the part formed out of the uro-genital cleft. The glans part of the urethra becomes canaliculized a short time before birth. The fossa navicularis and lacuna magna occur at the junction of the part of the urethra formed in the glans and the part formed by the union of the inner genital folds. The closure of the uro-genital lips to form the floor of the urethra in the penis may be arrested at any stage, giving rise to the condition of hypospadias, a condition normal in the female. It is not very rare to find the terminal half inch of the urethra in a hypospadiac condition. If the hypospadias is complete then probably the internal sexual organs have been arrested in their development, and the sex of the individual can be determined only by the microscopic examination of the genital glands. The development of the prostatic and membranous parts of the urethra from the uro-genital sinus has been already dealt with (see Figs. 91, 93, and 99).
Fig. 99. A section of the male bladder and urethra at birth, showing the structures derived from the intra-abdominal part of the Allantois and from the Cloaca.
The Scrotum is formed during the fourth month by the union of the external genital folds (labia majora in the female) between the penis in front and the perineal body behind. The line of union is marked by the raphe.
The Fate of the Allantois
(Fig. 99) The part of the, allantois outside the abdominal parietes forms (1) the inner or vascular layer of the chorion (p. 97), and part of the umbilical cord (Figs. 75 and 78). The cavity of the allantois within the cord disappears by the end of the third month.
The intra-abdominal part of the allantois forms :
- The Urachus between the umbilicus and bladder (Fig. 99). It is reduced to a fibrous cord before birth. It may remain patent in the middle and closed at both ends and thus give rise to a urachial cyst behind the abdominal wall. Or it may open at the umbilicus, or at the bladder or at both, thus giving rise to a urinary fistula at the umbilicus. Urachial cysts are rare.
- The apical part of the bladder.
- The Bladder - It was formerly customary to describe the bladder as a derivative of the basal part of the allantois but more recent researches show it to be part of the cloaca (Figs. 95 and 108).
At birth the neck of the bladder lies above and behind the symphysis pubis (Fig. 99). The cavity of the bladder is then fusiform in shape and mostly extra-pelvic in position. It differs from the adult bladder in having a very small trigone and its cavity, therefore, a very short posterior limb in the closed condition.
(2) The Female Urethra and the corresponding part in the male, viz., the part between the neck of the bladder and the sinus pocularis.
(3) The uro-genital sinus or canal, and the parts derived from it which have been already described (Fig. 93 and page 115).
(4) The terminal part of the rectum.
Ectopia vesicae is as yet unexplained. The condition is shown diagrammatically in Fig. 100 A. This condition appears to be produced thus :
- The development of the tail in the embryo thrusts the region of the primitive streak, situated on the body-stalk, towards the ventral aspect of the body. The uro-genital part of the perineal depression and the cleft into the uro-genital sinus are formed in the line of the primitive streak (Fig. 158, p. 192). The body-stalk which at first is a direct continuation of the body (Fig. 75, p. 96), is arrested in its migration towards the ventral surface of the body. It is arrested between the rami of the pubes, and prevents the formation of the symphysis.
- The uro-genital cleft is formed in front of and above the genital tubercle instead of below it. The cleft opens thus into the anterior Wall of the bladder instead, as it normally does, into the uro-genital sinus (Fig. 100 5).
Fig. 100. A. A section to show the condition of parts in Ectopia Vesicae. B. Section of the pelvis of an embryo (4th week) to show how the condition is probably produced.
It is also possible that the condition of ectopia vesicae is due to a dropsical condition of the allantois, with subsequent rupture, in the embryonic condition.
The Prostate is developed round the uro-genital sinus. It consists of glandular tissue and stroma.
(1) The glandular tissue is composed of tubular glands which open into the prostatic part of the urethra. They are developed in the 4th month, as series of solid buds from the epithelium lining the upper part of the uro-genital sinus (Fig. 101). The buds grow out as a right and left lateral mass, and form the glandular tissue of the lateral lobes. At first the two lateral lobes, as in mammals generally, lie separately behind the urethra. Then they fuse behind the urethra ; in man only do they meet to form a dorsal or pubic commissure over it. The third lobe (Fig. 91) appears later; the tubular buds which form it rise from the posterior part of the stalk of the bladder (Fig. 101) above the opening of the sinus pocularis. It is not unfrequently absent or very small Skene's tubules, which may be found opening into the urethra of the female, probably represent prostatic tubules.
Fig. 101. A diagram to show the position at which the Prostatic Tubules arise.
(2) The Stroma of the Prostate. — While the glandular tubes arise in three groups — two lateral and one posterior median — from the epithelium lining the uro-genital sinus, and stalk of the bladder — the muscular and fibrous elements arise from the mesoblastic tissue surrounding the terminal parts of the Wolffian and Müllerian ducts. The stroma surrounds the glandular tissue and forms the peripheral part of the gland. It contains muscular tissue which is especially developed in the pubic commissure.
Probably the stroma is similar in its nature and origin to the uterine muscle, for at a corresponding time of life it is apt to give rise to the same form of fibrous tumours as occurs in the uterus.
As regards the nature of the Prostate :
- It is purely genital, and develops only in the rutting season in such mammals as manifest a periodical sexual life.
- It remains comparatively undeveloped until puberty. At the age of seven it weighs only 3 grains ; after sexual life is established it weighs about 300 grains.
- It atrophies on castration, a fact which is utilised by the surgeon in cases of prostatic hypertrophy. Castration performed on old men frequently leads to insanity. Atrophy of the prostate is also produced by section of the vas deferens on both sides. In one man out of three over 55 years of age the prostrate hypertrophies, both the glandular and fibro-rnuscular elements participating. Hypertrophy of the third lobe causes a valvular elevation behind the vesical opening of the urethra.
The Glands of Cowper and Bartholin are produced as solid buds from the hypoblast lining the mouth of the uro-genital sinus. Hence in the female the ducts of Bartholin's glands open outside the hymen at each side of the vagina, for the hymen marks the junction of the Müllerian ducts with the uro-genital sinus. In the male the ducts of Cowper's glands open at the commencement of the bulbous part of the urethra. Their function is unknown, but they are certainly sexual in nature. The numerous glands of Littre, like Cowper's and Bartholin's glands, are produced by tubular outgrowths during the fourth month. In the male the glands of Littre are produced most numerously along the dorsal aspect of the urethra.
Descent and Development of the Testicle
The position of the testicle in a foetus of the third month is shown in Fig. 102. It is situated in the iliac fossa. The mesorchium, a fold of peritoneum, binds its attached border to the iliac fossa. At its outer side lies the genital part of the Wolffian body which forms the epididymis. It, also, is suspended by a mesentery — the Wolffian mesentery. The two mesenteries have a common base — the common uro-genital mesentery (see Fig. 80, p. 104). The upper part of the uro-genital mesentery forms the diaphragmatic fold or plica vascularis (Fig. 102). This in the female becomes the ovario-pelvic ligament (Fig. 59, p. 81).
Fig. 102. The Position of the Testis in a foetus of 2.5 months .
the inguinal fold or plica gubernatrix, A fold of peritoneum, continues the common uro-genital mesentery to the groin (Fig. 102). The gubernaculum testis is developed in the plica gubernatrix; in the corresponding fold in the female the round ligament of the uterus appears. The vas deferens (Wolffian duct) turns into the pelvis from the lower end of the epididymis (Wolffian body), and within the pelvis lies in the genital cord (Fig. 87, p. 112). A remnant of the Müllerian duct lies along the outer and ventral aspect of the epididymis.
The Development of the Testis
Its blood supply comes from the level of the 12th dorsal vertebra; its nerve supply from the 10th dorsal segment of the spinal cord. The testis is therefore developed in the genital ridge between the 10th and 12th dorsal segments. The development of the testis is similar to that of the ovary (Fig. 62, p. 83). The columnar germinal epithelium which covers the genital ridge contain between them larger genital cells — the primordial ova. Tubular buds of germinal epithelium grow into the tissue of the genital ridge and form the epithelial lining of the seminiferous tubules instead of, as in the female, the Graafian follicles. Primordial ova are carried down within the tubes of enclosing cells and these produce the spermatoblasts. The tunica albuginea is formed from the mesoblastic covering of the genital ridge. The visceral layer of the tunica vaginalis on the testicle is the covering of germinal epithelium which remains after the ingrowth of the genital cells. The vasa efferentia and coni vasculosi are formed from the genital Wolffian tubules. The tubuli recti' and rete testis are new formations. The epididymis is the elongated upper segment of the Wolffian duct (Figs. 82, 107). The Wolffian elements (see p. 102) are produced within the Wolffian ridge.
Formation of the Gubernaculum Testis
As shown in Fig. 102 there is no trace of the inguinal canal in the 3rd month; the various layers of the abdominal wall are unbroken. In the fourth month the deep muscular layer of the abdominal wall, composed of the internal oblique and transversalis, buds inwards and expands the plica gubernatrix with muscular and fibrous tissue. The tissue does not stop short at the uro-genital ridge and Müllerian duct as in the female, but grows up and seizes the caudal pole of the testis (Fig. 103). At the same time the tissues in the whole thickness of the abdominal wall bud obliquely inwards towards the scrotum. They are probably carried away by the downgrowth of the gubernacular bud which pushes its way to the scrotum (Figs. 103 and 104). The gubernaculum grows downwards as a solid fibro-muscular mass, until it reaches the subcutaneous tissue which at that time completely fills the scrotum. Its attachment to the scrotum is slight and easily broken. The gubernaculum, as it grows through the abdominal wall, carries with it : —
- A process of peritoneum (the processus vaginalis) ;
- The trans versalis fascia (the infundibuliform fascia) ;
- The internal oblique and transversalis muscles to form the cremaster ;
- The spermatic fascia from the external oblique ;
- The deep layer (Scarpa's) of the superficial fascia of the groin. All these layers are added to the primitive coverings of the scrotum, which until then is made up simply of skin and superficial fascia (Fig. 104).
Fig. 103. Showing the Position of the Testis at the 6th month, and the Formation of the Gubernaculum Testis.
Fig. 104. The manner in which the structures in the wall of the abdomen are carried out so as to form the Inguinal Canal and Coverings of the Testis.
It will be thus seen that the gubernaculum testis is an actively growing mass of fibro-muscular tissue, which starting from the inner muscular layer of the abdominal parietes in the groin, invades first the plica gubernatrix and then the abdominal wall itself, every layer of which it carries as a prolongation within the scrotum. .It is an invading army of cells. It draws with it into the scrotum the peritoneum in the iliac fossa, on which the testis is dragged like a log on a sledge.
The testis spends the seventh month of foetal life in its exodus through the abdominal wall. In the eighth month it leaves the inguinal canal and lies at the external abdominal ring. After birth it reaches the fundus of the scrotum. The atrophy of the gubernaculum pulls it down. A remnant of the gubernaculum can always be found in the adult behind the epididymis and testicle, within the mesorchium (Fig. 105).
The Processus Vaginalis
The processus vaginalis becomes occluded at two points about the time of birth (Fig. 105). The upper point of occlusion takes place at the internal abdominal ring ; the lower a short distance above the testicle. The part of the processus vaginalis between the points of occlusion is known as the funicular process ; the part surrounding the testicle becomes the tunica vaginalis. In quite 30 o /° of children the occlusion takes place at the internal abdominal ring some considerable time after birth or it fails to appear altogether. Occlusion may fail at the upper point, at the lower point, or at both. Or it may close at both points, but the funicular process, instead of disappearing, may remain open and form a cyst (Fig. 105).
Fig. 105. A diagram of the Processus Vaginalis.
Descent of the testicle may be arrested at any stage; often in the inguinal canal ; more frequently at the external abdominal ring. Arrest of descent is commonly a symptom of arrest of testicular development. On the other hand, the testicle may assume an ectopic position. The gubernaculum ends in the scrotum principally, but bands of it pass to end on the root of the penis, in the groin and in the perineum. These bands, normally slight, may be big enough to influence the direction of descent of the testicle ; hence cases occur in which the testis is found in the groin, by the side of the penis or drawn back in the perineum almost to the anus (Lockwood).
The testis and epididymis were suspended within the abdomen by the common uro-genital mesentery. In the course of the descent of the testis this becomes shortened and binds the testis and epididymis firmly by their posterior borders to the tunica vaginalis. Occasionally the uro-genital mesentery and mesorchium persist ; the testicle is then liable to become twisted and strangulated on its mesentery. I have seen three such cases recently. The digital fossa is situated between the mesorchium and mesentery of the Wolffian body.
The meaning of the descent of the testes is unknown. In many animals the testes descend only during the rutting season. The inguinal canal, formed by the descent of the testis, is a source of weakness in man (see p. 133).
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Human Embryology and Morphology (1902): Development or the Face | The Nasal Cavities and Olfactory Structures | Development of the Pharynx and Neck | Development of the Organ of Hearing | Development and Morphology of the Teeth | The Skin and its Appendages | The Development of the Ovum of the Foetus from the Ovum of the Mother | The Manner in which a Connection is Established between the Foetus and Uterus | The Uro-genital System | Formation of the Pubo-femoral Region, Pelvic Floor and Fascia | The Spinal Column and Back | The Segmentation of the Body | The Cranium | Development of the Structures concerned in the Sense of Sight | The Brain and Spinal Cord | Development of the Circulatory System | The Respiratory System | The Organs of Digestion | The Body Wall, Ribs, and Sternum | The Limbs | Figures | Embryology History
Cite this page: Hill, M.A. (2021, June 17) Embryology Book - Human Embryology and Morphology 9. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Human_Embryology_and_Morphology_9
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