Book - Aids to Embryology (1948) 12
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter XII The Urogenital System
Although in the adult human the urinary and genital systems perform quite different functions, they are intimately associated with each other during development. They take their origin from the same part of the mesoderm, the intermediate cell mass, and some structures originally belonging to one system may later be annexed for use by the other, as, for example, the utilization by the male of the duct of the temporary kidney (the mesonephros) as an efferent duct for the male germ cells. Nevertheless, for purposes of descriptive embryology it is convenient to discuss the two systems separately.
The Urinary System
Before the permanent kidney or metanephros becomes differentiated in the human embryo, two other excretory organs arise and disappear. The first is called the pronephros and it is a transitory structure. The second, the mesonephros, is a temporary one functioning during the second month of development. These three organs arise in cranio-caudal sequence in the embryonic body from the nephrogenic cord of the intermediate mesoderm.
The pronephros is found during the fourth and early fifth weeks of development as a short series of segmentally arranged cell cords in the intermediate cell mass of the future cervical region. In each segment the lateral part of the cord turns caudally, comes in contact with the cord of the succeeding segment and fuses with it. In this way a longitudinal pronephric duct is formed, and the lateral end of the last tubule continues to grow caudally until it reaches the lateral wall of the cloaca into which it opens. The proximal part of each cord is connected with the coelomic wall and, the cord becoming hollowed out, this represents a nephrostome. Adjacent to it the coelomic wall becomes invaginated as a knob of cells representing an external glomerulus. Before the caudal part of the pronephros has yet arisen, the cranial part is already undergoing degeneration.
Fig. 34. The Pronephros.
1, Spinal cord ; 2, myotome ; 3, proriephric duct ; 4, nephrostome ; 5, glomerulus ; 6, aorta ; 7, notochord.
As the cephalic tubules of the pronephros degenerate, the mesonephros commences to form in the upper thoracic segments as spherical masses of cells in series with the tubules of the pronephros. The cell masses become hollow as mesonephric vesicles, and a cord of cells grows towards the pronephric duct from the lateral side of each. The cord becomes S-shaped, develops a lumen and its free extremity fuses with the pronephric duct, which may now be called the mesonephric duct. The medial wall of each mesonephric vesicle becomes invaginated by a branch from the dorsal aorta to form a glomerulus, its thinned out wall becoming Bowman s capsule. The tubule shows a proximal secretory portion lined by tall columnar epithelial cells, and a distal collecting portion where the epithelium is cubical. Each body segment contains several mesonephric tubules and the organ extends as far caudally as the third lumbar segment. When fully developed, it bulges as the urogenital fold on each side into the coelom from the dorsal body wall, being attached thereto by a broad mesentery. The tubules of the mesonephros differ from those of the pronephros as follows :
- They are not segmentally arranged.
- They do not develop a duct of their own but make use of the existing pronephric duct.
- They possess internal glomeruli.
Degeneration at the cranial end of the mesonephros commences during the fifth week and proceeds caudally. The process is well advanced by the ninth week, but some of the caudal tubules persist in modified form in connection with the genital organs (see later). The work of Gersh (1937) has shown that the mesonephros is functional in lower mammalian forms such as the opossum pouch young, and rabbit, cat, and pig embryos. In these the mesonephros can excrete such substances as ferrocyanide and phenol red. As functional differentiation of the metanephros occurs so the ability of the mesonephros to excrete such substances diminishes.
The metanephros takes origin from two primordia ; the excretory part of it is derived from the caudal undifferentiated tissue of the nephrogenic cord, while the collecting tubules are formed from the ureteric bud of the mesonephric duct. The latter is the first to appear. In the fifth week there is an outgrowth from the dorsal wall of the mesonephric duct close to its entrance into the cloaca. This is called the ureteric bud, and it grows actively, at first dorsally, and then dorsally and cranially. This brings its tip into contact with the caudal undifferentiated mesoderm of the nephrogenic cord. This tissue forms a cap over the free, somewhat dilated extremity of the ureteric bud, and it is termed the metanephritic blastema. As the ureteric primordium elongates the blastemal cap is carried cranially upon it from the original position opposite the first and second sacral segments.
The tip of the ureteric bud becomes bi-lobed and the blastema divided also into two parts. Each of these gives off three to six tubular diverticula, the blastema at the same time dividing into a corresponding number of caps. Secondary and tertiary tubules are then formed by branching, and the process continues until some thirteen generations are present. The mesodermal caps divide so as to keep pace with the appearance of each new generation of tubules, so that each always has its own blastema. The first two tubules by their subsequent growth form the major calyces ; the secondary tubules connected with these form the minor calyces. The tubules of the third and fourth generations are absorbed into the walls of the minor calyces so that the tubules of the fifth generation ultimately open into them as the papillary ducts of the renal pyramids. The tubules of the remaining generations form the collecting tubules of the adult kidney.
The blastema at the tip of each collecting tubule is at first a solid mass of cells, but it soon becomes vesicular and then elongated in an S-shape with one extremity in contact with the tip of the collecting tubule. Communication between the lumina of the two tubules now occurs. The lower (distal) limb of the S-shaped tubule becomes invaginated by blood vessels to form a renal glomerulus, the neighbouring wall of the loop becoming thinned out as Bowmanâ€™s capsule. The portion of the tubule next this region elongates and forms the proximal convoluted tubule ; the portion next the collecting tubule forms the distal convoluted tubule, while the limb of the section between these two elongates greatly to become the loop of Henle. Not all of the embryonic renal tubules persist into ad,ult life for Kampmeier (1926) has shown that a number in the depths of the kidney, that is, early formed ones, have only a temporary existence.
Fig. 35. - Two Stages in the Development of the Secreting Tubules of the Kidney.
A. 1, Collecting tubule ; 2, main collecting tubule ; 3, distal convoluted tubule ; 4, glomerulus ; 5, loop of Henle.
B. 1, Collecting tubule ; 2, proximal convoluted tubule ; 3, distal convoluted tubule ; 4, glomerulus ; 5, loop of Henle.
Gersh (1937), comparing metanephric differentiation in the human embryo with evidence obtained from other mammals, considers that tubular function commences about the 32 mm. stage (9 weeks of age) while the loop of Henle is capable of water reabsorption at the 81 mm. stage (13 weeks).
Summary of the Development of the Kidney
- A series of three excretory systems is developed in the human embryo, the pro-, meso- and metanephros. The pronephros is non-functional, the mesonephros is temporary, and the metanephros forms the functional kidney of the adult.
- The metanephros is developed from two sources :
- an outgrowth (ureteric bud) from the mesonephric duct forms the ureter, pelvis and collecting tubules ;
- a mass of mesoderm at the caudal end of the nephrogenic cord forms the secretory system.
- Branching and re-branching of the extremity of the ureteric bud gives rise to about thirteen generations of tubules. These become connected with S-shaped tubules derived from caps of mesoderm over the extremity of each.
- The lower limb of each S-shaped tubule becomes invaginated by blood vessels to form a Bowmanâ€™s capsule and glomerulus.
- The transverse portion of the S-shaped tubule elongates towards the medulla to form the loop of Henle, while the remaining parts are transformed into the proximal and distal convoluted tubules.
Anomalies of Development of Kidneys and Ureters
- Congenital cystic kidney may be the result of failure of union of the secretory and collecting tubule systems, or it may be due to persistence of renal tubules which normally degenerate.
- Horseshoe kidney is produced by fusion of the two metanephric blastemae. The fusion is usually at the lower ends.
- Double ureter is caused by the growth of two ureteric buds. A cleft renal pelvis and partially cleft ureter are a sequel of early division of the ureteric bud.
- Pelvic kidney is due to failure of ascent of the organ from its primary developmental position opposite the first and second sacral segments.
Subdivision of Cloaca and Formation of Bladder
The caudal termination of the primitive gut is somewhat dilated as a blind sac in which the allantois also terminates. This is the cloaca, and the ventral surface of it is in contact with the surface ectoderm as the cloacal membrane. The growth of the tail causes this membrane at first to face ventrally and then to become reversed so that it looks towards the attachment of the umbilical cord to the body wall. The primitive streak is now just caudal to it, and proliferates mesoderm which passes on each side around the margins of the membrane to contribute to the formation of the infra-umbilical part of the anterior abdominal wall and external genitalia (Wyburn, 1937) During the fifth week the pronephric (later mesonephric) ducts reach the lateral wall of the cloaca and open into it. A coronally arranged septum now arises in the angle between the allantois and the hindgut and grows caudally towards the cloacal membrane. This is the urorectal septum and it splits the cloaca into two parts, a dorsal rectum and a ventral urogenital sinus which has the mesonephric ducts opening into it. The urorectal septum reaches and fuses with the entoderm of the cloacal membrane during the seventh week at the primitive perineum. Subsequent breakdown of the two subdivisions of the cloacal membrane puts the rectum and urogenital sinus in communication with the exterior. The site of entrance of the mesonephric ducts divides the urogenital sinus into an upper vesico-urethral primordium and a lower urogenital sinus proper. The cranial part of the vesico-urethral primordium dilates to form the bladder. During this process, a series of growth changes occur in the region of the lower ends of the mesonephric ducts. The terminal parts, of them are absorbed into the bladder so that the ureters and the mesonephric ducts acquire separate openings, and then the ureteric orifices are carried cranio-laterally by the growth of the bladder. The openings of the mesonephric ducts remain in the primitive urethral part of the primordium. Since the mesonephric ducts are of mesodermal origin absorption of them into the bladder wall contributes a mesodermal component to the otherwise entodermal structure. This mesodermal area is the trigone of adult anatomy. The allantois regresses as development proceeds and ultimately becomes a fibrous cord, the urachus, connecting the apex of the bladder with the umbilicus.
Fig. 36. - Four Diagrams to show the Changes in the Cloaca. (Adapted from Corning.)
1, Allantois; 2, opening of mesonephric duct; 3, cloaca; 4, ureteral opening ; 5, genital tubercle ; 6, cloacal membrane ; 7, bladder ; 9, rectum ; 10, urethra.
The primitive urethral part of the vesico-urethral primordium grows slowly and so remains relatively narrow. It forms most of the length of the urethra in the female. In the male it forms that part of the urethra extending from the bladder to the openings of the common ejaculatory ducts.
The Prostate Gland
This is first seen in the male embryo of three months as five solid cords of entodermal cells which grow from the distal part of the primitive urethra and proximal part of the urogenital sinus into the surrounding mesoderm. These become hollow, tortuous, and branched to form the prostatic gland tubules. The surrounding mesoderm contributes smooth muscle and connective tissue fibres, to the capsule. Corresponding entodermal cords in the female become the paraurethral glands of Skene.
Summary of the Development of the Bladder
- In the early stages there is a common cloaca in which terminate the hindgut, the allantois and the mesonephric ducts.
- A septum (urorectal) grows caudally from the angle between the gut and the allantois to fuse with the cloacal membrane at the primitive perineum. This separates the cloaca into the rectum and the urogenital sinus.
- The part of the early urogenital sinus above the openings of the mesonephric ducts becomes the bladder in both sexes ; in addition, it forms most of the female urethra, and that part of the male urethra from the bladder to the openings of the common ejaculatory ducts.
- The lower ends of the mesonephric ducts are absorbed into the vesico-urethral primordium in such a manner that the ureteric openings appear to migrate cranially to open into the bladder, while the mesonephric ducts open into the urethra. The bladder thus acquires a mesodermal component, the trigone.
Anomalies of Development of the Bladder
- Persistent cloaca is due to failure, in varying degrees, of fusion of the urorectal septum with the cloacal membrane.
- Ectopia vesicae is due to failure of mesoderm from the primitive streak to migrate around the cloacal membrane and form the infra-umbilical anterior abdominal wall. Breakdown of the upper part of the cloacal membrane then exposes the posterior wall of the bladder on the surface of the body.
- Cysts may occur in persisting remnants of the urachus. Rarely, a lumen may remain in the whole structure and urine may escape at the umbilicus at a urachal fistula.
- Abnormal opening of the ureter into the urethra, seminal vesicle, vagina or elsewhere, may be found.
The Genital Glands and Ducts
When the mesonephros has attained its full degree of development it projects into the coelom and is attached to the dorsal body wall by a broad mesentery. On its lateral aspect the mesonephric duct runs towards the urogenital sinus in what may be called the tubar subdivision of the urogenital fold. This designation is apt, since a second duct appears later in the tubar fold of each side, the paramesonephric duct (see later). The genital gland arises from a thickening of the c celomic epithelium on the medial side of the urogenital fold in about the middle two-fourths of its extent. Its growth causes it to bulge ventromedially from the mesonephros and to be attached to the latter by a mesentery, the genital mesentery. Cords of cells proliferate from the thickened coelomic epithelium into the subjacent mesenchyme. This first occurs in the fifth week, but it is not until nearly two weeks later that the genital gland becomes histologically recognisable as a testis or an ovary.
In the male, bundles of fibrous tissue appear in the genital gland primordium, and these split up the genital cells into testis cords. Another dense layer of fibrous tissue forms just under the coelomic epithelium covering the gland. This is the tunica albuginea and when it has formed no further addition to the testis cords can be made by the germinal epithelium. The testis cords extend towards the mesorchium to form the rete testis which later becomes connected with the mesonephric duct by some persisting mesonephric tubules. These solid testis and rete cords become canalized later in foetal life. In the female, the cords of genital cells become arranged in masses which are the primordial follicles. A rete ovarii forms in the mesovarium but a definite tunica albuginea is lacking.
Primordial germ cells have been described in various mammals as arising from the gut entoderm and migrating by way of the dorsal mesentery to the genital gland. Hamlett (1935) has reported such cells in the human embryo, but it has always been a matter of dispute whether they have any definite relation to the ultimate germ cells of the adult. Recent experimental work by Everett (1943) on the mouse, suggests that the adult germ cells are derived from these primordial entodermal elements. If the genital ridge was transplanted to the kidney of a host animal before primordial germ cells were present a testis or ovary was not differentiated although the associated sex ducts were formed. If transplants were made after primordial germ cells were present in the genital ridge, ovarian or testicular tissue was formed.
During the seventh and eighth weeks of development, the paramesonephric or Mullerian ducts are laid down on each side of the embryonic body. This duct is first seen as an invagination of the coelomic epithelium covering the cranial and lateral part of the mesonephros. The solid tip of the invagination burrows into the underlying mesoderm and grows caudally just lateral to the mesonephric duct until it reaches the lower pole of the mesonephros. Here it crosses ventral to the mesonephric duct to lie on its inner side in a transverse partition of mesoderm in the pelvis, the genital cord. This genital cord is situated just dorsal to the bladder. The duct traverses the genital cord in a caudal direction until its tip fuses with the dorsal epithelial wall of the urogenital sinus. Here the two ducts produce an elevation of the dorsal wall of the sinus, the Mullerian tubercle. The two ducts now fuse with each other in a caudo-cranial direction in the genital cord forming a median utero-vaginal canal. The further changes in the paramesonephric ducts are dependent on the sex of the developing embryo.
The Male Embryo
The mesonephric duct becomes transformed into the duct of the epididymis and the vas deferens. It is connected with the rete testis by a number of persistent tubules derived from the caudal part of the mesonephros, which thus become the vasa efferentia. The blind upper end of the mesonephric duct persists as the appendix of the epididymis, while a few blindly ending caudal mesonephric tubules form the paradidymis. Near the termination of the mesonephric duct in the urogenital sinus a small dilatation marks the site of the ampulla, and an outpocketing from this gives rise to the seminal vesicle. The paramesonephric ducts disappear almost completely in the male. The extreme cranial end forms the appendix of the testis. The caudal part of it may contribute to the so-called uterus masculinus, but the evidence on this point is not precise.
Fig. 37. - Fate of the Mesonephric and Paramesonephric Ducts in the two Sexes.
A. Male ; B., female. 1, Genital gland ; 2, mesonephric duct; 3, paramesonephric duct ; 4 a, uterus masculinus ; 4 b, uterus ; 5, vas deferens ; 6, testis ; 7, appendix testis ; 8, duct of Gartner ; 9, ovary ; 10, epoophoron.
The Female Embryo
Some of the persisting mesonephric tubules along with a part of the mesonephric duct form the epoophoron which lies in the broad ligament near the hilum of the ovary. Other caudal mesonephric tubules form the paroophoron in the medial part of the mesosalpinx. This normally degenerates soon after birth. The caudal portion of the mesonephric duct normally degenerates but may persist in varying degree as the duct of Gartner. The paramesonephric ducts form the genital tract of the female. They were earlier described as consisting of cranial separated portions, and caudal parts fused to form a median utero-vaginal canal. Each cranial part opened by a funnel into the c celomic cavity. This funnel becomes fimbriated and is the abdominal ostium of the uterine tube. The unfused part of each duct forms the uterine tube, and where this passes over into the genital cord, a fold of the coelomic wall is attached and runs anterolaterally to the abdominal wall. This is called the inguinal fold and its mesodermal core becomes the round ligament of the uterus. The lower end of the utero-vaginal canal is solid and ends at the dorsal wall of the urogenital sinus in continuity with the epithelium of the latter. The tubular portion of the canal forms the body and cervix of the uterus, which retain their simple columnar epithelial lining. With further growth the paramesonephric ducts appear to recede from the sinus, but in doing so, they retain their connection with it by the formation of a cord of epithelial cells, the vaginal plate. The source of the cells making up this plate has been the subject of much controversy : some workers (e.g., Hunter, 1930) consider them to be mesodermal having their origin from the paramesonephric ducts ; others think that the vagina is of compound origin, with a lower component derived from the entoderm of the urogenital sinus (Koff, 1933), or from the mesonephric ducts (Mijsberg, 1924). Certain abnormalities support the theory of compound origin (McKelvey and Baxter, 1935). Whatever may be the origin of the vaginal plate, its cells proliferate irregularly beginning at the end next to the sinus (Fig. 38, 5). Degeneration of the central cells of the vaginal cord then takes place and the lumen of the organ is formed. The upper limits of this lumen are extended upwards around the external os of the uterus to form the fornices which are thus the last part of the vagina to arise. The lower end of the vaginal cord is somewhat bulbous where it indents the dorsal wall of the urogenital sinus to form the Mullerian tubercle. After perforation of this in the canalization of the vagina, the hymen is left as the remnant of the former partition. Irregularity in the breakdown of the tissue of the tubercle results in the various forms of hymen. It should be noted that during development there is an "eversion" of the urogenital sinus so that the vaginal orifice approaches the body surface. The vestibule of the vagina represents, in the adult, the original tubular urogenital sinus. The mesodermal tissue of the genital cord becomes transformed into the fibrous tissue and muscular coats of the uterus and vagina. The lateral part of it becomes the broad ligament which has added to it the urogenital mesentery. The mesoderm of the urogenital folds provide the muscular and fibrous coats of the uterine tube.
Fig. 38. = Diagrams to show the Development of the Female Genital Tract. Note that the vagina is derived from a solid cord of cells the core of which breaks down.
1, Paramesonephric duct ; 2, mesonephric duct ; 3, duct of Gartner ; 4, vaginal cord ; 5, proliferation of vaginal cord ; 6, degeneration commencing at the peripheral end of cord ; 7, formed vagina.
Summary of the Development of the Genital System
- The genital glands develop from the coelomic epithelium on the medial aspect of the mesonephros. Some of the tubules of the mesonephros form the vasa efferentia in the male and part of the epoophoron in the female.
- The mesonephric duct is transformed into the duct of the epididymis and the vas deferens in the male. The paramesonephric duct disappears in this sex except for the cranial extremity which forms the appendix of the testis, and the caudal end, which may contribute to the uterus masculinus.
- The paramesonephric ducts in the female give rise to the paired uterine tubes. Their median fused parts form the uterus, while the vagina results from the canalization of a solid cord of cells extending from the cervix uteri to the dorsal wall of the urogenital sinus. The origin Rf this cord is disputed.
- The mesonephric duct largely disappears in the female. A little of the cranial end is incorporated in the epoophoron, and a varying segment of the caudal part may constitute the duct of Gartner.
- The hymen is the remains of the partition between the bulbous lower end of the vaginal cord and the lumen of the urogenital sinus. Its lower surface is therefore clothed with epithelium derived from sinus entoderm.
Anomalies of Development of the Genital System
- Absence of the testes or ovaries is a rare condition, due to failure of development of one or both genital ridges.
- Fused testes is also a rare anomaly.
- Anomalies of the uterus and vagina arise from non-union in varying degree, of the paired primordia. They range from complete duplication of the uterus (bicornuate uterus) with double vagina, through various stages of bipartite uterus to cases which merely show retention of the foetal form.
- The vaginal cord may fail to canalize.
- Imperforate hymen may occur, a condition which often passes unrecognised until after puberty .
Descent of the Genital Gland
Both the testis and the ovary undergo descent from their primary position during development. In the case of the testis there are two stages in the process : ( a ) The testis moves within the abdominal cavity from the fronto-medial aspect of the mesonephros to the deep inguinal ring. ( b ) Later, the gland passes through the inguinal canal to the scrotum.
The first phase of descent is commonly agreed to be due to growth changes in the dorsal abdominal wall, which increases in length rapidly while the testis grows slowly. The gonad thus appears to migrate caudally, and by the end of the third month is lying in the false pelvis near the deep inguinal ring.
A fibro-muscular band, the gubernaculum, runs from the lower pole of the testis to the anterior abdominal wall in the urogenital fold. It can be traced on through the layers of the anterior abdominal wall to the scrotum. An evagination of the peritoneum, the processus vaginalis, occurs along the track of the gubernaculum, commencing about the end of the third month. During the seventh month the testis passes along the inguinal canal lying behind the peritoneum of the processus vaginalis to reach its adult position in the scrotum. During this descent the gubernaculum shortens. It was formerly thought that atrophy and contraction of the gubernaculum drew the testis downwards from the abdominal cavity. Hunter (1926) is of the opinion that the gubernaculum acts as a kind of anchor to prevent the testis moving cephalad with the developing kidney, the testis remaining at the deep inguinal ring until the intra-abdominal pressure is raised to a certain point by the growth and enlargement of the abdominal viscera, and that it is finally forced away from the intra-abdominal position, along a preformed inguinal canal, to its normal adult position in the scrotum. The cavity of the portion of the processus vaginalis between the testis and the deep inguinal ring becomes obliterated, leaving the lower portion persisting as the tunica vaginalis.
A, Position of testis at the deep inguinal ring with the coneshaped processus vaginalis. B, Testis immediately after descent into the scrotum. C, Testis after obliteration of the processus, leaving only the tunica vaginalis.
Anomalies in the Descent of the Testis
- The testis may be retained within the abdominal cavity or it may undergo only partial descent, remaining in the inguinal canal. Many of such testes may be induced to complete the process of descent by the administration of androgens.
- The testis may descend to an abnormal position (ectopia testis) . It may be found in the perineum, the thigh, between the layers of the abdominal musculature or in a pre-penile position. These locations are considered to be associated with the development of abnormal bands of the gubernaculum.
- Failure of obliteration of the processus "vaginalis gives rise to a congenital inguinal hernia.
In the female the ovary descends to just below the pelvic brim and lies on the posterior surface of the broad ligament. A gubernaculum develops as in the male, except that it is fused, a short distance caudal to the ovary, with the supero-lateral angle of the developing uterus. This fusion divides the gubernaculum into two parts ; a proximal, which becomes the round ligament of the ovary, and a distal, which is converted into the round ligament of the uterus.
The External Genitalia
The external genitalia develop on the ventral surface of the body wall around the anterior, or urogenital portion of the cloacal membrane. In the mid-line a genital tubercle is formed while the urogenital membrane is bounded laterally by labio-scrotal swellings. When the membrane breaks down, a median shallow urethral grooc e appears extending on to the ventral surface of the genital tubercle, and this is flanked by urethral folds. The external (phallic) portion of the urogenital sinus is represented by this elongated groove which is continuous caudally with the opening of the pelvic portion.
The Male Genitalia
The urethral folds meet and fuse in the middle line to form a canal continuous with the opening of the urogenital sinus. The fused edges form a median raphe. The glans penis is marked out at the extremity of the genital tubercle by a coronal sulcus at which the anterior opening of the urethra is now situated. An ectodermal cord of cells sinks into the substance of the glans and becoming canalized completes the formation of the penile urethra. During this time, the scrotal swellings have migrated caudally towards the root of the penis and becoming approximated to each other, form the scrotum. The prepuce develops as a fold of tissue at the coronal sulcus, which grows distallv over the surface of the glans (Hunter, 1935). The bulbourethral glands arise as evaginations from the hinder end of the entodermal penile urethra.
The Female Genitalia
The genital tubercle becomes transformed into the clitoris, the distal extremity being marked off by a coronal sulcus. The labial swellings migrate somewhat caudally and join behind the urogenital opening to form the posterior commissure, the remainder of them forming the labia majora. The urethral folds remain separate as the labia minora, the urethral groove and the urogenital sinus as far as the Mullerian tubercle becoming the vestibule of the vagina. The greater vestibular glands arise as entodermal outgrowths from this vaginal vestibule.
Summary of the Development of the External Genitalia
- A genital tubercle forms at the anterior end of the elongated opening of the urogenital sinus. This opening is bounded laterally by labioscrotal swellings.
- A urethral groove runs backwards in the midline from the genital tubercle. Urethral folds arise on each side of this.
- The labio-scrotal swellings form, as their name implies, either the halves of the scrotum, or the labia majora.
- The penile urethra is a composite structure being derived from :
- an entodermal segment formed by the fusion of the urethral folds ;
- an ectodermal part formed by canalization of an epithelial cord in the glans penis.
- The urethral folds do not fuse in the female but are transformed into the labia minora.
- The bulbo-urethral glands in the male, and the greater vestibular glands in the female, arise as entodermal outgrowths from the lower part of the urogenital sinus.
Anomalies of the External Genitalia
- Double penis, a rare condition, is due to longitudinal division of the genital tubercle.
- Hypospadias is a condition in which the urethra opens on the under surface of the penis. It is caused by failure of fusion of the urethral folds.
- Epispadias is an uncommon anomaly in which there is a urethral groove on the upper surface of the penis. It is said by Wyburn (1937) to be caused by a minor failure of the mesoderm to develop in the region of the genital tubercle.
- Stenosis of the urethra is due to failure of the urethral cord to acquire a lumen.
- Phimosis is a condition of the prepuce in which there is an abnormally small opening.
- Hypertrophy of the labia minora and/or clitoris may be found.
Hermaphroditism is a condition in which the individual possesses the genital organs of both sexes. In the human it may be either true hermaphroditism or false hermaphroditism. The former condition is excessively rare, the subject having the genital glands of both sexes with external genitalia which may be of male, female or mixed type. False hermaphroditism is much more common and here the individual has genital glands of one sex with external genitalia and secondary sex characters of the other. Thus in one type, testes are present but the external genitalia are retarded in development and resemble those of the female. An opposite type will show an enlarged clitoris and perhaps fused labia minora, resembling thus a male while ovaries are also present.
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