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urogenital development

Monday 27 November 2006

The urinary and generative organs are developed from the intermediate cell-mass which is situated between the primitive segments and the lateral plates of mesoderm.

The permanent organs of the adult are preceded by a set of structures which are purely embryonic, and which with the exception of the ducts disappear almost entirely before the end of fetal life.

These embryonic structures are on either side; the pronephros, the mesonephros, the metanephros, and the Wolffian and Müllerian ducts.

The pronephros disappears very early; the structural elements of the mesonephros mostly degenerate, but in their place is developed the genital gland in association with which the Wolffian duct remains as the duct of the male genital gland, the Müllerian as that of the female; some of the tubules of the metanephros form part of the permanent kidney.

The Pronephros and Wolffian Duct

In the outer part of the intermediate cell-mass, immediately under the ectoderm, in the region from the fifth cervical to the third thoracic segments, a series of short evaginations from each segment grows dorsalward and extends caudalward, fusing successively from before backward to form the pronephric duct.

This continues to grow caudalward until it opens into the ventral part of the cloaca; beyond the pronephros it is termed the Wolffian duct.

The original evaginations form a series of transverse tubules each of which communicates by means of a funnel-shaped ciliated opening with the celomic cavity, and in the course of each duct a glomerulus also is developed. A secondary glomerulus is formed ventral to each of these, and the complete group constitutes the pronephros. The pronephros undergoes rapid atrophy and disappears.

The Mesonephros, Müllerian Duct, and Genital Gland

On the medial side of the Wolffian duct, from the sixth cervical to the third lumbar segments, a series of tubules, the Wolffian tubules, is developed; at a later stage in development they increase in number by outgrowths from the original tubules.

These tubules first appear as solid masses of cells, which later become hollowed in the center; one end grows toward and finally opens into the Wolffian duct, the other dilates and is invaginated by a tuft of capillary bloodvessels to form a glomerulus. The tubules collectively constitute the mesonephros or Wolffian body.

By the fifth or sixth week this body forms an elongated spindle-shaped structure, termed the urogenital fold, which projects into the celomic cavity at the side of the dorsal mesentery, reaching from the septum transversum in front to the fifth lumbar segment behind; in this fold the reproductive glands are developed.

The Wolffian bodies persist and form the permanent kidneys in fishes and amphibians, but in reptiles, birds, and mammals, they atrophy and for the most part disappear coincidently with the development of the permanent kidneys. The atrophy begins during the sixth or seventh week and rapidly proceeds, so that by the beginning of the fifth month only the ducts and a few of the tubules remain.

In the male the Wolffian duct persists, and forms the tube of the epididymis, the ductus deferens and the ejaculatory duct, while the seminal vesicle arises during the third month as a lateral diverticulum from its hinder end.

A large part of the head end of the mesonephros atrophies and disappears; of the remainder the anterior tubules form the efferent ducts of the testis; while the posterior tubules are represented by the ductuli aberrantes, and by the paradidymis, which is sometimes found in front of the spermatic cord above the head of the epididymis.

In the female the Wolffian bodies and ducts atrophy. The remains of the Wolffian tubules are represented by the epoöphoron or organ of Rosenmüller, and the paroöphoron, two small collections of rudimentary blind tubules which are situated in the mesosalpinx.

The lower part of the Wolffian duct disappears, while the upper part persists as the longitudinal duct of the epoöphoron or duct of Gärtner.

The Müllerian Ducts

Shortly after the formation of the Wolffian ducts a second pair of ducts is developed; these are named the Müllerian ducts. Each arises on the lateral aspect of the corresponding Wolffian duct as a tubular invagination of the cells lining the celom.

The orifice of the invagination remains patent, and undergoes enlargement and modification to form the abdominal ostium of the uterine tube. The ducts pass backward lateral to the Wolffian ducts, but toward the posterior end of the embryo they cross to the medial side of these ducts, and thus come to lie side by side between and behind the latter—the four ducts forming what is termed the genital cord.

The Müllerian ducts end in an epithelial elevation, the Müllerian eminence, on the ventral part of the cloaca between the orifices of the Wolffian ducts; at a later date they open into the cloaca in this situation.

In the male the Müllerian ducts atrophy, but traces of their anterior ends are represented by the appendices testis (hydatids of Morgagni), while their terminal fused portions form the utriculus in the floor of the prostatic portion of the urethra.

In the female the Müllerian ducts persist and undergo further development. The portions which lie in the genital core fuse to form the uterus and vagina; the parts in front of this cord remain separate, and each forms the corresponding uterine tube—the abdominal ostium of which is developed from the anterior extremity of the original tubular invagination from the celom.

The fusion of the Müllerian ducts begins in the third month, and the septum formed by their fused medial walls disappears from below upward, and thus the cavities of the vagina and uterus are produced.

About the fifth month an annular constriction marks the position of the neck of the uterus, and after the sixth month the walls of the uterus begin to thicken.

For a time the vagina is represented by a solid rod of epithelial cells. A ring-like outgrowth of this epithelium occurs at the lower end of the uterus and marks the future vaginal fornices; about the fifth or sixth month the lumen of the vagina is produced by the breaking down of the central cells of the epithelium. The hymen represents the remains of the Müllerian eminence.

Genital Glands

The first appearance of the genital gland is essentially the same in the two sexes, and consists in a thickening of the epithelial layer which lines the peritoneal cavity on the medial side of the urogenital fold.

The thick plate of epithelium extends deeply, pushing before it the mesoderm and forming a distinct projection. This is termed the genital ridge, and from it the testis in the male and the ovary in the female are developed.

At first the mesonephros and genital ridge are suspended by a common mesentery, but as the embryo grows the genital ridge gradually becomes pinched off from the mesonephros, with which it is at first continuous, though it still remains connected to the remnant of this body by a fold of peritoneum, the mesorchium or mesovarium.

About the seventh week the distinction of sex in the genital ridge begins to be perceptible.

The Ovary

The ovary, thus formed from the genital ridge, is at first a mass of cells derived from the celomic epithelium; later the mass is differentiated into a central part or medulla covered by a surface layer, the germinal epithelium.

Between the cells of the germinal epithelium a number of larger cells, the primitive ova, are found, and these are carried into the subjacent stroma by bud-like ingrowths (genital cords) of the germinal epithelium.

The surface epithelium ultimately forms the permanent epithelial covering of this organ; it soon loses its connection with the central mass, and a tunica albuginea develops between them.

The ova are chiefly derived from the cells of the central mass; these are separated from one another by the growth of connective tissue in an irregular manner; each ovum assumes a covering of connective tissue (follicle) cells, and in this way the rudiments of the ovarian follicles are formed.

According to Beard the primitive ova are early set apart during the segmentation of the ovum and migrate into the germinal ridge.

The Testis

The testis is developed in much the same way as the ovary. Like the ovary, in its earliest stages it consists of a central mass of epithelium covered by a surface epithelium. In the central mass a series of cords appear, and the periphery of the mass is converted into the tunica albuginea, thus excluding the surface epithelium from any part in the formation of the tissue of the testis.

The cords of the central mass run together toward the future hilus and form a network which ultimately becomes the rete testis. From the cords the seminiferous tubules are developed, and between them connective-tissue septa extend.

The seminiferous tubules become connected with outgrowths from the Wolffian body, which, as before mentioned, form the efferent ducts of the testis.

Descent of the Testes.—The testes, at an early period of fetal life, are placed at the back part of the abdominal cavity, behind the peritoneum, and each is attached by a peritoneal fold, the mesorchium, to the mesonephros. From the front of the mesonephros a fold of peritoneum termed the inguinal fold grows forward to meet and fuse with a peritoneal fold, the inguinal crest, which grows backward from the antero-lateral abdominal wall. The testis thus acquires an indirect connection with the anterior abdominal wall; and at the same time a portion of the peritoneal cavity lateral to these fused folds is marked off as the future saccus vaginalis. In the inguinal crest a peculiar structure, the gubernaculum testis, makes its appearance. This is at first a slender band, extending from that part of the skin of the groin which afterward forms the scrotum through the inguinal canal to the body and epididymis of the testis. As development advances, the peritoneum enclosing the gubernaculum forms two folds, one above the testis and the other below it. The one above the testis is the plica vascularis, and contains ultimately the internal spermatic vessels; the one below, the plica gubernatrix, contains the lower part of the gubernaculum, which has now grown into a thick cord; it ends below at the abdominal inguinal ring in a tube of peritoneum, the saccus vaginalis, which protrudes itself down the inguinal canal. By the fifth month the lower part of the gubernaculum has become a thick cord, while the upper part has disappeared. The lower part now consists of a central core of unstriped muscle fiber, and outside this of a firm layer of striped elements, connected, behind the peritoneum, with the abdominal wall. As the scrotum develops, the main portion of the lower end of the gubernaculum is carried, with the skin to which it is attached, to the bottom of this pouch; other bands are carried to the medial side of the thigh and to the perineum. The tube of peritoneum constituting the saccus vaginalis projects itself downward into the inguinal canal, and emerges at the cutaneous inguinal ring, pushing before it a part of the Obliquus internus and the aponeurosis of the Obliquus externus, which form respectively the Cremaster muscle and the intercrural fascia. It forms a gradually elongating pouch, which eventually reaches the bottom of the scrotum, and behind this pouch the testis is drawn by the growth of the body of the fetus, for the gubernaculum does not grow commensurately with the growth of other parts, and therefore the testis, being attached by the gubernaculum to the bottom of the scrotum, is prevented from rising as the body grows, and is drawn first into the inguinal canal and eventually into the scrotum. It seems certain also that the gubernacular cord becomes shortened as development proceeds, and this assists in causing the testis to reach the bottom of the scrotum. By the end of the eighth month the testis has reached the scrotum, preceded by the saccus vaginalis, which communicates by its upper extremity with the peritoneal cavity. Just before birth the upper part of the saccus vaginalis usually becomes closed, and this obliteration extends gradually downward to within a short distance of the testis. The process of peritoneum surrounding the testis is now entirely cut off from the general peritoneal cavity and constitutes the tunica vaginalis.

Descent of the Ovaries.—In the female there is also a gubernaculum, which effects a considerable change in the position of the ovary, though not so extensive a change as in that of the testis. The gubernaculum in the female lies in contact with the fundus of the uterus and contracts adhesions to this organ, and thus the ovary is prevented from descending below this level. The part of the gubernaculum between the ovary and the uterus becomes ultimately the proper ligament of the ovary, while the part between the uterus and the labium majus forms the round ligament of the uterus. A pouch of peritoneum analogous to the saccus vaginalis in the male accompanies it along the inguinal canal: it is called the canal of Nuck. In rare cases the gubernaculum may fail to contract adhesions to the uterus, and then the ovary descends through the inguinal canal into the labium majus, and under these circumstances its position resembles that of the testis.

The Metanephros and the Permanent Kidney.—The rudiments of the permanent kidneys make their appearance about the end of the first or the beginning of the second month. Each kidney has a two-fold origin, part arising from the metanephros, and part as a diverticulum from the hind-end of the Wolffian duct, close to where the latter opens into the cloaca (Figs. 1115, 1116). The metanephros arises in the intermediate cell mass, caudal to the mesonephros, which it resembles in structure. The diverticulum from the Wolffian duct grows dorsalward and forward along the posterior abdominal wall, where its blind extremity expands and subsequently divides into several buds, which form the rudiments of the pelvis and calyces of the kidney; by continued growth and subdivision it gives rise to the collecting tubules of the kidney. The proximal portion of the diverticulum becomes the ureter. The secretory tubules are developed from the metanephros, which is moulded over the growing end of the diverticulum from the Wolffian duct. The tubules of the metanephros, unlike those of the pronephros and mesonephros, do not open into the Wolffian duct. One end expands to form a glomerulus, while the rest of the tubule rapidly elongates to form the convoluted and straight tubules, the loops of Henle, and the connecting tubules; these last join and establish communications with the collecting tubules derived from the ultimate ramifications of the diverticulum from the Wolffian duct. The mesoderm around the tubules becomes condensed to form the connective tissue of the kidney. The ureter opens at first into the hind-end of the Wolffian duct; after the sixth week it separates from the Wolffian duct, and opens independently into the part of the cloaca which ultimately becomes the bladder (Figs. 1117, 1118).

The secretory tubules of the kidney become arranged into pyramidal masses or lobules, and the lobulated condition of the kidneys exists for some time after birth, while traces of it may be found even in the adult. The kidney of the ox and many other animals remains lobulated throughout life.

The Urinary Bladder.—The bladder is formed partly from the entodermal cloaca and partly from the ends of the Wolffian ducts; the allantois takes no share in its formation. After the separation of the rectum from the dorsal part of the cloaca (p. 1109), the ventral part becomes subdivided into three portions: (1) an anterior vesico-urethral portion, continuous with the allantois—into this portion the Wolffian ducts open; (2) an intermediate narrow channel, the pelvic portion; and (3) a posterior phallic portion, closed externally by the urogenital membrane (Fig. 1118). The second and third parts together constitute the urogenital sinus. The vesico-urethral portion absorbs the ends of the Wolffian ducts and the associated ends of the renal diverticula, and these give rise to the trigone of the bladder and part of the prostatic urethra. The remainder of the vesico-urethral portion forms the body of the bladder and part of the prostatic urethra; its apex is prolonged to the umbilicus as a narrow canal, which later is obliterated and becomes the medial umbilical ligament (urachus).

The Prostate.—The prostate originally consists of two separate portions, each of which arises as a series of diverticular buds from the epithelial lining of the urogenital sinus and vesico-urethral part of the cloaca, between the third and fourth months. These buds become tubular, and form the glandular substance of the two lobes, which ultimately meet and fuse behind the urethra and also extend on to its ventral aspect. The isthmus or middle lobe is formed as an extension of the lateral lobes between the common ejaculatory ducts and the bladder. Skene’s ducts in the female urethra are regarded as the homologues of the prostatic glands.
The bulbo-urethral glands of Cowper in the male, and greater vestibular glands of Bartholin in the female, also arise as diverticula from the epithelial lining of the urogenital sinus.

The External Organs of Generation (Fig. 1119).—As already stated (page 1109), the cloacal membrane, composed of ectoderm and entoderm, originally reaches from the umbilicus to the tail. The mesoderm extends to the midventral line for some distance behind the umbilicus, and forms the lower part of the abdominal wall; it ends below in a prominent swelling, the cloacal tubercle. Behind this tubercle the urogenital part of the cloacal membrane separates the ingrowing sheets of mesoderm.
The first rudiment of the penis (or clitoris) is a structure termed the phallus; it is derived from the phallic portion of the cloaca which has extended on to the end and sides of the under surface of the cloacal tubercle. The terminal part of the phallus representing the future glans becomes solid; the remainder, which is hollow, is converted into a longitudinal groove by the absorption of the urogenital membrane.
In the female a deep groove forms around the phallus and separates it from the rest of the cloacal tubercle, which is now termed the genital tubercle. The sides of the genital tubercle grow backward as the genital swellings, which ultimately form the labia majora; the tubercle itself becomes the mons pubis. The labia minora arise by the continued growth of the lips of the groove on the under surface of the phallus; the remainder of the phallus forms the clitoris.

In the male the early changes are similar, but the pelvic portion of the cloaca undergoes much greater development, pushing before it the phallic portion. The genital swellings extend around between the pelvic portion and the anus, and form a scrotal area; during the changes associated with the descent of the testes this area is drawn out to form the scrotal sacs. The penis is developed from the phallus. As in the female, the urogenital membrane undergoes absorption, forming a channel on the under surface of the phallus; this channel extends only as far forward as the corona glandis.
The corpora cavernosa of the penis (or clitoris) and of the urethra arise from the mesodermal tissue in the phallus; they are at first dense structures, but later vascular spaces appear in them, and they gradually become cavernous.
The prepuce in both sexes is formed by the growth of a solid plate of ectoderm into the superficial part of the phallus; on coronal section this plate presents the shape of a horseshoe. By the breaking down of its more centrally situated cells the plate is split into two lamellæ, and a cutaneous fold, the prepuce, is liberated and forms a hood over the glans. “Adherent prepuce is not an adhesion really, but a hindered central desquamation” (Berry Hart, op. cit.).

The Urethra.—As already described, in both sexes the phallic portion of the cloaca extends on to the under surface of the cloacal tubercle as far forward as the apex. At the apex the walls of the phallic portion come together and fuse, the lumen is obliterated, and a solid plate, the urethral plate, is formed. The remainder of the phallic portion is for a time tubular, and then, by the absorption of the urogenital membrane, it establishes a communication with the exterior; this opening is the primitive urogenital ostium, and it extends forward to the corona glandis.

In the female this condition is largely retained; the portion of the groove on the clitoris broadens out while the body of the clitoris enlarges, and thus the adult urethral opening is situated behind the base of the clitoris.

In the male, by the greater growth of the pelvic portion of the cloaca a longer urethra is formed, and the primitive ostium is carried forward with the phallus, but it still ends at the corona glandis. Later it closes from behind forward. Meanwhile the urethral plate of the glans breaks down centrally to form a median groove continuous with the primitive ostium.

This groove also closes from behind forward, so that the external urethral opening is shifted forward to the end of the glans.

Animations

- Development of the female urogenital system

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Production: Maryam Razmpoosh, Alexande Oligny, Luc Oligny; Department of Pathology, CHU Sainte-Justine, Université de Montréal, QC.

- Development of the male urogenital system

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Production: Maryam Razmpoosh, Alexande Oligny, Luc Oligny; Department of Pathology, CHU Sainte-Justine, Université de Montréal, QC.

See also

- genital ducts
- genitourinary development
- gonadal ridge
- urogenital fold