Urogenital fold and genital ridge

The intraembryonic celomic cavity is formed by about the fourth week of gestation by the separation of the primitive mesoderm into a core that is covered with celomic mesothelium , some of which will become the germinal epithelium on the urogenital ridge ( Fig. 17-1 A).

A, Cross section. B, Oblique view.

(Adapted from Jírasek JE: Atlas of Human Prenatal Morphogenesis. Boston, Martinus Nijhoff Publishers, 1983.)

As the mesonephros grows, the mesoderm from the posterior body wall becomes extruded into the celomic cavity , principally by proliferation of the celomic epithelium covering its medial surfaces ( Fig. 17-1 B). This forms the urogenital ridge , which will later contain the mesonephros, müllerian (para mesonephric) and wolffian (mesonephric) ducts , and the reproductive gland. The testes develop from celomic mesothelium and underlying mesenchyme.

The postcardinal vein lies dorsally and the wolffian duct, posterolaterally.

Mesonephric and genital ridges

The urogenital ridge is divided longitudinally into a medial genital ridge and a lateral mesonephric ridge . It becomes partially separated from the body wall by the formation of a urogenital mesentery ( Fig. 17-2 ). The genital portion will subsequently acquire its own mesentery, the mesorchium or mesovarium.

A, Cross section. B, Oblique view.

Primordial germ cells

The germinal cells in the gonad arise from primordial germ cells (primary gonocytes) early in ontogenesis. These first appear in the caudal part of the yolk sac adjacent to the allantoic duct and migrate by ameboid movement dorsally to the angle between the median dorsal mesentery and the genital ridge ( Fig. 17-3 A). Proliferating celomic epithelium lies over the portion of the urogenital ridge destined to become the genital ridge.

Cords of epithelial cells grow onto the mesenchyme of the ridge (the gonadal blastema ), forming the primary sex (gonadal) cords into which the germ cells become incorporated ( Fig. 17-3 B).

Formation of the seminiferous elements

Between 7 and 8 weeks, in the presence of H-Y antigen, the gonad rapidly differentiates into a testis while the müllerian (paramesonephric) duct degenerates.

The sex cords by epithelial proliferation form a cortex that surrounds the mesenchymal medulla derived from the gonadal blastema. Extensions from the mesenchyme will separate the cords from the celom and become the tunica albuginea, which extends into the gonad as trabeculae ( Fig. 17-4 A).

As the sex cords lengthen and extend into the medulla to form seminiferous tubules , they join to form the rete testis ( Fig. 17-4 B). The rete connects the seminiferous tubules to the efferent ductules derived from the few mesonephric tubules lying nearby, some of which may remain as appendages to the testis and epididymis. The efferent ductules are extensions of the epididymal duct (derived from the wolffian duct) that provide egress from the sex cords.

The attachment of the sex cords (seminiferous cords) to the germinal epithelium is lost by the interposition of the thick mesenchymal tunica albuginea that forms trabeculae .

Sustentacular (Sertoli) cells arise from germinal epithelium of the surface of the gland. The interstitial (Leydig) cells form from fibroblast-like cells in the local mesenchyme around the seminiferous cords and possibly from loose epithelial cells not taken up in formation of the tubules. Thus, not only does testicular differentiation occur early, but so does the secretion of hormones from these two types of cells that are essential for the differentiation of the genital ducts and external genitalia.

It is not until the 14th to 18th week that the sex cords acquire lumens and differentiate into seminiferous tubules, tubuli recti, and a more organized rete testis. The rete connects 15 to 20 residual mesonephric tubules, which are the efferent ductules, with the mesonephric duct to form the epididymis. The gonocytes from the primitive germ cells move to the periphery of the testis and become spermatogonia in increasing numbers up to the 17th week, with the multiplication ceasing around the 20th week. The germinal epithelium flattens to form the mesothelium surrounding the testis.

By the second postnatal year, the spermatogonia start differentiating into spermatocytes, maturation that is delayed in cryptorchidism. In the 11th and 12th years, the testis shows significant growth and subsequently matures with puberty.

Urogenital union occurs after 12 weeks, when the paragenital portion of the mesonephric tubules sends out collecting tubules into the indifferent reproductive gland. There, they become surrounded by the inner epithelial cords that will form the rete testis. The tubules of the rete then join the collecting tubules, now termed efferent ductules, completing the union. Those paragenital tubules not taken up in the union remain as vestigial structures.

The blood supply to the primitive testis is derived from the 30 pairs of mesonephric arteries that arise from the aorta (see Fig. 12-7 ), about one-third of which contribute to the urogenital arterial rete that supplies the kidneys and the developing testes. The lowest of the pairs does not become obliterated and forms the testicular artery. That this vessel branches from the urogenital arterial rete accounts for the variability of its origin. The testicular artery runs through the mesonephric fold above the testis, then passes caudally as an unbranched medial descending limb to enter the tunica albuginea at the lower pole. From there, it circles the lower pole to run cranially as the branched lateral ascending limb on the ventral and dorsal surfaces of the testis. Further branching is shown in Figure 17-38 . In the female, the artery meets the ovary at hilus, from which it distributes branches.

Mesorchium and gubernaculum

The epithelium on the medial surface of the urogenital ridge thickens to form two longitudinal projections into the celom: (1) the gonadal (genital) ridge medially and (2) the mesonephric ridge laterally ( Fig. 17-5 ).

The gonadal ridge may be divided longitudinally into a lateral tubal portion, containing the wolffian and müllerian ducts, and a medial gonadal portion. At the upper end, the portions fuse to form the thin diaphragmatic ligament (urogenital mesentery) and, below, they join to form the testicular ligament , the two ligaments together constituting the mesorchium. The testicular ligament will become the gubernaculum, a structure that is continuous with the mesenchyme of the future inguinal canal (see Fig. 17-6 ).

Gubernacular growth is an end effect of the hormonal chain: hypothalamic gonadotropic-releasing hormone, pituitary-luteinizing hormone, Leydig cell testosterone, and enzymatically reduced dihydrotestosterone formed locally, to the cells of the gubernaculum by a steroid receptor complex.

The gubernaculum is present before the development of the abdominal musculature, a sequence that promotes formation of the inguinal canal. The gubernaculum appears to hold the testis near the canal during the rapid growth of the lumbar vertebral column that has the effect of carrying the kidney cephalad. Without the gubernaculum, the testis would lie high in the retroperitoneal space. Thus, the pelvic position of the testis comes not so much from retroperitoneal descent of the testis as from its distal fixation as the kidney rises.

Development of the duct system in the male

The gubernaculum attached to the lower pole of the testis will extend to the inguinal canal. The müllerian duct has fused, and a remnant remains near the testis.

Under the influence of testosterone, the wolffian (mesonephric) duct gains a thick layer of muscle from which the canal of the epididymis, the vas deferens, and the ejaculatory duct are formed. The upper part of the epididymal canal becomes coiled, and the lower portion expands to form the ampulla. There, the mesonephric duct gives off the primordia of the seminal vesicle , with the ejaculatory duct lying distally ( Fig. 17-6 A).

Through the action of 5-alpha reductase, dihydrotestosterone that is produced locally from testosterone induces the mesenchyme of the endodermal urethral epithelium of the urogenital sinus to differentiate into the prostate and bulbourethral glands ( Fig. 17-6 B) (also see Figure 14-4 ). The müllerian elements disappear as a result of the action of müllerian-inhibiting substance, except for a remnant on the end of each testis as the appendix testis and in the posterior urethra in the verumontanum as the prostatic utricle or vagina masculina.

Homologies with female urogenital structures are shown in Table 17-1 .

Appendages of the testis and epididymis

Appendages derived from vestigial portions of the wolffian (mesonephric) and müllerian (paramesonephric) systems are organized in Table 17-2 ( Fig. 17-7 ).

The cranial aberrant ductules (appendix epididymis) represent the persistence of the one or two most cranial mesonephric tubules ( Figs. 17-8 and 17-9 ). It is almost always pedunculated. The next five or six mesonephric tubules form the definitive efferent ductules of the testis and the lobules of the head of the epididymis. In the female, they form the tubules of the epoöphoron. The superior and inferior caudal aberrant ductules that form the vas aberrans (Haller), the diverticulum of the epididymal tail , and the paradidymis (the organ of Giraldés) come from the most caudal mesonephric tubules. The paradidymis is homologous with the paroöphoron in the female.

Appendages of testis and epididymis. In the image at left, an appendix testis and a single appendix epididymis are present. In the image at right, the dark-colored pedunculated structure and the small pedunculated structure indicated by the fine forceps are both epididymal appendices; the structure held by the large forceps is an appendix testis.

Appendix epididymis. This wolffian remnant is found in about one-third of testes. It is a pedunculated cystic epithelial-lined spherical or ovoid structure that arises from the anterosuperior pole of the head of the epididymis. The wall of the cyst is loose connective tissue; its outer surface is at least partially lined by cells of the tunica vaginalis.

A vestige of the müllerian duct remains as the appendix testis (hydatid of Morgagni) ( Figs. 17-8 and 17-10 ) on the anterior superior pole of the testis. It may be on a stalk and even may be fimbriated like a small fallopian tube. In one autopsy study, it was found to be pedunculated in 82% of cases, at least on one side. It differs from the other appendages in its greater vascularity and in having ducts that communicate with the tunic. Painful torsion of the appendix testis is not uncommon and is probably fostered by a particularly long pedicle. The less-developed but pedunculated appendix epididymis and the other appendages only infrequently become symptomatic ( Figs. 17-11 and 17-12 ).

Appendix testis. This müllerian remnant is present in more than 90% of testes, as a sessile or polypoid mass, 2 to 4 mm in size, located at the superior pole of the testis near the epididymis. It consists of an epithelial-lined fibrovascular core composed of loose connective tissue, and is covered by simple cuboidal or low columnar müllerian-type epithelium that merges with the adjacent tunica vaginalis.

Torsion of appendix epididymis. This 12-year-old boy was found at surgery to have a large edematous and hemorrhagic appendix epididymis, indicated by the tip of the forcep.

(Image courtesy of Edward Cherullo, MD.)

Torsion of appendix epididymis. A cross-section of the lesion shown in Figure 17-11 is at top. Lesion is centrally cystic, with a thin fibrous wall; the yellow-tan material is fibrinous exudate. At bottom, a section from the wall shows residual epithelium overlying edematous and markedly hemorrhagic stroma.

The derivations of the vestigial structures are shown in Table 17-3 .

Testis and inguinal canal

The first movement by the testis as it lies suspended by the mesorchium from the dorsal abdominal wall occurs when the cranial end of the mesorchium, the diaphragmatic ligament , degenerates and leaves the testis in a lower position. The testis becomes attached to the lower ventral abdominal wall by an inguinal fold of peritoneum containing its blood and nerve supply. The caudal end of the mesorchium, a cord of mesenchyme destined to become the gubernaculum , lies inside a peritoneal fold that will extrude to join the skin of the lower part of the anterior abdominal wall at the genital swellings, the future site of the scrotum. Up to this time, the gubernaculum has been an intraperitoneal organ, but as the scrotum develops, it becomes surrounded by the muscles of the anterior abdominal wall that make up the developing inguinal canal .

By the 12th week, the anterior abdominal wall elongates to accommodate the intestines and the body bends into an upright position at the same time that the lumbar segments grow. The umbilical artery, which is anchored between the aorta and the umbilicus, becomes elevated and lifts a fold of overlying peritoneum medial to the testis. This forms a partition separating the primitive true pelvis from lateral inguinal fossa. It is from this fossa that the peritoneum evaginates into the scrotum as the processus vaginalis, ventral to the gubernaculum. The defect in the transversus abdominis constituting the internal inguinal ring moves laterally, with the result that the gubernaculum must take an oblique course to reach the relatively fixed external inguinal ring.

As the peritoneum evaginates to form the processus vaginalis , it carries with it the elements of the anterior body wall so that the testis, with the tunica vaginalis, becomes covered with a succession of layers during its passage through the inguinal canal ( Fig. 17-13 ). The inner layer, the internal spermatic fascia, is continuous with, but not part of, the outer stratum of retroperitoneal tissue of the pelvis from which the transversalis fascia arises, thus accounting for the belief by some anatomists that it is derived from the transversalis fascia. Strictly speaking, the internal spermatic fascia is not a true fascial layer, but is the connective tissue from the intermediate stratum that embeds the cord structures. It lies within the cremasteric layer, to which it is densely adherent. The second covering, the cremasteric fascia and cremasteric muscle, is a continuation of the internal oblique and transversus abdominis . The outer coat, the external spermatic fascia, is an extension from the innominate fascia associated with the external oblique (see Fig. 9-6 ).

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