Normal Development and Anatomy
Until the seventh week of gestation, the embryos of both sexes undergo identical development. During this time, genital ridges form on both sides of the midline. These ridges are composed of mesenchyme, and extend from the sixth thoracic to the second sacral segments. They are covered by proliferating mesothelium that subsequently forms the primitive sex cords. The primordial germ cells that form in the wall of the yolk sac migrate along the hindgut and dorsal mesenteric root into the genital ridge.
Under the influence of testis-determining factor (encoded by the Y chromosome), the primitive "indifferent" gonad begins to form testicular elements beginning at 42 to 44 days of gestation. Sertoli cells that are derived from coelomic epithelium aggregate around the germ cells in the form of "sex cords." The primitive sex cords form the seminiferous tubules. Therefore, the seminiferous tubules contain elements from 2 distinct cell lines. The mesothelium gives rise to the Sertoli or supporting cells, whereas the germ cells become the spermatogonia. The mesenchyme between the seminiferous tubules differentiates into the Leydig (interstitial) cells. Testicular descent into the pelvis occurs between the 7th and 12th weeks, and descent through the inguinal canal begins at approximately 36 weeks.1
Under the influence of testosterone secreted by the Leydig cells, the embryonic mesonephric ducts differentiate into the epididymis, vas deferens, seminal vesicles, and ejaculatory ducts. The mesonephros is a common factor in renal and internal genital organ development, and alterations during this early phase of embryogenesis likely account for combined anomalies of these structures. The Sertoli cells secrete Müllerian inhibiting factor, which causes the paramesonephric ducts (Müllerian ducts) to regress. Normal rudimentary structures derived from the paramesonephric ducts include the appendix testis and the prostatic utricle.2,3
The testis consists of seminiferous tubules separated by thin fibrous septa. The tunica albuginea is a dense fibrous capsule that surrounds the testis. The tunica vaginalis is a flattened layer of mesothelium that covers the tunica albuginea. The seminiferous tubules coalesce posteriorly to form larger ducts (the tubuli recti), which drain into the rete testis. The rete testis forms efferent ductules that penetrate through a thickened area of the tunica albuginea to form the head of the epididymis. Within the epididymis, the ductules converge to form a single tubule that exits the epididymis as the vas deferens. The thickened portion of the tunica albuginea at the head of the epididymis invaginates into the testis to form the mediastinum testis, which is the site of entry for the testicular ducts, nerves, and vessels. The Sertoli cells aid spermatogenesis by providing a support structure for maturing germ cells and by facilitating phagocytic removal of degenerating germ cells. The space between the seminiferous tubules consists of connective tissue, lymphatics, blood vessels, mast cells, and Leydig cells. The Leydig cells are the principal source of testosterone production.