DESCENT OF THE TESTIS
The early genital ridge on the posterior wall of the coelomic cavity contains the primordial testis and extends from the sixth thoracic to the second sacral segment. At 8 weeks’ gestation, the testis, lying beneath the mesothelium (primitive peritoneum), becomes an elongated, spindle-shaped organ projecting into the coelomic cavity (future abdominal cavity).
The mesothelium is thrown into two folds: the upper, diaphragmatic or cranial suspensory ligament extends to the diaphragm, whereas the lower, inguinal ligament or future gubernaculum terminates in the lower abdominal wall at a site where the inguinal bursa (future inguinal canal) is to develop. A pouch-like peritoneal evagination of the abdominal wall, termed the processus vaginalis, emerges during the sixth month. It grows to become the inguinal bursa, which, by the end of the seventh month is large enough to admit the testis. Concurrently, as a result of an involution of the cranial and adjacent mesonephros, the testis becomes mobile and is left suspended from the epididymis by the mesorchium, a fold of primitive peritoneum. By 7 months, the gonad is located several millimeters above the groin, with its long axis oriented obliquely or at right angles to the embryo.
At the end of the seventh month, the testes pass inferiorly through the inguinal canal. However, it is not uncommon to ﬁnd them in the canal at birth, with ﬁnal descent occurring postnatally. At the time of testis descent into the processus vaginalis within the inguinal bursa and scrotum, the portion of this processus vaginalis superior to the testis becomes obliterated sometimes weeks or months after birth. Persistence of the processus vaginalis after birth can result in what is called a communicating hydrocele, in which peritoneal ﬂuid freely enters the tunical vaginalis space within the scrotum. This type of hydrocele is characterized by dramatic changes in size when assuming an upright or supine position.
The gubernaculum, originally discernible as a ﬁbrous band in early fetal life, develops as the lower inguinal ligament and increases in size through the seventh month of gestation. It connects the upper end of the wolfﬁan duct (epididymis), and with it the testis, to the lower abdominal wall. The distal attachment of the gubernaculum extends to the region of the inguinal bursa where the future external oblique layer of the abdominal wall develops.
The role of the gubernaculum in the descent of the testis is incompletely understood. What is known is that testis descent occurs in two stages: transabdominal migration and inguinoscrotal descent. Failure of either stage results in varying degrees in what is termed undescended testis or cryptorchidism. Initially, the gubernaculum contracts and thickens to guide migration of the testis toward the internal inguinal ring. In mice, this migration appears to be controlled by a testisderived insulin-like/relaxin-like peptide (Insl-3). The human homologue of the mouse Insl3 gene has been identiﬁed as an insulin and relaxin-like molecule (INSL3) and is produced by Leydig cells. In studies of cryptorchid boys, mutations in the INSL3 gene only occur in 1% to 2% of cases, suggesting that other factors must also play a role in testis descent. The second phase of descent, transinguinal to scrotal, is thought to be androgen dependent. This is surmised from conditions such as androgen insensitivity (faulty androgen receptor activity) and Kallmann syndrome (defective androgen production) in which there is trans-abdominal but not inguinoscrotal descent observed. It also follows that endocrine disruptors that alter androgen balance in the third fetal trimester may also predispose male infants to cryptorchidism.