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.
