At 3 weeks of development, the germ cells migrate from the yolk sac to the genital ridge. From the 4th to the 8th week, in male embryos with a normal sex determining region on the short arm of the Y chromosome, the germ cells coalesce to form the primordial testis. Under the influence of human chorionic gonadotropin , the Leydig cells of the developing testis begin to secrete testosterone.
At about 9 weeks of development, the labioscrotal swellings fuse to form the scrotum (see movie). Testosterone also induces development of the mesonephric (Wolfian) duct to form the epididymis, vas deferens and seminal vesicles. During this stage of development, the testis moves from the genital ridge across the pelvis to lie at the internal inguinal ring.
The processus vaginalis appears at about 13 weeks of development as an outpouching of the parietal peritoneum. This developing tunnel moves medial and caudal between the internal and external abdominal oblique muscles and into the scrotum. The testis stays at the opening of the processus vaginalis, the internal inguinal ring, for 10 to 12 weeks. This herniation of the patent processus is at least partially dependent on the abdominal wall musculature to generate an elevated intra-abdominal pressure. If the abdominal muscles cannot increase intra-abdominal pressure, the patent processus vaginalis may not progress through the inguinal canal and the testis may not descend into the scrotum
At 26 to 36 weeks of development the epididymis precedes the testis into the processus vaginalis. These structures descend into the scrotum and become fused with the posterior layers of the scrotum, providing an anchor which prevents the testis from rotating. At 37 to 40 weeks (full term), the processus vaginalis closes, eliminating any communication between the peritoneum and the inguinal canal or scrotum.
|As the mesonephric duct develops into the epididymis, a
proximal remnant (or more than one remnant) may persist as a small appendage,
the appendix epididymis. This tissue is most often attached to the caput
epididymis (the most proximal and cephalad portion). Occasionally, such
an appendix can twist and become inflamed. See
a case history.
Simultaneously, the paramesonephric structures (Müllerian) regress under the influence of Müllerian inhibiting substance secreted by the Sertoli cells of the developing testis. The most proximal remnant of the Müllerian duct persists as the appendix testis.
See abnormal testis/scrotal development.
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©David A. Hatch, M.D., 1996