Anatomical and Functional Aspects of Testicular Descent and Cryptorchidism*

Abstract

TESTICULAR descent to the scrotum is a profound example of sexual dimorphism that cannot be fully explained as yet. It is not a simple process, but appears to be multistaged, with various anatomical factors and hormonal influences. During mammalian evolution the male gonad has assumed a progressively lower position relative to that of the ovary, eventually taking up an extraabdominal location within a scrotum in most modern mammals (1, 2). The exact site and structure of the scrotum varies among species with, for example, some modern marsupials having a prepenile scrotum (3). The essential physiological feature, however, is that the scrotum is a specialized, low-temperature environment that allows the extraabdominal testis to be maintained at a temperature below that of the rest of the body (4). 1. Sexual development. The urogenital ridge in humans is identical morphologically in males and females up to 7–8 weeks of gestation (5). Sexual differentiation begins with the testis-determining gene (SRY) on the Y chromosome triggering testicular differentiation by unknown mechanisms (6). Recent studies suggest a role for steroidogenic factor 1 (the Ad4-binding protein) in gonadal development (7, 8) and regulation of the Müllerian inhibiting substance (MIS) or anti-Müllerian hormone gene (9). MIS itself also has been proposed to have a role in gonadal differentiation (10). Once a testis is formed, both MIS and testosterone are involved in altering the anatomy of the male embryo (11, 12). MIS causes regression of the Müllerian ducts whereas testosterone secretion directly into the Wolffian duct permits its continuing development into epididymis, vas deferens, and seminal vesicles (13, 14).