Testicular differentiation in mammals under normal and experimental conditions

Gonadal differentiation begins with the establishment of a sexually undifferentiated gonad, in which gonadal cords are formed by condensation of somatic cells and deposition of basal laminar components around the cluster of epithelial-like cells. The first event of sexual differentiation is the invasion of mesenchymal and endothelial cells into the genital ridge in the XY gonad. As a consequence of this event, the gonadal cords become conspicuous, recognized as seminiferous cords (or testis cords). Cytological differentiation of Sertoli cells follows these stromal changes. In the XX gonad, by contrast, the invasion of the mesenchyme is absent and gonadal cords remain associated with the surface epithelium. In the B6.YDOM XY ovotestis, seminiferous cords and ovarian gonadal cords are often enveloped by common basal laminae, confirming that both structures share the embryonic origin. It has been recently reported that seminiferous-like cords are formed after loss of oocytes in the rat XX ovary cultured in the presence of Müllerian inhibiting substance or after long-term culture in the basic medium alone. These results are comparable with our observation on the persistent gonadal cords in the ovary of busulphan-treated rats or W/WV mutant mice, in which oogonia are absent or scarce. Ultrastructural evidence for Sertoli cell differentiation from XX cells has been presented, so far, only in the fetal mouse ovary that has been grafted beneath the kidney capsule of adult male mice. Possible mechanism of gonadal sex determination is discussed based on these morphological studies.