The contribution of the mesonephros to the development of the sheep fetal testis.

In this study, which was performed on 52 sheep fetuses aged 24 to 85 days, we examined the relationship between testicular development and the "giant" mesonephric nephron, a peculiar structure consisting of a large glomerulus and multiple tubules. The development of the testis occurred in two phases. The preparatory phase began at day 24 of gestation, evolved simultaneously with the involution of the glomerulus of the giant nephron, and was characterized by mobilization of the glomerular cells and by their colonization of the genital ridge. By day 31, a prominent mass of migrating mesonephric cells had developed; it extended uninterrupted from the giant glomerulus into the gonad where the mesonephric cells associated with the germinal cells forming the cellular template from which the testicular cords later became assembled. At complete involution of the glomerulus (day 52), the migratory mass implanted on the tubules of the giant nephron, which thus became continuous with the gonad. During the organizational phase, which began at day 29 with the formation of the tunica albuginea, the various components of this continuum became progressively organized along a testis-to-mesonephros direction into seminiferous cords, tubuli recti, the cords of the rete testis, and the ductules efferentes. These observations show that, in the sheep, the precursors of the Sertoli cells are mesonephric in origin, and that the genital tract proximal to the epididymis differentiates from a single mesonephric nephron.