The role of the mesonephros in cell differentiation and morphogenesis of the mouse fetal testis

In mouse fetal gonads, the adjacent mesonephros is required for seminiferous cord formation in vitro (Buehr et al., Development 117: 273-281, 1993). Here, we have investigated the role of mesonephric cells in seminiferous cord formation and in differentiation of Sertoli and Leydig cells. Undifferentiated male gonads with and without mesonephros at 11.5 dpc were cultured and immunocytochemical staining of Müllerian-inhibiting substance (MIS) was used as the criterion for Sertoli cell differentiation. For Leydig cells, testosterone (T) radioimmunoassay and cytochemical detection of delta 5-3 beta-hydroxysteroid-dehidrogenase (HSD) were undertaken. An ultrastructural study was also performed. In our culture conditions, the timing of differentiation of both Sertoli and Leydig cells was similar to that in the fetus. Although mesonephros is required for seminiferous cord formation, differentiation of Sertoli and Leydig cells proceeds in its absence at 11.5 dpc. Moreover, when 3H-thymidine labeled mesonephroi were grafted to unlabeled gonads, endothelial and peritubular myoid-like cells migrated into the gonad. We propose that these cells might be the mesonephric cells required for seminiferous cord formation.