THE ROLE OF THE MESONEPHROS IN CELL-DIFFERENTIATION AND MORPHOGENESISOF THE MOUSE FETAL TESTIS

In mouse fetal gonads, the adjacent mesonephros is required for semini ferous cord formation in vitro (Buehr et al., Development 117: 273-281 ,1993). Here, we have investigated the role of mesonephric cells in se miniferous cord formation and in differentiation of Sertoli and Leydig cells. Undifferentiated male gonads with and without mesonephros at 1 1.5 dpc were cultured and immunocytochemical staining of Mullerian-inh ibiting substance (MIS) was used as the criterion for Sertoli cell dif ferentiation. For Leydig cells, testosterone (T) radioimmunoassay and cytochemical detection of DELTA5-3beta-hydroxysteroid-dehidrogenase (H SD) were undertaken. An ultrastructural study was also performed. In o ur culture conditions, the timing of differentiation of both Sertoli a nd Leydig cells was similar to that in the fetus. Although mesonephros is required for seminiferous cord formation, differentiation of Serto li and Leydig cells proceeds in its absence at 11.5 dpc. Moreover, whe n 3H-thymidine labeled mesonephroi were grafted to unlabeled gonads, e ndothelial and peritubular myoid-like cells migrated into the gonad. W e propose that these cells might be the mesonephric cells required for seminiferous cord formation.