Mesonephric stromal cells differentiate into Leydig cells in the mouse fetal testis.

Circumstantial evidence has suggested that Sry expression probably occurs in pre-Sertoli cells, implying that they produce signals required for testis differentiation. From experiments involving gonad/mesonephros grafts it has been shown that, at 11.5 days postcoitum stromal cells from the mesonephros invade the male gonad. Although in the grafted testes, Leydig cells appeared among the stromal cells, in these studies their origin remained elusive. In the current study, we reconstructed urogenital ridges in organ culture by grafting morphologically undifferentiated male genital ridges from CD-1 embryos, to mesonephroi from ROSA26 transgenic embryos whose cells express the bacterial beta-galactosidase. With an improved technique for the detection of beta-gal enzyme activity in electronmicrographs, we studied cell migration and differentiation of mesonephric cells into the testis in reconstructed urogenital ridges with XY or XX mesonephroi. It was found that, in addition to differentiation of myoid and connective cells, some migratory mesonephric cells acquired ultrastructural features of steroidogenic Leydig cells. Several beta-gal positive cells differentiated as Leydig cells in gonads grafted with either male or female mesonephros. The results suggest that mesonephric cells responded to putative signal(s) produced in the male gonad and participate in morphogenesis and cell differentiation of the fetal testis.