MESONEPHRIC STROMAL CELLS DIFFERENTIATE INTO LEYDIG-CELLS IN THE MOUSE FETAL TESTIS

Circumstantial evidence has suggested that Sry expression probably occ urs in pre-Sertoli cells, implying that they produce signals required for testis differentiation. From experiments involving gonad/mesonephr os grafts it has been shown that, at 11.5 days postcoitum stromal cell s from the mesonephros invade the male gonad. Although in the grafted testes, Leydig cells appeared among the stromal cells, in these studie s their origin remained elusive. In the current study, we reconstructe d urogenital ridges in organ culture by grafting morphologically undif ferentiated male genital ridges from CD-1 embryos, to mesonephroi from ROSA26 transgenic embryos whose cells express the bacterial beta-gala ctosidase. With an improved technique for the detection of beta-gal en zyme activity in electronmicrographs, we studied cell migration and di fferentiation of mesonephric cells into the testis in reconstructed ur ogenital ridges with XY or XX mesonephroi. It was found that, in addit ion to differentiation of myoid and connective cells, some migratory m esonephric cells acquired ultrastructural features of steroidogenic Le ydig cells. Several beta-gal positive cells differentiated as Leydig c ells in go. nads grafted with either male or female mesonephros. The r esults suggest that mesonephric cells responded to putative signal(s) produced in the male gonad and participate in morphogenesis and cell d ifferentiation of the fetal testis. (C) 1998 Academic Press.