Experimental control of the differentiation of Leydig cells in the rat fetal testis.
In the developing fetal testis, in vitro as well as in vivo, two kinds of endocrine cells differentiate successively: Sertoli cells, which produce the Müllerian inhibitor (or anti-Müllerian hormone) and aggregate with germ cells into seminiferous cords; and Leydig cells, which release androgens. Serum added to the synthetic culture medium prevents the morphogenesis of the seminiferous cords but not the cytodifferentiation of the endocrine cells. L-Azetidine 2-carboxylic acid (LACA), a proline competitor, introduced into the medium also prevents differentiation of seminiferous cords. In the present experiments, the effects of LACA on the endocrine cells were studied. It did not suppress production of the Müllerian inhibitor, but it opposed differentiation of Leydig cells. Histochemically detectable 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) was virtually absent and the release of testosterone, delta 4-androstenedione, 17-hydroxyprogesterone, or progesterone into the medium became undetectable. Moreover, dibutyryl cAMP added to the medium during the final day in vitro had very little effect on the parameters of steroidogenesis. An excess of proline added to the LACA-containing medium permitted normal morphogenesis of seminiferous cords, normal steroidogenesis, and normal response to cAMP. LACA did not prevent the appearance of 3 beta-HSD activity in the adrenals, nor did it reduce the expression of laminin and fibronectin (data not shown) in the mesonephric structures as much as in the testes. The differentiation of the testis and especially of the Leydig cells appears to have special requirements for proline.