Steroidogenesis in the gonads of rainbow trout fry (Salmo gairdneri) before and after the onset of gonadal sex differentiation.

Gonadal homogenates of rainbow trout from D(ay) 60, D100 and D200 after fertilization have been incubated in vitro in the presence of dehydroepiandrosterone-3H to demonstrate 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and delta 5,4-isomerase activity, of pregnenolone-3H to assess the synthesis of "progestins" and of androstenedione-3H to determine oestrogen synthesis in the ovaries and the formation of androgens in the testes. Histologically indifferent gonads from D50 contained 3 beta-HSD, delta 5,4-isomerase and 17 alpha-hydroxylase, indicating a capacity to synthesize progestins. Ovaries from D100 possessed the same enzymes as D50 gonads; those from D200 had, in addition, 17 beta-HSD and aromatase, indicative of oestrogen synthesis. Unlike the D50 gonads, the D100 testes also contained 17 alpha, 20-desmolase and 11 beta-hydroxylase, showing a capacity to synthesize androgens. 17 beta-HSD was only demonstrated in D200 testes. The possible role of androgens and progestins in the regulation of gonadal sex differentiation in rainbow trout has been discussed. Ultrastructural and enzymecytochemical investigations have demonstrated stroma (Leydig) cells as sources of steroidogenesis in rainbow trout testis from D100 onwards. The appearance of big mitochondria with many tubular cristae in those cells synchronized with the appearance of 11 beta-hydroxylase activity. Obvious steroidogenic sites could not be demonstrated in younger gonads and developing ovaries.