Cellular Sources of Sex Steroids in Teleost Gonads

Abstract

Histochemical and ultrastructural data on the cellular source of gonadal steroids are reviewed, together with some recent observations on in vitro steroidogenesis by the ovarian follicles of the amago salmon, Oncorhynchus rhodurus. The interstitial cells of the testis appear to be homologous with mammalian Leydig cells and are considered to be the major site of androgen synthesis. Studies on lobule boundary cells suggest that in some species they may be involved in phagocytosis and transport of metabolites while in others they may be steroidogenic. The granulosa cells and the special thecal cells are the major sites of steroid synthesis in the teleost ovary. Studies on the amago salmon using isolated thecal and granulosa layers indicate that both follicle layers are necessary for estradiol-17β production in vitro in response to salmon gonadotropin. The role of the thecal layer during the major part of vitellogenesis is postulated to be the production of estrogen precursors which are converted to estradiol-17β in the granulosa layer. Histochemical and ultrastructural evidence from a number of species for steroid biosynthesis by postovulatory follicles is supported by our recent radioimmunoassay data from white-spotted char, Salvelinus leucomaenis, and the amago salmon showing that young postovulatory follicles can secrete progesterone; the physiological significance of this observation remains to be explained.Key words: teleosts, gonad, steroidogenesis, in vitro, morphology