Cyclic changes in the ovary of the rainbow trout, Salmo gairdneri, with special reference to sites of steroidogenesis.

In the maturation cycle of ovaries of cultured rainbow trout, three periods can be distinguished: (1) a period of ovulation and previtellogenesis (January-May), (2) a period of exogenous vitellogenesis (May-November/December), and (3) a period of maturation of oocytes (November/December-January). Enzyme cytochemical and electron microscopical data indicate that stroma cells (i.e., interstitial cells and special theca cells) and granulosa cells represent sources of steroids. Steroidogenesis in stroma cells is found throughout the annual cycle, reaching a peak activity in January and February. Weak steroidogenic activity is observed in the granulosa cells of exogenous vitellogenic follicles and young postovulatory follicles. Possible functions of steroids secreted by stroma cells and granulosa cells are discussed.