REGULATION OF OOCYTE GROWTH AND MATURATION IN FISH

This chapter discusses the current status of the investigations on the hormonal regulation of oocyte growth and maturation in fish. Pituitary gonadotropins are of primary importance in triggering these processes in fish oocytes. In both cases, however, the actions of gonadotropins are not direct but are mediated by the follicular production of steroidal mediators, estradiol-17β (oocyte growth), and 17α,20β-dihydroxy-4-pregnene-3-one (DP) or 4-pregnen-17,20β,21-triol-3-one (20β-S) (oocyte maturation). It has been established that both estradiol- 17β and 17α,20β-DP are biosynthesized by salmonid ovarian follicles through an interaction of two cell layers—namely, the thecal and the granulosa cell layers (two-cell-type model). The granulosa cell layers are the sites of production of these two steroidal mediators, but their production depends on the provision of the precursor steroids by the thecal cell layers. © 1995, Academic Press Inc.