Cytodifferentiation of ovarian follicle cells during oocyte growth and maturation.

Inasmuch as 17 alpha,20 beta-diOHprog was identified as the maturation-inducing hormone, we now have two known biologically important mediators of oocyte growth and maturation in salmonids, estradiol-17 beta and 17 alpha,20 beta-diOHprog. It is now established that the granulosa cells are the site of production of these two mediators, but production by the ovarian follicle depends on the provision of precursor steroids by the thecal cell (two-cell type hypothesis). A dramatic switch in the steroidogenic pathway from estradiol-17 beta to 17 alpha,20 beta-diOHprog occurs only in ovarian follicle cells immediately prior to oocyte maturation. This switch is a prerequisite step for the growing oocyte to enter the maturation phase. Resolution of the molecular events regulating this switch will provide new insight into the hormonal events regulating oocyte growth and maturation.