Role of FSH and epidermal growth factor (EGF) in the initiation of steroidogenesis in granulosa cells associated with follicular selection in chicken ovaries.

In chicken ovaries, one small yellow follicle (SYF) is selected daily from a pool of follicles of similar size and becomes a preovulatory follicle. FSH induces follicular growth and steroidogenesis. Epidermal growth factor (EGF), an intraovarian hormone, suppresses granulosa cell differentiation. This study demonstrates that recruitment of SYFs into the hierarchy of preovulatory follicles is associated with a change in steroidogenic activity in granulosa cells regulated, at least in part, by FSH and EGF. Abundance of P450 side-chain cleavage (P450scc) mRNA was higher in the smallest preovulatory follicle (F6) compared with SYF, whereas FSH and EGF receptor (FSHr and EGFr, respectively) mRNA abundance was similar. FSH increased P450scc mRNA abundance and progesterone secretion and decreased FSHr mRNA in cultured granulosa cells, whereas EGF attenuated or suppressed P450scc mRNA and decreased FSHr mRNA abundance. None of the hormones influenced EGFr mRNA abundance. When used in combination, EGF attenuated or suppressed the stimulatory effect of FSH on the expression of P450scc mRNA and production of progesterone in a dose-dependent manner. The results indicate that (1) selection is associated with an increase in P450scc mRNA; (2) FSH stimulates expression of P450scc mRNA and progesterone secretion in granulosa cells of SYF; and (3) induction of P450scc mRNA and progesterone secretion by FSH is attenuated or blocked by EGF.