Extraovarian and intraovarian control of follicular selection in the chicken ovary

Follicular selection is the basis for regular ovulation. In the chicken ovary, one small yellow follicle (SYF) is selected daily from a pool of similar size follicles and becomes a preovulatory follicle. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) are primary regulators of follicular growth and differentiation. Epidermal growth factor (EGF), an intraovarian hormone, suppresses granulosa cell differentiation. However, the precise regulation and interactions of these hormones on follicular selection and the expression of their receptors have not been determined. The objectives of this study we (1) develop and validate a suitable culture system for granulosa cells from SYF; (2) investigate if FSH, LH and EGF alone or in combination regulate the abundance of FSH, LH and EGF receptor (FSHr, LHr and EGFr, respectively) and P450 side chain cleavage (P450scc) mRNA and progesterone (P₄) production in granulosa cells from SYF; (3) determine the differential abundance of FSHr, LHr, EGFr and P450scc mRNA in granulosa cells as follicles approach the stage of selection and immediately after selection and (4) determine if the differential expression of these genes in granulosa cells varies during the ovulatory cycle. The major findings were: (1) granulosa cells from SYF proliferate and remain viable (i.e. do not undergo apoptosis) in vitro for at least 24 h when cultured under appropriate conditions; (2) FSH induces selection by increasing P450scc mRNA and P4 secretion in granulosa cells of SYF, whereas LH has little impact at this stage of development; (3) EGF counteracts the positive effects of FSH and LH on granulosa cell differentiation by reducing the abundance of FSHr and LHr mRNA and by suppressing the expression of P450scc mRNA and production of P₄; (4) FSHr mRNA abundance is maximum in 8mm and F6 follicles compared to 6 mm; (5) LHr mRNA abundance increases in F6 follicles; (6) P450scc mRNA is highest in 8mm and F6 follicles at all times of the ovulatory cycle; (7) P450scc mRNA is higher in 6mm at 21 and 3 h before the next ovulation, and (8) EGFr mRNA levels are similar in all follicles studied. These data clearly indicate that extraovarian and intraovarian hormones are involved in follicular selection. Our working model implies that the SYF that is more responsive to FSH overcomes the suppressive effect of EGF on granulosa cell differentiation and is selected.