Requirement for follicle-stimulating hormone action in the formation of primordial follicles during perinatal ovarian development in the hamster.

Whereas FSH action is critical for the growth of preantral follicles, its role in the development of primordial follicles is controversial. The objective of the present study was to evaluate whether perinatal (fetal through early postnatal) FSH action is needed for the formation of primordial follicles, which first appear in the hamster ovary on the 7th to 8th day of postnatal life. A single dose of FSH-specific polyclonal antibody was injected into pregnant hamsters on the 12th, 13th, or 14th day of gestation and into newborn hamsters. Some of the antibody-exposed postnatal hamsters were injected with a single dose of equine CG (eCG) to check the reversibility of the antibody action. Ovaries were collected on D8pn or D12pn, and the percentage of primordial, primary, and secondary follicles was quantitated morphometrically. Ovaries of 8-day-old hamsters that were born to mothers treated with a single s.c. dose of the anti-FSH-antibody on day 12 of gestation had significantly reduced numbers of primordial follicles, compared with those treated with preimmune serum or saline (2.4% vs. 25%); however, the antibody inhibition was nearly completely reversed (approximately 18%) by a single injection of eCG on the first day of life. Delaying antibody treatment during late gestation caused a time-dependent block in granulosa cell differentiation, with a consequent proportional increase in the percentage of primordial follicles. This indicates that FSH-induction of primordial follicle development begins at a critical time of ovarian development. On the other hand, shortening the postnatal duration of eCG exposure reduced the degree of reversal, suggesting that prolonged perinatal FSH action is essential for developing the full gamut of primordial follicles. These results provide the first direct evidence that FSH action during fetal ovarian development is critical for the onset of primordial follicle formation.