Intrafollicular pressure within preovulatory follicles of the pig

Antral pressure was measured within the follicles of unstimulated ovaries in prepubertal pigs and following an ovulatory stimulus with exogenous gonadotropins. No increase in intrafollicular pressure (IFP) was observed as the time of ovulation approached. A wide range of IFP was noted within follicles of the unstimulated ovary. In many follicles IFP was greater than 30 cm H2O, suggesting that the antral fluid was not in hydrostatic equilibrium with the surrounding thecal capillaries. IFP of unstimulated follicles could be increased to more than 400 cm H2O by antral injection of mineral oil without follicular rupture--a demonstration of the need for stigma formation in the release of the ovum from the follicle. Stimulated follicles were found to be more distensible than unstimulated follicles. The follicles also fell into two groups--those in which sustained versus transient elevation in IFP occurred following oil injection. It is postulated that the follicle wall develops the ability to undergo stress relaxation during follicular maturation and that this process plays a role in regulating IFP.