Hypoxanthine Promotes the Acquisition of Meiotic Competence in Pig Oocytes from Early Antral Follicles during Growth Culture

Abstract

Pig early antral follicles (0.5-1.0 mm in diameter) contain oocytes approximately 100 μm in diameter with limited competence for meiotic resumption. Oocyte-cumulus complexes containing parietal granulosa cells (OCGs) were dissected from the ovarian follicles, embedded in collagen gels and cultured in Waymouth's medium containing 5% fetal calf serum. In the first experiment, OCGs were cultured in medium containing 0, 1, 10 or 100 ng/ml of FSH for 8 days, and the viability and growth of oocytes were examined. The mean diameter of the surviving oocytes in each experimental group significantly increased, although the percentage of surviving oocytes was highest in the 10 ng/ml FSH-supplemented group (41% vs. 16-22%). In the second experiment, OCGs were cultured in medium containing 10 ng/ml FSH with or without 2 mM hypoxanthine for 7 days. Regardless of hypoxanthine supplementation, about 45% of the oocytes in each group had normal morphology after the culture and grew to almost the full size of 120 μm. All of these were at the germinal vesicle stage. Recovered oocytes were further cultured for 48 hours so that their meiotic competence could be compared. Of the oocytes cultured in hypoxanthine-free medium, 13% underwent germinal vesicle breakdown, although no oocytes progressed beyond metaphase I. On the other hand, 47% of the oocytes cultured in hypoxanthine-supplemented medium underwent germinal vesicle breakdown, and 9% progressed to metaphase II. These results suggest that hypoxanthine promotes the acquisition of meiotic competence in growing pig oocytes in the medium containing FSH, which supports oocyte viability.