Summary. Cumulus expansion and cumulus cell-oocyte coupling during in-vivo and in-vitro maturation of pig oocytes were studied by measuring [3H]uridine uptake. In vivo, cumulus expansion started before germinal vesicle breakdown (GVBD) (16 h versus 20 h after hCG) but no significant change occurred in the coupling index until 32 h after hCG. Intercellular coupling was decreasing at 32 h after hCG in oocytes at anaphase I and telophase I. Complete uncoupling was closely correlated with corona radiata expansion. In vitro, partial uncoupling was observed in oocyte—cumulus cell complexes from prepubertal and PMSG-stimulated gilts cultured for 16 and 32 h, respectively. The addition of FSH caused cumulus expansion, and the functional coupling between the cumulus cells and the oocyte was maintained up to at least 16 h of culture in complexes from prepubertal gilts.
We conclude that, under our conditions, neither hormone-free nor FSH-supplemented medium ensured the same [3H]uridine uptake and uncoupling kinetics as during in-vivo maturation.