Meiotic resumption and gap junction modulation in the cultured rat cumulus-oocyte complex

Intercellular communication within the ovarian follicle has been implicated in the control of meiotic arrest and maturation in the mammalian oocyte. We have shown that a rapid down-regulation of cumulus cell gap junctions is correlated temporally with meiotic resumption in the intact rat follicle [Larsen et al., Dev Biol, 113:517-521]. Here this relationship has been analyzed further by incubating isolated rat cumulus-oocyte complexes (COCs) with agents known to influence germinal vesicle breakdown (GVBD) or that have been shown to modulate gap junction turnover in vitro. Quantitative freeze-fracture analysis revealed that cumulus cell gap junction membrane decreased significantly prior to the initiation of GVBD in COCs incubated in medium lacking serum or other additives. The addition of serum and follicle-stimulating hormone, an experimental condition that delayed GVBD, accelerated and augmented gap junction loss at both the cumulus cell and oocyte surface. The continuous elevation of cyclic adenosine monophosphate levels, which stimulates gap junction formation in other systems, maintained meiotic arrest but did not interfere with gap junction loss. Conversely, the complete inhibition of junctional loss by a microfilament destabilizing agent, dihydrocytochalasin B, did not alter the course of GVBD normally seen in its absence. Subsequent freeze-fracture analysis and dye coupling experiments confirmed that cumulus and oocyte gap junctions in these preparations were intact and functional during the period of meiotic resumption. These findings suggest that factors other than cumulus and oocyte gap junction turnover are required for the control of meiotic arrest and maturation in the isolated COC. However, these results do support our earlier suggestion that gap junction loss within the cumulus oophorus is instrumental in isolating the oocyte from the regulatory influence of its underlying membrana granulosa cells during meiotic maturation in the intact preovulatory follicle [Larsen et al., Dev Biol, 122:61-71].