Oocyte developmental competence after in vitro maturation depends on the persistence of cumulus-oocyte comunications which are linked to the intracellular concentration of cAMP

Oocyte and cumulus cells are interconnected by an extensive network of gap junctions (GJ) formed by connexin 43. In the present study the functional and morphological status of GJ was investigated during in vitro maturation of bovine oocytes using media known to induce different rates of maturation and developmental competence as well as different levels of cumulus expansion and intracellular concentrations of cAMP. GJ functional condition was studied by microinjection of the fluorescent dye Lucyfer Yellow in cumulus-enclosed oocytes and the intracellular distribution of connexin 43 was examined by immunofluorescence immunocytochemistry. Both functional and morphological analysis of GJ between oocytes and cumulus cells indicated that high maturation and development rates are accompanied by the prolonged persistence of permeable communications, which, however, are independent from cumulus expansion. On the contrary, the premature interruption of such communications was linked to low maturation and development. When these results were correlated to the measurements of intracellular levels of cAMP it was observed that cumulus expansion, GJ permeability and good quality maturation require the highest levels of cAMP. Intermediate cAMP levels determine GJ permeability and good quality maturation in the absence of cumulus expansion while below a threshold level of cAMP, oocyte maturation is defective with a premature interruption of communications through GJ and lack of cumulus expansion which lead to poor development.