Chromatin-mediated cortical granule redistribution is responsible for the formation of the cortical granule-free domain in mouse eggs

A cortical granule-free domain (CGFD) overlies the metaphase chromatin in fully mature mouse eggs. Although a chromatin-induced localized release of cortical granules (CG) during maturation is thought to be a major contributing factor to its formation, there are indications that CG redistribution may also be involved in generating the CGFD. We performed experiments to determine the relative contributions of CG exocytosis and redistribution in generating the CGFD. We found that the CGFD-inducing activity was not specific to female germ cell chromatin and was heat stable but sensitive to DNase and protease treatment. Surprisingly, chelation of egg intracellular Ca(2+) levels did not prevent CGFD formation in response to microinjection of exogenous chromatin, suggesting that development of the CGFD was not a result of CG exocytosis. This finding was confirmed by the lack of CG exudate on the plasma membrane surface of the injected eggs and the absence of conversion of ZP2 to ZP2(f) during formation of the new CGFD. Moreover, clamping intracellular Ca(2+) did not prevent the formation of the CGFD during oocyte maturation, but did inhibit the maturation-associated release of CGs between metaphase I and II. Results of these experiments suggest that CG redistribution is the dominant factor in formation of the CGFD.