Mouse oocytes penetrated by sperm at GV or GVBD stage lose the ability to fuse with additional spermatozoa.

Mouse oocytes penetrated by spermatozoa during germinal vesicle (GV) breakdown undergo maturation and are arrested at metaphase of the second meiotic division despite the presence of sperm nuclei within the ooplasm. When these oocytes were re-inseminated, none was penetrated by newly added spermatozoa. When GV oocytes were inseminated and cultured in the presence of dibutyryl cAMP, the oocytes remained at GV stage, yet they did not permit entry of additional spermatozoa. These observations suggest that the plasma membrane of maturing oocytes is modified by precociously penetrating spermatozoa independently from cortical granule exocytosis. Sperm components incorporated into the oocytes seem to be responsible for the modification of the oocyte's plasma membrane.