Activation of hamster zona-free oocytes by homologous and heterologous spermatozoa.

Spermatozoa of a wide variety of species can fuse with zona-free hamster oocytes. Zona-free hamster oocytes were inseminated with spermatozoa of homologous (hamster) and other (mouse, guinea-pig and human) species, and their responses were closely examined to determine whether such interspecific sperm-oocyte fusion always induces normal oocyte activation. While guinea-pig and human spermatozoa could activate hamster oocytes as efficiently as hamster spermatozoa, mouse spermatozoa could not. Mouse spermatozoa fused readily with hamster oocytes, yet most oocytes remained inactivated at least during the first 1.5-2 h. The amount of M-phase (metaphase) promoting factor was reduced in hamster oocytes fused with one or several mouse spermatozoa; however, repetitive Ca2+ transients failed to occur unless oocytes were inseminated with a concentrated sperm suspension and penetrated by very many spermatozoa. These observations suggest that sperm-oocyte membrane fusion per se is not sufficient to trigger oocyte activation, and that putative sperm-derived oocyte activating factors show some degree of species specificity.