ACTIVATION OF HAMSTER ZONA-FREE OOCYTES BY HOMOLOGOUS AND HETEROLOGOUS SPERMATOZOA

Spermatozoa of a wide variety of species can fuse with zona-free hamst er oocytes. Zona-tree hamster oocytes were inseminated with spermatozo a of homologous (hamster) and other (mouse, guinea-pig and human) spec ies, and their responses were closely examined to determine whether su ch interspecific sperm-oocyte fusion always induces normal oocyte acti vation. 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 o ocytes remained inactivated at least during the first 1.5-2 h. The amo unt of M-phase (metaphase) promoting factor was reduced in hamster ooc ytes 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 sper m-derived oocyte activating factors show some degree of species specif icity.