Polyspermic mouse eggs can dispose of supernumerary sperm

Zona-free mouse eggs inseminated with capacitated epididymal sperm in a modified Krebs-Ringer bicarbonate medium exhibited unusual kinetics of sperm incorporation. At a sperm concentration of 10 5 cells/ml or higher, the mean number of sperm per egg reached a maximum and then decreased with time. This decrease was correlated with the abstriction of sperm in cytoplasmic blebs which formed during or slightly after second polar body abstriction, 1.5–2.5 hr postinsemination. A correlation was apparent between the degree of polyspermy and the total number of sperm lost by this mechanism. Of 82 dispermic eggs studied, 36 underwent sperm loss by blebbing, a process that restored the monospermic condition. The sequential steps in the abstriction process are depicted in micrographs of whole mounts of fixed eggs. A sperm head or male pronucleus could be seen in isolated blebs. The prevention of bleb formation by exposure of penetrated eggs to cytochalasin B largely eliminated any difference in sperm number when the mean number of sperm per egg was compared at 2, 4, and 6 hr postinsemination. Sperm abstriction may be a novel sperm exclusion mechanism employed by mammalian eggs. Evidence is also presented that an unknown mechanism of sperm exclusion is operative in mouse eggs, since sperm loss by abstriction did not account for all sperm loss.