Establishment of the mammalian membrane block to polyspermy: Evidence for calcium-dependent and -independent regulation

One crucial result of egg activation is the establishment of blocks on the zona pellucida and the egg plasma membrane to prevent fertilization by additional sperm. The mechanism(s) by which a mammalian egg regulates the establishment of the membrane block to polyspermy is largely unknown. Since Ca²⁺ signaling regulates several egg activation events, this study investigates how sperm-induced Ca²⁺ transients affect the membrane block to polyspermy, building on our previous work (Biology of Reproduction 67:1342). We demonstrate that mouse eggs that experience only one sperm-induced Ca²⁺ transient establish a membrane block that is less effective, than in eggs that experience normal sperm-induced Ca²⁺ transients but that is more effective than in eggs with completely suppressed [Ca²⁺]_cyt increases. Sperm-induced increases in [Ca²⁺]_cyt regulate the timing of membrane block establishment, as this block is established more slowly in eggs that experience one or no sperm-induced Ca²⁺ transients. Finally, our studies produce the intriguing discovery that there is also a Ca²⁺-independent event that is associated with fertilization in the pathway leading to membrane block establishment. Taken together, these data indicate that Ca²⁺ plays a role in facilitating membrane block establishment by regulating the timing with which this change in egg membrane function occurs, and also that the membrane block differs from other post-fertilization egg activation responses as Ca²⁺ is not the only stimulus. The membrane block to polyspermy in mammalian eggs is likely to be the culmination of multiple post-fertilization events that together modify the egg membrane's receptivity to sperm.