Actin polymerization in sperm cells occurs primarily during:
Actin polymerization in sperm is temporally restricted to acrosomal reaction. G-actin monomers stored bound to profilin in sperm head rapidly assemble into F-actin bundle forming slender acrosomal process that extends bindin to reach vitelline envelope. Calcium influx and pH rise activate actin nucleation via formin and depolymerization of capping proteins, generating force to protrude process through egg jelly remnants. Cytochalasin B or latrunculin blocking polymerization prevents process formation and fertilization fails. Cortical reaction involves secretory exocytosis in egg, fertilization
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Actin polymerization driving acrosomal process extension in sea urchin sperm.