Practice question

During acrosomal reaction, sperm subacrosomal region contains concentrated pool of globular actin monomers maintained unpolymerized by profilin. Upon Ca2+ influx triggered by egg jelly contact, actin rapidly polymerizes into bundle of filamentous F-actin pushing acrosomal membrane forward forming slender elongated acrosomal process about one micrometer long, thrusting bindin-bearing tip through viscous egg jelly toward vitelline envelope receptor field. Cytochalasin D experiments blocking actin polymerization abolish process extension and prevent fertilization. Fertilization cone actin polymerization occurs in egg cortex, cortical reaction involves egg exocytosis, not sperm actin. Thus actin polymerization in sperm cells occurs primarily during acrosomal reaction creating penetration organelle.