cortical reaction

Meaning: Fusion of a sperm with the egg membrane triggers a rise in cytoplasmic Ca²⁺, released from the egg's endoplasmic reticulum. Ca²⁺ makes cortical granules, vesicles lying just under the egg plasma membrane, fuse with it and release their enzymes by exocytosis. The enzymes alter the zona pellucida (the vitelline layer in sea urchins), so sperm receptors are modified and the layer hardens. Additional sperm can no longer bind. In sea urchins the layer also lifts away as a fertilization envelope. This is the slow block to polyspermy, working over minutes. The fast block is a change in membrane potential within seconds, well shown in sea urchins and less clear in mammals.

On the MCAT: A passage may describe eggs treated with a Ca²⁺ chelator, a Ca²⁺ ionophore, or a mutation that removes cortical granules. Predict the result: without the reaction, several sperm may enter and the zygote ends up with extra chromosome sets, so development fails. A Ca²⁺ rise without any sperm can still trigger the reaction. Questions may also ask you to place it in the sequence of fertilization.

Common confusion: The cortical reaction is not the acrosomal reaction. The acrosomal reaction occurs in the sperm before penetration and releases enzymes that digest a path through the egg's outer layers. The cortical reaction occurs in the egg after fusion and blocks further sperm.