Practice question

Unfertilized sea urchin egg is surrounded by vitelline membrane, thin extracellular coat containing sperm receptors, overlaid by thick jelly coat. Upon calcium-triggered cortical granule exocytosis, structural proteins and enzymes detach vitelline membrane and harden it via cross-linking by transglutaminase and ovoperoxidase using hydrogen peroxide from respiratory burst. Elevated hardened coat becomes fertilization envelope. Plasma membrane remains beneath, zona pellucida is mammalian equivalent not present in urchins, egg jelly dissolves. Thus fertilization envelope originates directly from modified vitelline membrane repurposed as protective barrier essential for embryonic development.