In sea urchins, the sperm acrosomal reaction begins when sperm contacts:
The acrosome reaction is triggered before sperm contacts vitelline envelope or egg plasma membrane, ensuring bindin exposure at correct time. Initial stimulus comes from fucose-sulfate-rich glycoconjugates and low molecular weight peptides within egg jelly surrounding egg. Jelly components induce calcium influx via voltage-gated channels and activation of Na+/H+ antiport raising intracellular pH, causing outer acrosomal membrane fusion with plasma membrane releasing acrosomal enzymes and polymerizing actin to project acrosomal process. Vitelline envelope interaction follows jelly-induced priming; egg membrane fusion occurs afterwards, and nuclear contact never directly triggers reaction.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Acrosome reaction triggered by egg jelly polysaccharides.