Induction of the Acrosome Reaction in Starfish

Abstract

When starfish spermatozoa reach the jelly coat of homologous eggs, they immediately undergo the acrosome reaction that is a prerequisite for fertilization. Three organic components of the egg jelly are responsible for this phenomenon; namely, an extremely large sulfated glycoprotein named acrosome reaction-inducing substance (ARIS), a group of sulfated steroid saponins named Co-ARIS, and a sperm activating oligopeptide(s) (SAP). ARIS induces the acrosome reaction species-specifically in high Ca2+ or high pH sea water, but it requires Co-ARIS for the induction in normal sea water. The acrosome reaction induced by ARIS and Co-ARIS is not accompanied with a transient increase in the intracellular pH (pHi), which is generally accepted to be inevitable for triggering the acrosome reaction, and proceeds slower than the jelly-induced acrosome reaction. All the three, but nothing else, are required to reconstruct the acrosome reaction-inducing ability of the egg jelly.