Egg signals for triggering the acrosome reaction in starfish spermatozoa

Upon encountering the jelly coat of an egg, starfish spermatozoa undergo the acrosome reaction. To induce the acrosome reaction, 3 jelly components act in concert on the spermatozoa: a sulphated glycoprotein named acrosome reaction-inducing substance (ARIS), a group of steroidal saponins named Co-ARIS, and an oligopeptide presumably having an ability to increase the intracellular pH of the spermatozoon. All three are required to mimic the full ability of jelly coat to induce the acrosome reaction instantaneously. A combination of ARIS and Co-ARIS is enough for induction in normal sea water, although its action is almost 2 min slower than the jelly. ARIS can induce the acrosome reaction by itself in high Ca2+ or high pH sea water. When spermatozoa meet the jelly coat, the acrosome reaction is eventually induced because ARIS and Co-ARIS co-operatively increase the intracellular Ca2+ by stimulating verapamil- and maitotoxin-sensitive Ca2+ channels and the oligopeptide increases the intracellular pH by stimulating Na+/H+ exchange systems.