Fig. 4

Acrosome reaction in P. pectinifera spermatozoa and the sperm-induced Ca²⁺ responses in the egg. a Scanning electron microscopy micrographs of spermatozoa fixed in natural seawater without eggs. No sign of acrosome reaction is present. bP. pectinifera spermatozoa on the egg surface after acrosome reaction has taken place (see the long filamentous process, AP). c Ca²⁺ signals in a mature egg of Astropecten aranciacus following the interaction of the tip of the AP with the egg surface. A simultaneous increase of Ca²⁺ over the entire periphery of the egg (cortical flash, CF) is observed when the fertilizing sperm head (marked in red dot) is still in the outer jelly coat (JC) (t = 0:17). This Ca²⁺ increase is followed by a Ca²⁺ wave that starts from the sperm-egg interaction and propagates to the opposite side of the egg, while the head of the sperm is still far away from the egg surface (t = 0:20). Only after the Ca²⁺ wave has spread globally to the opposite side do sperm become incorporated into the egg by passing through the FE