Sperm Attachment and Acrosome Reaction on the Egg Surface of the Polychaete, Tylorrhynchus heterochaetus

Sperm binding to the egg envelope (chorion) was examined in fixed eggs and isolated chorions of the polychaete, Tylorrhynchus heterochaetus. Sperm binding included two successive steps: attachment (acrosomal outer surface-chorion binding) before the acrosome reaction and adhesion (acrosomal process-chorion binding) after the acrosome reaction. The attachment between sperm head-tip and the outermost layer of the chorion was observed in Ca-free seawater, in which the acrosome reaction did not occur. The surface of the chorion was stained with phosphotungstic acid (PTA). Sperm did not attach to pronase-treated eggs, in which the PTA-positive layer disappeared. When isolated chorions were soaked in distilled water for several hours, they lost the capacity for sperm attachment, and the PTA-positive layer thinned. The acrosome reaction was induced by material that was dissolved from the chorions into distilled water. This suggests that both the receptor for sperm attachment and the inducer of the acrosome reaction are involved in the PTA-positive layer.