Acrosome reaction in ascidians induced by a calcium ionophore

Ca ionophore A23187 induces an acrosome reaction in Ciona intestinalis and Halocynthia roretzi spermatozoa. This process occurs by the fusion of the acrosomal outer membrane along its peripheral margin with the plasma membrane overlying the acrosome. The fusion process leads to the exocytosis of the acrosomal contents and results in the formation of a vesicle bounded by membrane made up of the acrosomal outer membrane and the overlying plasma membrane that is eventually shed. The acrosomal inner membrane is exposed and becomes a part of the plasma membrane enclosing the anterior region of the sperm head. The chemical nature and the precise role of the acrosomal contents at fertilization must await further studies.