Acrosome reaction in Octopus tankahkeei induced by calcium ionophore A23187 and a possible role of the acrosomal screw

The octopod sperm is unique especially in two aspects: the screw-shaped acrosome and its inner layered substructure (striation). The present study aims to investigate morphological changes of Octopus tankahkeei spermatozoa during the acrosome reaction (AR) and to pursue functions of the internal substructures revealed by inducing AR with the calcium ionophore A23187. Gradual changes of the spermatozoa were traced using fluorescence and electron microscopy. The AR process included the bulging, vesiculation, and dehiscence of the plasma membrane around the acrosome and the nucleus, as well as the vesiculation of the mitochondrial sheath. Membrane vesiculation outside the nucleus has never been reported in the order Octopoda. The rigid screw and the inner striation of the acrosome remained intact surmounting the nucleus, suggesting that these two structures have potential functions during fertilization. In addition, the detachment of the sperm head and the tail was commonly observed in this study, both in intact and acrosome-reacted sperm. Fluorescence microscopy revealed that the detached mitochondrial sheath usually gave weaker and more dispersive signals than the joint ones. This phenomenon implied that the intense energy release might promote the detachment of the mitochondrial sheath.