An acrosome reaction in natantian sperm

Sperm of the natantian Sicyonia ingentis are highly polarized with a conical cap and single spike extending from a spherical main body. The acrosome consists of a spiraled spike and two glycoprotein cap components: the anterior granule and the membrane pouches. During sperm‐egg interaction, the spike is lost and the cap membrane folds back, exposing the two glycoprotein acrosomal components. At this point, the anterior granule lies directly adjacent to the oolemma and has replaced the spike as the anterior end of the sperm. Treatment of sperm with ionophore A23187 initiates the acrosome reaction, thereby facilitating the experimental sequencing of the reaction. The ionophore‐induced reaction begins with the spike's loss of its spiraled nature, the appearance of filaments in the spike core, and the immediate depolymerization of the spike into a granular mass. As the spike retracts, the membrane pouches swell. The cap membrane in the area of the retracted spike ruptures and begins to fold back. As seen during natural sperm‐egg interaction, the anterior granule becomes the new anterior end of the sperm after loss of the spike. Several known artificial activators of sperm were tested, but only the divalent cation ionophore A23187 successfully induced the acrosome reaction. The use of artificial seawater with different ionic properties showed that the acrosome reaction is dependent on the influx of Ca ++ ions.