An ultrastructural analysis of the gametes and early fertilization in two bivalve molluscs, Chama macerophylla and Spisula solidissima with special reference to gamete binding.

An ultrastructural investigation of the gametes and their interaction during the early events of fertilization in molluscs has been performed. A gamete binding event involving large numbers of sperm has been identified and examined in detail. The surface of the oocyte is projected into numerous microvilli which extend through the vitelline envelope. Tufts of fibrillar material radiate from the tips of these microvilli, forming a layer external to the vitelline envelope. The acrosomal vesicle of the mature spermatozoon contains two major components, which function differently during fertilization. The vesicle is indented at its adnuclear surface, constituting a preformed acrosomal tubule. This tubule does not elongate during the acrosome reaction. Completion of the reaction results in the formation of an extracellular coat, derived from on component of the acrosomal vesicle, on the anterior surface of the sperm. Sperm-egg binding is accomplished by an association of the extracellular coat on the reacted sperm and the fibrous tufts on the tips of the microvilli of the oocyte. Evidence that gamete membrane fusion occurs by fusion of the acrosomal tubule and a microcillus is presented. These observations provide a generalized pattern of molluscan fertilization.