Fertilization in ascidians: apical processes and gamete fusion in Ciona intestinalis spermatozoa

This paper describes the fine structure of the spermatozoan apex and its morphological changes during the process of fertilization in Ciona intestinalis. At the apex of the head an acrosome is present in the form of a flattened vesicle containing moderately electron-dense material with an electron-dense plate in its centre. Contiguous to the acrosome, an apical substance is located in the anterior-most tip of the sperm head. The spermatozoa that bind to and penetrate the chorion of both normal and caffeine-treated eggs have an intact acrosome and do not show an observable alteration of the plasmalemma enclosing the apex of the head. On the other hand, spermatozoa in the perivitelline space of both normal and caffeine-treated eggs lack an acrosome. Instead of an acrosome, apical processes are observed at the apex of the spermatozoa in the perivitelline space of both normal and caffeinetreated eggs. Gamete fusion seems to occur between the egg membrane and some of these apical processes at the tip of the sperm head. The apical process reported here is thought to be analogous to the acrosomal process in other marine invertebrates. The apical processes that are induced in the perivitelline space have not been previously described in ascidians.