The fine structure of the sperm and spermatid differentiation in the brown mussel Perna perna

Abstract

The mature sperm of Perna perna is 50-55 µm long and comprises three regions: a head, a mid-piece and a tail. The head incorporates a round electron-dense nucleus and an elongated cone-shaped acrosoma The mid-piece consists of a ring of five mitochondria, in the centre of which is the distal centriole from which the tail arises. The early spermatid is characterized by a large nucleus and perinuclear cytoplasm containing numerous mitochondria and proacro- somal vesicles. By mid-spermiogenesis the nuclear chromatin begins to condense, the proacrosomal vesicles coalesce to form the acrosome and the mitochondria are reduced to five in number. At this stage the tail first appears. During late spermiogenesis the acrosome elongates and in-vaginates on its adnuclear surface. The structure of the sperm of P. perna is therefore similar to that of other mytilaceans, thus supporting the contention that sperm ultrastructure could be used in studies on bivalve phylogeny.