Spermatogenesis inPlatynereis massiliensis (Polychaeta: Nereidae)
© Biologische Anstalt Helgoland 1989
Stage 1 of spermatogenesis in the protandrous polychaetePlatynereis massiliensis is represented by clusters of about 60 spermatogonia which appear in the coelomic cavity. There are no testes inP. massiliensis. The origin of the spermatogonial clusters is not known. Subclusters of approximately 20 primary spermatocytes each represent stage 2. The appearance of synaptonemal figures in the spermatocyte nuclei marks the beginning of stage 3. Cells tend to lose their tight packing during stage 3 but interdigitate with cellular processes. Then very small subclusters of 4 to 8 spermatocytes appear. Meiosis is completed during stage 4, giving rise to secondary spermatocytes and then to spermatid tetrads. Spermatogonia and primary spermatocytes are interconnected by structurally specialized fusomes while secondary spermatocytes and spermatids, which are also in cytoplasmic continuity, show rather simple cell bridges. Synthesis of acrosomal material starts during stage 2. During spermiogenesis the proacrosomal vesicles of Golgi origin travel from the posterior part of the cell to its anterior part to form the acrosome proper. Acrosome formation, nuclear condensation, shaping of the long and slender sperm nucleus, and development of the sperm tail are the main events during spermiogenesis. Sperm morphology is briefly discussed wity respect to its phylogenetic bearings.