Electron microscopy reveals the ultrastructure and the chromatin organization of Holothuria scabra male germ cells

Holothuria scabra release their gametes into seawater where external fertilization rapidly takes place. H. scabra sperm are generated through the steps of germ cell proliferation and differentiation within the wall of the testicular tubule. The outer most layer of the tubular wall supports the developing germ cells. All germ cells are classified according to the heterochromatization pattern of the cell nuclei. Based on the degrees of chromatin condensation, the two spermatid stages are classified as St1 and St2. The St1 exhibits a small, round nucleus with heterochromatin clumping around the nuclear envelope which, in turn, is surrounded by the mitochondrial aggregation. In the St2, thick chromatin fibers appear. Large mitochondria fuse and the tail rudiment also starts to form at the posterior, whereas acrosome and subacrosomal structures are fully formed at the anterior region. It is speculated that a coiling of nucleosomes organized by the sperm-specific histones causes the chromatin condensation in spermatids and sperm nuclei of H. scabra, much like what happens in sea urchin sperm which indicates a highly conserved phylogenetic process. The appearance of numerous phagosomes in the Sertoli-like cells suggests their possible roles in providing nutrients to the proliferating germ cells and phagocytic removal of cytoplasmic debris from the maturing germ cells.