Spermiogenesis in the bluegill (Lepomis macrochirus): a study of cytoplasmic events including cell volume changes and cytoplasmic elimination.

The process involved in the reduction of both nuclear and cytoplasmic volume was investigated in the bluegill (Lepomis macrochirus), a teleost fish. Young spermatids contained centrally positioned nuclei which, with time, moved toward the cell surface to become eccentrically positioned. Chromatin condensation was initiated from a region near the implantation fossa, whereas at the opposite pole of the nucleus an area sparse in heterochromatin (clear area) was noted. The nuclear membrane lying adjacent to the clear area dissolved and subsequently reformed, yielding a nucleus with a reduced volume. During this process, packets of cytoplasm surrounded by a double membrane were formed along the future midpiece. The packets of cytoplasm migrated toward the cell surface, protruded from the surface, and were extruded into the spermatocyst lumen. These structures, termed residual bodies, were subsequently endocytosed, accumulated into large phagocytic vacuoles, and eventually degraded by the nearby Sertoli cell. When the spermatocyst ruptured, spermatozoa containing sparse cytoplasm were released into the excurrent duct system. During spermiogenesis, both the nuclear and cytoplasmic volumes decreased substantially (80%, 92% respectively) leading to an overall 87% reduction in total cell volume.