Ultrastructural localization of a nuclear autoantigenic sperm protein in spermatogenic cells and spermatozoa

This study demonstrates the ultrastructural localization of rabbit nuclear autoantigenic sperm protein (NASP) in spermatogenic cells and spermatozoa. NASP is present in rabbits, rats, mice, and human testes and spermatozoa. It has recently been sequenced in rabbits and humans and characterized as an acidic, histone binding protein. Currently it has been proposed that NASP may play a role in regulating early events of spermatogenesis through its ability to bind and translocate testicular histone variants to nucleosomes. The ultrastructural localization of NASP confirms that it is initially present in primary spermatocytes in their Golgi regions and nucleus. In round spermatids it is present in the nucleus as well as in the acrosome and subacrosomal space. In later spermatids, testicular spermatozoa, and ejaculated spermatozoa, NASP is concentrated over the nucleus, although some is still present in the acrosome. It is likely that NASP would be carried into the ovum with the sperm nucleus at fertilization.