The ultrastructure of nuclear differentiation during spermiogenesis in the salmon

Progressive condensation of nuclear material occurs during spermiogenesis in the salmon. Nuclei of early spermatid stages contain randomly distributed, contorted fibers, similar in appearance, distribution, and mean diameter (175 Å) to chromatin fibers of salmon somatic cell and spermatocyte nuclei. These fibers give way in nuclei of later spermatid stages to oriented, unusually thick fibers (with mean diameter 340 Å), and to highly unusual, irregular, ribbonlike elements. In late spermatid stages nuclear fibers are no longer clearly visible; only short lengths of fibers and ribbonlike elements remain. Mature sperm nuclei are extremely electron-dense, showing little substructure. The unusually thick fibers and irregular ribbons which form during spermiogenesis in the salmon are clearly different from the typical fibers found in somatic cell nuclei and chromosomes and in the interphaselike nuclei of early spermatids. At least some of the striking structural changes which occur during nuclear condensation in salmon spermiogenesis probably are related to replacement of histones by protamine.