Nucleons, I: A model for studying the mechanism of sperm nucleus swelling in vitro

The nucleon, a highly organized chromatin structure, was studied to learn if its swelling takes place by the action of heparin/GSH, without the participation of any mechanism provided by sperm membranes, subcellular organelles, or other proteins foreign to the sperm nucleus. Sperm suspensions of guinea pigs and rats were incubated with 9 mM DTT and 1% CTAB. The nucleons obtained from washed epididymal spermatozoa appear under a phase-contrast microscope to preserve their original nucleus shape and to completely lack the acrosome, middle piece, and tail. In an electron microscope, nucleon thin sections show a slight nuclear chromatin decompressed from the periphery toward the center. An outstanding result was that the nucleon swelling pattern by heparin/GSH showed the same classic organization into hub-like nuclear bodies joined by a network of chromatin fibers ranging in thickness from 25 to 1.5 nm. Under the conditions of this study there was no need of any membrane or subcellular structure. At stage IV, all the thick fibers disappear, leaving only thin bead fibers on a string. With respect to nuclear swelling there is no doubt that the sperm chromatin is organized in a special form that decides a specific required pattern of unpacking.