Redox regulation of spermatogenesis and fertilization. During the spermatogenic process, histones are converted to protamines via transition proteins in sperm nuclei. The maturation of spermatozoa proceeds in the epididymis. Oxidation mediated by sulfoxidase is involved in the packaging of chromatin into the small nucleus via disulfide bridge formation between protamines. After fertilization, the sperm head expands to the male pronucleus by reducing the disulfide bond in the oocyte.