HIGHER-ORDER NUCLEAR-STRUCTURE IN MAMMALIAN SPERM REVEALED BY IN-SITUHYBRIDIZATION AND EXTENDED CHROMATIN FIBERS

The higher order organization of chromosomes in human and mouse sperm- cell nuclei has been visualized by fluorescence in situ hybridization. In mouse testicular sperm, centromeres form several linear higher ord er structures that are attached to a heterochromatic chromocenter at t he center of the nucleus. Telomeres of the acrocentric mouse chromosom es are associated with either the heterochromatic center or the nuclea r membrane. Whole chromosome domains are arranged parallel to the hete rochromatic chromocenter and occasionally wrapped around it. We propos e that constitutive heterochromatin serves as the nucleation point for a cell-type-specific organization of mouse sperm chromatin. In human mature sperm, individual chromosomes also appear as elongated territor ies. When human sperm nuclei are lysed with high salt and detergent, t he normally condensed sperm chromatin unravels into linear arrays that exhibit a discrete nodular substructure (of <300 nm diameter). These beads may represent a basic packaging unit of sperm chromatin. Larger superbead-like structures are also seen along the extended chromatin f ibers and these could contain multiples of the basic packaging unit. T hese observations indicate that mammalian sperm nuclei have a highly d efined nuclear architecture. The implications of an ordered organizati on of DNA in sperm are unknown, but it is possible that functional com partmentalization of the sperm-cell nucleus influences the initiation and regulation of paternal gene activity in the early embryo. (C) 1995 Academic Press, Inc.