Well-defined genome architecture in the human sperm nucleus

Using fluorescence in situ hybridization, conventional epifluorescence microscopy, and laser scanning confocal microscopy followed by three-dimensional reconstruction we describe a well-defined higher order packaging of the human genome in the sperm cell nucleus. This was determined by the spatial localization of centromere and telomere regions of all chromosomes and supported by localization of subtelomere sequences of chromosome 3 and the entire chromosome 2. The nuclear architecture in the human sperm is characterized by the clustering of the 23 centromeres into a compact chromocenter positioned well inside the nucleus. The ends of the chromosomes are exposed to the nuclear periphery where both the subtelomere and the telomere sequences of the chromosome arms are joined into dimers. Thus chromosomes in the human sperm nucleus are looped into a hairpin-like configuration. The biological implications of this nuclear architecture in spermatogenesis and male pronuclear formation following fertilization are discussed.