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To investigate the arrangement of chromosomes within pronuclei-stage mouse zygotes.

In vitro study.

Academic medical center. Patient(s): None. Intervention(s): None. Main Outcome Measure(s): Location of major α-satellite DNA, centromeres, and telomeres, and relative location of chromosomes. Result(s): Chromosomes appeared to be oriented inward by centromeres and to be interconnected by major α-satellite DNA, which appeared to be the sole DNA component of the nucleoli. This chromosomal arrangement persisted throughout interphase. Chromosomal painting failed to identify chromosomal ordering within pronuclei. Conclusion(s): Pronuclear nucleoli are represented by α-satellite sequences of interconnecting chromosomes that hold all chromosomes together during interphase. Chromosomes within the pronucleus are randomly positioned relative to each other.