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Pronuclear development and the first cleavage division in polyspermic mouse eggs

Summary. Zona-free F_, (C57B1/10 x CBA/H) mouse eggs were fertilized in vitro with F_, spermatozoa and examined in whole mounts 4·5–5 h or 17–19 h after mixing the gametes. The fertilization rate was nearly 90% with about one-third of eggs undergoing monospermic and two-thirds polyspermic fertilization (2–8 spermatozoa). In all monospermic eggs the sperm head developed into a large pronucleus. In polyspermic eggs either all or only some of the spermatozoa had completed the process of transformation, while the remainder were arrested at the early phase of pronuclear development. During the first cleavage division these pronuclei condensed but separate chromosomes did not differentiate and therefore could not contribute to the genome of the embryo. The condensed pronuclei were passively displaced to one or both blastomeres. The number of decondensed sperm heads present in the egg shortly after fertilization was therefore not always equivalent to the ploidy of the future embryo. The duration of the first cell cycle appeared to be related to ploidy (number of functional = 'advanced' pronuclei), with the monospermic eggs dividing first.

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