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the haploid nucleus of a sex cell, or gamete. During fertilization, the diploid nucleus of the zygote, or synkaryon, is formed by the fusion of two pronuclei; the chromosome sets of both pronuclei are joined in the synkaryon.
In multicellular organisms and a number of protozoans that are anisogamous, there are female and male pronuclei. The female pronucleus is the nucleus of a mature egg cell, or macro-gamete. The male pronucleus is formed by the swelling of the nucleus of a spermatozoid, or microgamete, after it has become embedded in the cytoplasm of an egg cell. In isogamous unicellular organisms, the fusing pronuclei, like the gametes themselves, are morphologically indistinguishable. During autogamy, pronuclei that have been formed in the same cell fuse; they are usually sister nuclei. During conjugation in infusorians, two pronuclei form in each partner—a stationary pronucleus and a migrating pronucleus. There is first an exchange of migrating pronuclei between partners through a cytoplasmic bridge, which is followed by the fusion of the pronuclei in each of the partners.