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A pronucleus (plural: pronuclei) is the nucleus of a sperm or an egg cell during the process of fertilization, after the sperm enters the ovum, but before they fuse. Sperm and egg cells are haploid, meaning they carry half the number of chromosomes. The male and female pronuclei don't fuse, although their genetic material does. Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes. Their chromosomes can then combine and become part of a single diploid nucleus in the resulting embryo, containing a full set of chromosomes.
The appearance of two pronuclei is the first sign of successful fertilization as observed during in vitro fertilisation, and is usually observed 18 hours after insemination or ICSI. The zygote is then termed a two-pronuclear zygote (2PN). Two-pronuclear zygotes transitioning through 1PN or 3PN states tend to develop into poorer-quality embryos than ones who remain 2PN throughout development, and may be significant in embryo selection in IVF.