The process of fertilization in the ovum of a mouse.

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. When the pronucleus of a sperm fuses with the pronucleus of an egg, their chromosomes 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.

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