pronucleus

The haploid nucleus of a sperm or egg before fusion of the nuclei in fertilization.

1. One of two nuclei undergoing fusion in karyogamy.
2. The haploid nucleus of a sperm or egg before fusion of the nuclei in fertilization.

Pl. pronuclei; the haploid nucleus of a sex cell.

• female p. — the haploid nucleus of the fully mature ovum which loses its nuclear envelope and liberates its chromosomes to meet the synapsis with those from the male pronucleus.
• male p. — the nuclear material of the head of a spermatozoon, after it has penetrated the ovum and acquired a pronuclear membrane.

The noun has one meaning:

Meaning #1: the nucleus of the ovum or sperm after fertilization but before they fuse to form the nucleus of the zygote

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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