Zygote

Zygote is the initial cell formed when two gametes fuse during sexual reproduction. It is a diploid cell containing a complete genome derived from both parents. In humans, the haploid sets from the sperm and oocyte merge to form a 46-chromosome nucleus. The zygote contains maternal transcripts and proteins deposited in the oocyte, and paternal and maternal genomes that will guide embryonic development after genome activation.

Fertilization marks the start of embryogenesis. The zygote undergoes rapid mitotic divisions called cleavage, increasing cell number without growth in size. These cleavages produce a multicellular structure that progresses through the morula and then the blastocyst stage. The blastocyst implants into the uterine lining, with the outer trophectoderm contributing to the placenta and the inner cell mass forming the embryo.

A key developmental transition is zygotic genome activation, when control shifts from maternal gene products to the zygote’s own genome. In humans, major ZGA occurs at about the 4- to 8-cell stage, with earlier minor transcriptional activity around the 1- to 2-cell stage.

Zygotes are typically considered totipotent in the earliest stage, capable of giving rise to all embryonic and extraembryonic tissues. Questions of centriole inheritance and other cytoplasmic attributes vary among species. In reproductive medicine, zygotes are routinely produced by in vitro fertilization and cultured to the blastocyst stage before transfer into the uterus.