Human fertilization

Human fertilization is the fusion of a sperm cell with an egg cell, producing a zygote. It usually occurs in a uterine tube and initiates development of a new organism.

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Sperm capacitation: A set of changes in the female reproductive tract that enables sperm to fertilize an egg. Capacitation prepares sperm to respond to the egg's coverings and undergo the acrosome reaction.
Spermatozoon: A mature male reproductive cell specialized to deliver genetic material to an egg. Its motility and surface proteins support travel to, and interaction with, the egg.
In vitro fertilization: A fertility treatment in which eggs and sperm are brought together outside the body, with resulting embryos transferred to the uterus. It enables fertilization when conception through intercourse has not occurred or is not feasible.
Zygote: The single cell formed when two gametes unite. It is the immediate product of human fertilization and begins embryonic development.
Conception: A term commonly used for fertilization or, in some contexts, the beginning of pregnancy. Its shifting usage can blur fertilization with implantation, which occurs later.
Acrosome reaction: The release of enzymes from a sperm's acrosome as it approaches an egg. This reaction helps sperm penetrate the egg's outer protective layer.
Oocyte: A female germ cell that develops into an egg and can be fertilized. The human cell typically fertilized is a secondary oocyte arrested in meiosis.
Intracytoplasmic sperm injection: An assisted-reproduction technique in which a single sperm is injected directly into an egg. It bypasses several sperm–egg interaction steps when fertilization is difficult.
Cleavage (embryology): A series of rapid cell divisions that partitions an early embryo without initially increasing its overall size. The zygote enters cleavage as it travels toward the uterus.
Implantation: The attachment and embedding of an early embryo in the uterine lining. It follows fertilization and embryo development; it is not the fusion of gametes.