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Embryology · General embryology: weeks 1 to 8

Fertilization: capacitation, acrosome reaction and zona reaction !

  • Process

Sperm meets egg in the uterine tube: only one gets in, and a new individual with 46 chromosomes begins.

Updated 2026-11-01

See it in 3D: Ampulla of uterine tube · Uterine tubes · Infundibulum and fimbriae

What it is

Fertilization usually happens in the ampulla of the uterine tube within about 24 hours of ovulation. The sperm must be capacitated, pass the corona radiata, bind the zona pellucida and release its acrosome; when it fuses with the oocyte, the cortical reaction blocks other sperm. The oocyte completes meiosis II and the two pronuclei form the zygote.

Key facts

  • Site: ampulla of the uterine tube.
  • The oocyte can be fertilized for about a day; sperm survive several days in the female tract.
  • Capacitation: in the female tract, glycoproteins and seminal proteins are removed from the sperm membrane.
  • Acrosome reaction: on binding the zona pellucida (classically ZP3) the sperm releases enzymes such as acrosin.
  • Cortical and zona reactions: cortical granules change the zona pellucida and prevent polyspermy.
  • Genetic sex is set by the sperm (X or Y); the embryo's mitochondria are maternal.

Step by step

  1. Sperm capacitation in the uterus and tube.
  2. Passage through the corona radiata.
  3. Binding to the zona pellucida and acrosome reaction; the sperm crosses the zona.
  4. Fusion of the sperm and oocyte membranes.
  5. Cortical reaction: the zona pellucida becomes impenetrable to other sperm.
  6. The oocyte completes meiosis II, extrudes the second polar body and forms the female pronucleus.
  7. The pronuclei replicate their DNA and meet on the first cleavage spindle: a 46-chromosome zygote.

Markers

  • ZP3
  • Acrosin
  • Cortical granules

Clinical notes

  • Ectopic pregnancy: if the zygote does not move along the tube, it can implant there, most often in the ampulla.
  • Polyspermy (two sperm): a cause of triploidy and partial mole.
  • In vitro fertilization and intracytoplasmic sperm injection bypass steps of this process in infertility.
  • Emergency contraception works mainly by delaying ovulation.

Test yourself

Think of the answer, then tap the question to see it.

Where does fertilization usually take place?

In the ampulla of the uterine tube.

What stops a second sperm from entering?

The cortical reaction: cortical granules release their contents and change the zona pellucida (zona reaction), which then rejects sperm.

What happens to the oocyte's meiosis at fertilization?

Meiosis II is completed: the second polar body is extruded, leaving the haploid female pronucleus.

What is capacitation and where does it occur?

The final maturation of sperm in the female tract: surface proteins are removed so the sperm can undergo the acrosome reaction.

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The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 3: First Week of Development: Ovulation to Implantation (Wolters Kluwer, 2015); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); How do cells divide?: MedlinePlus Genetics (MedlinePlus (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.