Fertilisation
Also known as: fertilization, conception, capacitation, acrosome reaction, cortical reaction
The fusion of a spermatozoon with a secondary oocyte, usually in the ampulla of the uterine tube within a day of ovulation, which restores the diploid chromosome number, determines genetic sex and initiates embryonic development.
Overview
Of the tens to hundreds of millions of sperm deposited in the vagina, only a few hundred reach the ampulla, helped by cervical mucus channels, uterine and tubal contractions and their own motility. Fertilisation proceeds in stages:
1. Capacitation (several hours in the female tract): cholesterol and seminal glycoproteins are removed from the sperm membrane over the acrosome; sperm develop hyperactivated motility. 2. Penetration of the corona radiata, aided by sperm-surface hyaluronidase and tubal enzymes. 3. Zona binding and the acrosome reaction: binding to the zona pellucida (ZP2 in humans) triggers exocytosis of acrosomal enzymes (acrosin), allowing the sperm to cut a path through the zona. 4. Membrane fusion: the sperm protein Izumo1 binds the oocyte receptor Juno and the membranes fuse; the sperm head (and tail) enter the oocyte. 5. Oocyte activation: sperm phospholipase C zeta triggers Ca²⁺ oscillations, causing the cortical reaction — cortical granules release enzymes that modify the zona (zona reaction), preventing further sperm binding (block to polyspermy) — and the resumption of meiosis II, with extrusion of the second polar body. 6. Pronuclei: the sperm nucleus decondenses (protamines replaced by histones) to form the male pronucleus; the oocyte chromosomes form the female pronucleus. Both replicate their DNA; their envelopes break down and the chromosomes assemble on the first mitotic spindle — the zygote.
- Location
Usually the ampulla of the uterine tube.
Function
- Restores the diploid chromosome number (46).
- Determines chromosomal sex (X- or Y-bearing sperm).
- Generates genetic variation.
- Activates the oocyte and initiates cleavage.
- Physiology
The fertile window is ≈6 days: sperm survive up to ≈5 days and the oocyte ≈12–24 h. Polyspermy (two sperm) produces a triploid embryo, which is non-viable (e.g. partial hydatidiform mole, 69,XXY).
Clinical significance
Assisted conception
IVF: oocytes are fertilised in the laboratory and embryos transferred to the uterus. ICSI injects a single sperm into the oocyte, bypassing capacitation, zona binding and fusion — used for severe male-factor infertility.
Twinning
Dizygotic twins result from fertilisation of two oocytes (familial, IVF, maternal age). Monozygotic twins result from splitting of one embryo: days 0–3 → dichorionic diamniotic; days 4–8 → monochorionic diamniotic; days 8–12 → monochorionic monoamniotic; after ≈day 13 → conjoined twins.
Test yourself
Where does fertilisation normally take place?
- Infundibulum of the uterine tube
- Ampulla of the uterine tube
- Isthmus of the uterine tube
- Body of the uterus
Answer: Ampulla of the uterine tube
The ampulla — the widest and longest segment of the tube — is the usual site of fertilisation and also the commonest site of tubal ectopic pregnancy.
Compiled from standard anatomy and physiology references — see the Reproductive System sources. For education only; not medical advice.