Why does only one sperm normally fertilize an egg?

Fertilization triggers fast electrical and slower structural changes to the egg surface — the blocks to polyspermy — that prevent additional sperm from fusing, so the zygote keeps the correct chromosome number.

What does egg activation mean?

It refers to the changes, especially a rise in calcium inside the egg, that wake the egg from its arrested state and start the metabolic and cell-cycle events of development.

Fertilization and Egg Activation

Definition

Fertilization is the process by which a sperm and an egg recognize each other, fuse, and combine their genomes to form a diploid zygote; egg activation is the accompanying set of changes — notably a rise in intracellular calcium — that ends the egg's dormancy and triggers the onset of development.

Scope

This topic covers gamete recognition and binding, the acrosome reaction, sperm–egg membrane fusion, the blocks to polyspermy, and the calcium-driven activation of egg metabolism that initiates development. It also addresses the union of the two haploid genomes to form a diploid zygote. Reproductive-medicine applications are framed as significance rather than clinical guidance.

Core questions

How do sperm and egg of the same species recognize and bind each other?
How does the egg prevent fertilization by more than one sperm?
What signal activates the egg and restarts the cell cycle after sperm entry?
How do the maternal and paternal genomes come together in the zygote?

Key concepts

Species-specific gamete recognition
Acrosome reaction
Fast and slow blocks to polyspermy
Calcium wave and egg activation
Pronuclear fusion and restoration of diploidy

Mechanisms

Capacitated sperm bind the egg's extracellular coat (zona pellucida or vitelline envelope) through species-specific receptors and undergo the acrosome reaction, releasing enzymes that allow penetration. Membrane fusion delivers the sperm nucleus and triggers a rise in egg intracellular calcium, often as a propagating wave. This calcium signal drives egg activation: cortical granule exocytosis modifies the egg coat to block additional sperm (the slow block to polyspermy), the metabolic and translational machinery is switched on, and the meiotic cell cycle resumes. The sperm and egg pronuclei then replicate their DNA and combine to restore the diploid genome of the zygote.

Clinical relevance

Mechanisms of gamete recognition and egg activation underpin assisted-reproduction techniques and explanations of certain infertility and abnormal-fertilization outcomes. This entry is educational and does not provide diagnostic or treatment guidance.

History

Fertilization was first described microscopically in the nineteenth century, when the fusion of sperm and egg nuclei was observed in sea urchins and other marine animals; later work identified the calcium signal and cortical reaction that activate the egg.

Seminal works

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Frequently asked questions

Why does only one sperm normally fertilize an egg?: Fertilization triggers fast electrical and slower structural changes to the egg surface — the blocks to polyspermy — that prevent additional sperm from fusing, so the zygote keeps the correct chromosome number.
What does egg activation mean?: It refers to the changes, especially a rise in calcium inside the egg, that wake the egg from its arrested state and start the metabolic and cell-cycle events of development.