Fertilization

Core Concepts

External fertilization: the aquatic strategy

External fertilization — gametes released into water and meeting outside the body — is common in aquatic animals such as corals, sea urchins, many fishes, and frogs. Water is essential: sperm swim to eggs through it, and the gametes never dry out. Because sperm and eggs are released into a vast environment, most never meet, so animals that use this strategy produce enormous numbers of gametes — a female cod can release millions of eggs — and often synchronize release using environmental cues (temperature, tides, lunar phase) or chemical signals (pheromones), so that eggs and sperm are present at the same time and place. Parental care is usually minimal or absent; the strategy trades quantity and environmental luck for low energy investment per offspring. Coral spawning events, where an entire reef releases gametes on the same night, are the spectacular visible result.

Internal fertilization: the terrestrial strategy

Internal fertilization — sperm deposited inside the female reproductive tract — is the rule in terrestrial animals (mammals, reptiles, birds, insects) and also occurs in some aquatic ones (sharks, some fish). It requires either copulation or transfer of a sperm package (a spermatophore, as in some salamanders and arthropods). Internal fertilization protects gametes from desiccation, greatly reduces the number of gametes needed (a human ejaculate contains millions of sperm, but a female produces only one egg per cycle — both tiny numbers compared with broadcast spawning), and sets the stage for internal development (viviparity) and parental care. The trade-off is the mirror image of external fertilization: few offspring, high investment in each.

Species recognition: the molecular handshake

Sperm and egg of different species do not normally fuse, because fertilization begins with specific molecular interactions. In sea urchins, a sperm protein (bindin) binds to a receptor on the egg surface; in mammals, sperm bind to the zona pellucida, the glycoprotein coat around the egg, via species-specific receptors (the protein ZP3 is the classic example). This specificity is an important reproductive isolating mechanism — one of the barriers that keeps species separate — and a good answer to "what stops a horse sperm from fertilizing a donkey egg?" (a mule, when it happens between close relatives, but only because the handshake is similar enough).

The fast and slow blocks to polyspermy

Once one sperm fuses with the egg, the egg must prevent others from entering. The sea urchin provides the textbook two-step model:

1. Fast block (membrane depolarization). Within milliseconds of sperm fusion, sodium ions rush in and the egg membrane's electrical potential jumps from negative to positive, repelling additional sperm. This block is quick but temporary.
2. Slow block (cortical reaction). Seconds later, cortical granules just beneath the egg membrane fuse with the membrane and release enzymes into the space around the egg. The enzymes modify the vitelline layer, lifting it away from the egg to form a hardened fertilization envelope that sperm can no longer penetrate. This permanent barrier seals the egg.

The same logic applies in mammals: the cortical reaction modifies the zona pellucida so no further sperm can bind. The result is a diploid zygote — the male and female pronuclei have fused — ready to begin cleavage.

Common Confusions