3.11—Mammalian gamete specialisation

Mammalian gametes are haploid cells produced by meiosis. Their structures are specialised for meeting, penetrating and supporting the next stage of reproduction.

Sperm have a flagellum for movement, many mitochondria to supply ATP and an acrosome containing enzymes that digest the zona pellucida. An egg is much larger, contains nutrient reserves, and has a zona pellucida that can become a barrier after fertilisation.

The acrosome helps one sperm reach the egg membrane; once fertilisation begins, changes to the egg’s zona pellucida help prevent additional sperm nuclei entering.

The egg’s size is not simply “for movement” and the sperm’s acrosome is not a nucleus. A gamete is haploid; the diploid chromosome number is restored only when nuclei fuse.