Human fertilization

The acrosome reaction for a sea urchin, a similar process.

Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube.

There is a specific sequence of events that occur in fertilization:

The sperm passes through the corona radiata, the outermost cell layer of the egg.
The sperm breaks through the zona pellucida.
- This occurs with the aid of several enzymes possessed by the sperm that break down the proteins of the zona pellucida, the most important one being acrosin.
- When the sperm penetrates the zona pellucida, the Acrosome reaction occurs. This makes the egg impermeable to any other sperms and prevents fertilization by more than one sperm.
The cell membranes of the egg and sperm fuse together.
The female egg, also called a secondary oocyte at this stage, completes its second meiotic division. This results in a mature ovum.
The sperm's tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.
The male and female pronuclei fuse to form a new nucleus that is a combination of the genetic material from both the sperm and egg.

[edit] Diseases

Various disorders can arise from defects in the fertilization process.

Dudek, Ronald W. High-Yield Embryology, 2nd ed. (2001). ISBN 0-7817-2132-6
Moore, Keith L. and T.V.N. Persaud. The Developing Human: Clinically Oriented Embryology, 7th ed. (2003). ISBN 0-7216-9412-8