Human fertilization

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The acrosome reaction for a sea urchin, a similar process.
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 cortical 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.
  • For Fertilization to occur, Semen must be ejaculated deep within the vaginal cavity, to allow for easier passage of individual sperm.
  • Semen outside of the body generally has a life expectancy of 30-60 seconds.

Various disorders can arise from defects in the fertilization process.

  • Polyspermy results from multiple sperm fertilizing an egg.

  • 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
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