Conception

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.
  

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