Human Fertilization

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•  Fertilization involves mechanisms that prevent polyspermy

The process of fertilization in humans involves a number of key processes, including:

  • Capacitation – biochemical changes which occur post ejaculation to improve sperm motility
  • Acrosome reaction – the release of hydrolytic enzymes which softens the zona pellucida (jelly coat)
  • Cortical reaction – hardening of the jelly coat post fertilization to prevent potential polyspermy

1.  Capacitation

Capacitation occurs after ejaculation, when chemicals released by the uterus dissolve the sperm’s cholesterol coat

  • This improves sperm motility (hyperactivity), meaning sperm is more likely to reach the egg (in the oviduct)
  • It also destabilises the acrosome cap, which is necessary for the acrosome reaction to occur upon egg and sperm contact

Overview of Capacitation


2.  Acrosome Reaction

When the sperm reaches an egg, the acrosome reaction allows the sperm to break through the surrounding jelly coat

  • The sperm pushes through the follicular cells of the corona radiata and binds to the zona pellucida (jelly coat)
  • The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix
  • The sperm then pushes its way through the softened jelly coat and binds to exposed docking proteins on the egg membrane
  • The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the egg

Overview of the Acrosome Reaction

acrosome reaction

3.  Cortical Reaction

The cortical reaction occurs once a sperm has successfully penetrated an egg in order to prevent polyspermy

  • Cortical granules within the egg’s cytoplasm release enzymes (via exocytosis) into the zona pellucida (jelly coat)
  • These enzymes destroy sperm binding sites and also thicken and harden the glycoprotein matrix of the jelly coat
  • This prevents other sperm from being able to penetrate the egg (polyspermy), ensuring the zygote formed is diploid 

Overview of the Cortical Reaction

cortical reaction