Fertilisation - Anatomy & Physiology

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Fusion with the Oocyte

  • When the Spermatozoan completely penetrates the Zona Pellucida and reaches the Perivitelline Space, it settles into a bed of microvilli formed by the Oocyte plasma membrane.
  • Oocyte plasma membrane fuses with the equitorial segment and the fertilizing Spermatozoon is engulfed.
  • Nucleus of the Spermatozoon is within the Oocyte cytoplasm.
  • Sperm nuclear membrane disappears.
  • Sperm nucleus decondenses.


Electron Scanning Micrograph of Spermatozoa in the Uterine Horn. Copyright RVC 2008 (Courtesy of John Bredl (RVC))
















Cortical Reaction - Block to Polyspermy

  • During the first and second meiotic divisions of Oogenesis small,dense cortical granules move to the periphery of the Oocyte cytoplasm.
  • Cortical granules consist of:
    • Mucopolysaccharides
    • Proteases
    • Peroxidase
  • Cortical granules undergo exocytosis, releasing their contents into the perivitteline space.
  • Contents of cortical granules cross-links zona proteins to make them impenetrable to further Spermatozoa.
  • This is known as the zona block, it prevents polyspermy (fertilization by more than one sperm)which would result in embryo death.


  • The cortical reaction also reduces the ability of the Oocyte plasma membrane to fuse with additional spermatozoa.
  • This is the vitelline block to polyspermy.


  • If two spermatozoa enter the perivitelline space simultaneously, they both contact the oocyte and proteins are not cross-linked rapidly enough to block penetration of the Zona Pellucida.
  • Thus, the block depends on limiting the number of Spermatozoa in the vicinity of the Oocyte.
  • Only a small sub-population are released from the store in the female tract over the period of ovulation so that 7-10 out of the original millions are around the Oocyte at the period of fertilization.



Formation of Pronuclei