Fertilisation - Anatomy & Physiology

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.
  • Brought about by a fusion protein that is inactive prior to the acrosome reaction.
• 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))

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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.

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