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))

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.