Fertilisation - Anatomy & Physiology

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

Fusion with the Oocyte

• When the spermatozoon 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.

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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Formation of Pronuclei

After the sperm nucleus enters the Oocyte it becomes the male pronucleus. Before this can occur, the sperm nucleus must undergo marked changes within the Oocyte cytoplasm.

• Disulphide cross-links render the nucleus almost inert. This is important when exposed to the hostile environment of the female tract during transport and also on penetration through the Zona Pellucida.
• In the Oocyte cytoplasm, the nucleus must decondense in order for male and female chromosomes to pair.
• Disulphide cross-links are rapidly reduced in the Oocyte cytoplasm primarily by Glutathione.

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Syngamy

• Male and female pronuclei fuse.
• Sperm delivers a haploid complement of chromosomes, but also activates the Oocyte by:
  • A unique phospholipase Cζ (zeta) causes release of calcium from the endoplasmic reticulem to the cytosol. This is then re-sequestered and bound in the endoplasmic reticulem to give regular 'spikes' or oscillations of calcium that signal rapidly across the Oocyte. The calcium signal activates the Oocyte from meiotic arrest.
  • Provides the centriole contribution to initial cell division, allowing formation of the two-celled embryo.

N.B: Although sperm mitochondria enters the Ooctye cytoplasm, they carry a ubiquitin signal. They are subsequently destroyed by Ubiquiitinase and do NOT contribute to mitochondrial DNA. Therefore mitochondial DNA is entirely from the maternal side.