Egg activation

After the fusion of the sperm plasma membrane and the egg plasma membrane after fertilization, animal eggs go through a process called egg activation to prepare the egg for development.

There are three functions of egg activation:

1. Block to polyspermy
2. Activation of egg metabolism
3. Resumption of the cell cycle

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Sperm trigger of egg activation

The sperm may trigger egg activation via the interaction between a sperm protein and an egg surface receptor. It is possible that a G protein receptor is activated by the sperm binding which activates a tyrosine kinase which then activates PLC. The inositol signaling system has been implicated as the pathway involved with egg activation. IP₃ and DAG are produced from the cleavage of PIP₂ by phospholipase C (PLC). However, another hypothesis is that a soluble 'sperm factor' diffuses from the sperm into the egg cytosol upon sperm-oocyte fusion. The results of this interaction could activate a signal transduction pathway that uses second messengers. A novel PLC isoform, PLC zeta, may be the equivalent of the mammalian sperm factor. A recent paper shows that mammaliam sperm contain PLC zeta which can start the signaling cascade^[1]

Fast and slow block to polyspermy

Polyspermy is the condition when multiple sperm fuse with a single egg. This results in duplications of genetic material. In sea urchins, the block to polyspermy comes from two mechanisms: the fast block and the slow block. The fast block is an electrical block to polyspermy. The resting potential of an egg is -70mV. After contact with sperm, an influx of sodium ions from the sea water increases the potential up to +20mV. The slow block is through a biochemical mechanism triggered by a wave of calcium increase. The rise of calcium is both necessary and sufficient to trigger the slow block. In the cortical reaction, cortical granules directly beneath the plasma membrane are released into the space between the plasma membrane and the vitelline membrane (the perivitelline space). An increase in calcium triggers this release. The contents of the granules contain proteases, mucopolysaccharides, hyalin, and peroxidases. The proteases cleave the bridges connecting the plasma membrane and the vitelline membrane and cleave the bindin to release the sperm. The mucopolysaccharides attract water to raise the vitelline membrane. The hyalin forms a layer adjacent to the plasma membrane and the peroxidases cross-link the protein in the vitelline membrane to harden it and make it impenetrable to sperm. Through these molecules the vitelline membrane is transformed into the fertilization membrane or fertilization envelope. In mice, the zona reaction is the equivalent to the cortical reaction in sea urchins. The terminal sugars from ZP3 are cleaved to release the sperm and prevent new binding.

Late responses

The combined increase of calcium signal and pH leads to a dramatic increase in protein synthesis. The protein synthesis uses mRNAs that are already present in the oocyte. These proteins direct the next events of embryogenesis such as the fusion of the egg and sperm pronuclei to form the diploid embryonic nucleus. The fusion is driven by microtubule-dependent movement of the female pronucleus to the male pronucleus.

References

1. ^ Saunders C, Larman M, Parrington J, Cox L, Royse J, Blayney L, Swann K, Lai F (2002). "PLC zeta: a sperm-specific trigger of Ca(2+) oscillations in eggs and embryo development.". Development 129 (15): 3533-44. PMID 12117804.

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