Question

What is a cortical reaction? What is the mechanism for slow block to prevent polyspermy? What would be the consequences if calcium release from smooth endoplasmic reticulum is blocked?

Answer 1

• The cortical reaction is an initiative process during fertilization which is facilitated by the release of cortical granules from the egg. Cortical reaction helps in preventing the polyspermy (union of multiple sperms with one egg).
• Polyspermy is a condition which is the fusion of multiple sperms with one egg. The mechanism of preventing polyspermy is the blockage of sperm in the fertilized egg.
• The mechanism for the slow block of polyspermy takes place by 'cortical reaction'. This reaction modifies the zona pellucida (extracellular coat of egg), which is modified by the serine proteases released from the cortical granules.
• The cortical reactions occurs due to oscillations in the PLC-zeta (sperm-specific trigger of calcium oscillations in the development of egg) and when the spermatozoa gets inside the oocyte, PLZ-zeta gets activated by the basal calcium concentrations of oocyte, which in turn initiates the formation of IP3 (Inositol trisphosphate) and releases calcium from endoplasmic reticulum. This release generates oscillations in calcium concentration which blocks polyspermy by activating the oocyte. This is the mechanism for 'slow block of polyspermy' to prevent it.
• If the calcium release from the smooth endoplasmic reticulum is blocked, it leads to apoptosis (death of cell). The decreased calcium levels results in improper posttransational processing, folding and exit of endoplasmic reticulum proteins.

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