PPT Slide
1. In marine invertebrates (e.g. SU), a fast block to polyspermy occurs. The FBPS involves the opening of Na+ channels in the egg PM. Na+ flows into the egg cell, depolarizing the membrane. This depolarization prevents additional sperm from fusing to the egg PM. The egg PM is restored to its normal -60mV potential within minutes of fusion as the Na+ channels close, other + ions flow out of the cell, and Na+ is pumped out.
2. Fusion is a signal that is transduced into two longer term cellular responses (SU + vertebrates). Two compounds are produced from a common precursor molecule in the plasma membrane.
- A. The slow block to polyspermy. The SBPS occurs within 10 secs. of fusion. One of the compounds (IP3) causes the release of Ca++ from intracellular stores in the endoplasmic reticulum. This Ca++ elicits the fusion of cortical vesicles (CV’s) with the egg PM, releasing their contents into the perivitelline space(PVS). This raises the vitelline membrane (VM), and enzymes so released inactivate bindin receptors on the VM. Thus, any additional sperm are released from the VM and no more bind.
- B. Intracellular pH is increased. The second compound (DAG) causes protein phosphorylation cascades to be initiated with one result being the phosphorylation and activation of a PM Na+:H+ ion exchanger. Na+ is pumped into the cell and H+ out. pH inside the cell increases. This has effects on gene expression (activated), translation of mRNA into protein (~8 min. post fertilization), and initiation of DNA synthesis (~30 min. post fertilization).