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Inhibition of the fast and slow blocks to polyspermy

Objective: to test the effects of inhibiting the fast and slow blocks to polyspermy on sea urchin development.

Sea urchin eggs release components of the egg jelly that attract sperm and induce the acrosome reaction (Gilbert, 2000). In the laboratory, a single egg many be covered with dozens or even hundreds of sperm burrowing through the egg jelly.

Once a sperm contacts the viltelline membrane, it binds to a receptor protein that triggers the events of fertilization. Eventually, the plasma membranes fuse, and the sperm nucleus and centriole enter the egg. However, if a single egg is fertilized by multiple sperm (polyspermy), additional chromosomes and centrioles are brought insideThis can cause defects at the first cleavage division as the chromosomes are segregated randomly between multiple spindles (Just, 1919, Gilbert, 2000).

Two mechanisms have evolved to reduce the occurrence of polyspermy. Sperm binding initially triggers an influx of Na+ ions leading to depolarization, which transiently blocks binding of additional sperm (known as the fast blockto polyspermy). This depolarization can be blocked by the addition of nicotine (Jaffe, 1980). In addition, sperm binding also stimulates the release of Ca++ ions from the endoplasmic reticulum, leading to the fusion of a series of exocytic vesicles (the cortical granules) with the plasma membrane. This releases their contents, including a trypsin-like protease, which destroys the sperm binding sites on the vitelline membrane and detaches it from the plasma membrane (Vacquier, et al, 1973). Water rushes into the space, causing the vitelline membrane to rise up and become the fertilization envelope. This produces a more permanent block to polyspermy (the slow block).

© 2001 Cebra-Thomas

Last Modified: 20 April, 2001

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