Prevention of Polyspermy in Sea Urchins

The entry of more than one sperm into the egg cell of an organism results in disastrous consequences for the developing embryo (Gilbert 2010). Known as polyspermy, many organisms have evolved mechanisms to prevent the entrance of multiple sperm during fertilization (Gilbert 2010). In sea urchins, two mechanisms are employed at the surface of the egg to ensure that monospermy occurs (Schuel 1984). A fast block occurs through an alteration in the electrical charge of the egg cell membrane, and a slow block occurs through the release of specialized secretory organelles (Gupta 2005). The fast block supports the slow block by limiting the entry of multiple sperm into the egg until these specialized secretory organelles, cortical granules, are released (Schuel 1984, Subramoniam and Subramoniam 2003).

The egg cell of a sea urchin contains a low concentration of sodium ions and a high concentration of potassium ions (Gilbert 2010). The plasma membrane of the egg cell limits the flow of sodium ions into the cell and the flow of potassium ions out of cell which results in a resting membrane potential of about -70mV (Gilbert 2010). Located on the outer surface of the egg cell membrane is an extracellular matrix called the vitelline envelope which is involved in the recognition and binding of sperm to the egg (Gilbert 2010). After the binding of first sperm to the vitelline envelope, an acrosomal protein activates the opening of sodium channels (Gupta 2005). The flow of sodium ions into the egg cell rapidly depolarizes the membrane potential to about +20mV (Gupta 2005). As a result, sperm cease to bind with the egg cell membrane because of their inability to fuse with membranes having a positive membrane potential (Gilbert 2010).

In sea urchins, the fast block to polyspermy is temporary (Gilbert 2010). The membrane potential of the egg cell remains positive for about a minute allowing polyspermy to occur if sperm remain bound to the vitelline envelope (Gilbert 2010). However, these remaining sperm are removed by a second slow block mechanism that forms a mechanical block to polyspermy (Gilbert 2010).

Following the binding and fusion of the first sperm to the egg cell of a sea urchin, soluble factors activate phospholipase C which splits phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-triphosphate (IP3) and diacylglycerol (Gilbert 2010). Binding of IP3 to specific receptors on calcium channels in the egg cell endoplasmic reticulum initiates the release of calcium ions near the site of sperm contact (Gilbert 2010). Even if an additional sperm is able to enter the egg cell, calcium is released at both sites of entry (Gilbert 2010). This increase in the intracellular concentration of calcium ions occurs in a self-propagating wave across the cell as calcium ions further stimulate the release of calcium by binding to calcium-sensitive receptors in neighboring areas of the endoplasmic reticulum