Sperm penetration into the egg: apical enzymes and apical reactions
Only a few minutes after the first sperm penetrates through the light membrane, calcium ions penetrate inside, the cell initiates a shell reaction that pours special substances into the periovulatory cavity.
The parietal body contains a large amount of Hyaluronidase and proteolytic enzymes. Hyaluronidase degrades the polymerization of hyaluronic acid - the component of intercellular binding - between the granulosa cells. Proteolytic enzymes destroy proteins in the layers of tissue cells while preserving the egg.
When the egg is released into the fallopian tube, it still carries many layers of granulosa cells. Before sperm can fertilize an egg, they must pass through layers of granulosa cells, then the thick transparent membrane that surrounds the egg. To do that requires the release of enzymes in the apical body. It is believed that the enzyme Hyaluronidase plays a particularly important role in paving the way between the granulosa cells that help sperm reach the egg.
When the sperm finds the transparent membrane, its anterior membrane will bind to the specific receptor on the transparent membrane of the egg. Next, the entire apical body rapidly dissolves, releasing all the enzymes in it. Within minutes, those enzymes had opened a pathway that helped the sperm head penetrate the transparent membrane into the egg. Over the next 30 minutes, the cell membrane at the tip of the sperm along with the cell membrane of the egg fuses together to form a single cell. At the same time, the chromosomes of the egg and sperm combine to form a completely new genome, with equal contributions from both parents. This is the fertilization process.
With so many sperm involved in the fertilization process, why can only 1 sperm go in to fertilize the egg? The reason is not completely understood. However, just a few minutes after the first sperm penetrates through the light membrane, calcium ions penetrate inside, the cell initiates a shell reaction that pours special substances into the periovulatory cavity. These particles penetrate the transparent membrane, preventing the attachment of other sperm, and even causing the attached sperm to fall off. As such, it is nearly impossible for more than one sperm to fertilize an egg.