Question

Where does fertilization take place in a woman? What helps the sperm to gain an entry into the ovum and how? What is the significance of the point of entry of the sperm?

Answer 1

In women, fertilization takes place in the ampullary-isthmic junction of the fallopian tube. When the sperm surrounds the egg cell, the egg releases the hormone progesterone which causes the sperm to undergo capacitation (experience an increase in activity and shed some outer protein layers). This helps the sperm to penetrate the outer layer of the egg called the corona radiata and come in contact with the zona pellucida layer. When the sperm comes in contact with the zona pellucida layer of the ovum, it induces changes in the membrane that block the entry of additional sperms so only one sperm can fertilize the ovum. The sperm releases the enzymes of the acrosome which digest the zona pellucida layer and helps the sperm plasma membrane to fuse with the egg plasma membrane. This fusion causes the release of calcium ions which in turn, causes the cortical granules inside the egg to fuse with the plasma membrane. As they fuse, these granules release their contents outside of the cell, toward the remains of the zona pellucida. The enzymes of the cortical granules further digest the zona pellucida, making it unable to bind more sperm, while other molecules found in the granules create a new protective layer around the fertilized egg.