Patient's question:
I got married last year. Recently, I've been experiencing lower abdominal pain, especially during my period. I'm very worried about it affecting fertility, and I also feel extremely cold and afraid of the cold. How does sperm enter the fallopian tube and combine with the egg?Doctor's answer:
When a mature egg departs from its mother's body—the ovary—it is embraced by the fimbriae of the fallopian tube. Due to the beating of cilia in the epithelial cells, it is transported to the abdominal opening of the fallopian tube within minutes, reaching the ampulla. There, it pauses briefly, allowing the tightly packed corona cells in its outer protective layer to relax, awaiting the arrival of sperm. When sperm break through multiple barriers to encounter the egg in the fallopian tube, the two cannot immediately fuse. Two crucial processes must occur before fertilization:First Process: Penetration of the Egg's Outer Protective Layer
This layer is composed of rich mucopolysaccharides and glycoproteins, which prevent sperm from binding to the egg. Therefore, sperm must release a series of enzymes to digest the mucopolysaccharides and proteins, dissolving the three protective layers. These enzymes are normally stored in the acrosome of the sperm head and are only released after the acrosome membrane ruptures—a process called the acrosome reaction. The acrosome enzyme system contains several types of enzymes, such as acid hydrolases, hyaluronidase, radiated corona dispersing enzymes, and acrosin, all of which are essential for dissolving the protective layer.
After the sperm passes through the cumulus oophorus and corona radiata, it loosely adheres to the surface of the zona pellucida. At this point, receptors on both the inner and outer surfaces of the zona pellucida recognize the sperm, ensuring the specificity of human egg-sperm binding. After recognition, the zona pellucida degrades, allowing the sperm to enter the egg.
Second Process: Fusion
During fusion, microvilli on the egg membrane first surround the sperm. The two cell membranes gradually merge, releasing cortical granules, which harden the zona pellucida to prevent additional sperm from entering, avoiding multiple sperm binding to a single egg. After entering the egg cytoplasm, the sperm undergo a second polar body formation, followed by the development of male and female pronuclei. When the two pronuclei contact each other, the membranes at the contact point become finger-like and interlock until they fuse, initiating the first cleavage. At this point, fertilization is complete, and a new life begins.