Final answer:
When the first sperm fuses with the oocyte, it deploys two mechanisms to prevent penetration by more than one sperm, which is critical to prevent triploid organisms. The egg releases proteins to prevent multiple sperm from fusing with it. Only a few hundred sperm out of millions that are ejaculated actually reach the egg, as they face various challenges in the female reproductive tract and need to undergo capacitation.
Step-by-step explanation:
When the first sperm fuses with the oocyte, the oocyte deploys two mechanisms to prevent polyspermy, which is penetration by more than one sperm. This is critical because if more than one sperm were to fertilize the oocyte, the resulting zygote would be a triploid organism with three sets of chromosomes. This is incompatible with life.
Many sperm travel towards the egg due to chemical attraction. However, only one sperm will succeed in fertilizing the ovum (egg), by penetrating its cell membrane and depositing the genetic material into the egg, where the two nuclei fuse. The fertilized ovum (zygote) immediately becomes resistant to penetration by any other sperm arriving later.
During ejaculation, hundreds of millions of sperm are released into the vagina. Only a few hundred actually reach the oocyte. The number of sperm that reach the oocyte is greatly reduced because of conditions within the female reproductive tract. Many sperm are overcome by the acidity of the vagina, others are blocked by mucus in the cervix, whereas others are attacked by phagocytic leukocytes in the uterus. Those sperm that do survive undergo a change in response to those conditions. They go through the process of capacitation, which improves their motility and alters the membrane surrounding the acrosome, the cap-like structure in the head of a sperm that contains the digestive enzymes needed for it to attach to and penetrate the oocyte.