The Fertilization Process
The long journey to fertilization must be completed within 12 to 48 hours, before the sperm die. They must first cross the barrier of the cervix, which will be thin and watery if the woman has just ovulated (for our purposes, we will assume that sexual intercourse has occurred within a couple of hours after ovulation).
Once the sperm have traversed the cervical mucus, they travel up the moist lining of the uterus into the Fallopian tubes (only one of the Fallopian tubes contains an egg, so many sperm travel in the wrong direction). Fewer than 1,000 sperm out of the millions in the semen actually reach the Fallopian tubes. Many sperm surround the egg in the Fallopian tube. The head of each sperm (acrosome) releases enzymes that begin to break down the outer, jelly-like layer of the egg's membrane, trying to penetrate the egg. Once a single sperm has penetrated, the cell membrane of the egg changes its electrical characteristics (depolarizes). This electrical signal causes small sacs just beneath the membrane (cortical granules) to dump their contents into the space surrounding the egg. The contents swell, pushing the other sperm far away from the egg (cortical reaction). The other sperm die within 48 hours. The cortical reaction ensures that only one sperm fertilizes the egg
The fertilized egg is now called a zygote. The depolarization caused by sperm penetration results in one last round of division in the egg's nucleus, forming a pronucleus containing only one set of genetic information. The pronucleus from the egg merges with the nucleus from the sperm. Once the two pronuclei merge, cell division begins immediately. The dividing zygote gets pushed along the Fallopian tube. By approximately four days after fertilization, the zygote has about 100 cells and is called a blastocyst. When the blastocyst reaches the uterine lining, it floats for about two days, finally implanting in the uterine wall by the sixth day after fertilization. Once implanted, the blastocyst secretes hCG, which rescues the corpus luteum and signals that a successful pregnancy has begun.
