Fertilization is the process in which a male sex cell (sperm) joins with a female sex cell (egg) to form a single new cell called a zygote, which is the start of a new organism. In humans, this usually happens in the fallopian tube, not in the uterus.
Basic definition
- Fertilization is the fusion of the nucleus of a sperm with the nucleus of an egg so that their genetic material combines into one cell.
- This new cell, the zygote, has a full set of chromosomes and can begin dividing to eventually form an embryo.
Step-by-step in humans
- During ovulation, an ovary releases an egg, which is picked up by the nearby fallopian tube.
- After intercourse, millions of sperm travel from the vagina, through the cervix and uterus, into the fallopian tubes; only a few hundred reach the egg.
- One sperm manages to penetrate the egg’s outer layers and fuse with it; once this happens, the egg’s surface changes so no other sperm can enter.
- The sperm and egg nuclei then merge, forming the zygote.
After fertilization
- The zygote travels down the fallopian tube toward the uterus, dividing into more and more cells as it goes.
- A few days later it reaches the uterus as a blastocyst and implants in the uterine lining, where it can continue developing into an embryo and then a fetus.
