Natural conception (or fertilization) requires sexual intercourse between a male and female. Healthy men typically release around 300 million sperm into the vagina during ejaculation. Sperm are the male gamete (reproductive cell) and have pinched, oval-shaped heads. A sperm swims using a flagellum tail that continuously whips back and forth. Despite this ideal structure, about 60% of sperm are imperfect even in the healthiest men. Many have multiple tails, are unable to move, or are otherwise defective. These sperm never escape the vagina, destroyed by its acidic 4.5 pH within hours. Their more successful siblings head toward the cervix.
Sperm are often imagined as relentless warriors questing towards the egg (the female gamete); this is not true. In reality, the woman’s body does most of the work. Usually the cervix – the narrow channel connecting the vagina and uterus – blocks intruding cells with a thick mucus plug. In the days preceding ovulation (the brief period when an egg is released to be fertilized) this mucus becomes watery, allowing sperm to pass through. Once inside the uterus (or womb), it would take the sperm about two days to reach the egg in the fallopian tubes. However, thanks to undulations of the uterine wall, the sperm are swept upward, making the trip in about half an hour.
Once in the fallopian tubes, most sperm are trapped and sustained in the hair-like fallopian cilia, where they undergo capacitation. When sperm first enter the woman, they cannot fuse with the egg; capacitation – chemical and molecular changes to the sperm head – enables fertilization. The cilia sustain capacitated sperm up to a week, releasing them towards the egg a few hundred at a time.
This prolonged release of sperm is vital, as the egg might not actually be there during intercourse. After a 3-5 day fertile period, an ovary releases an egg during ovulation; it remains in the fallopian tube for only a day. The egg is far larger than a sperm and well protected. To fertilize, sperm must pass a layer of selective chaperone-support cells to reach the zona – a thick, protein shell encasing the egg that blocks all but one sperm. That lucky sperm cannot simply muscle it’s way in: after precisely matching membrane-bound proteins to the zona, the sperm head transforms into an enzymatic drill, boring through. Upon finally touching the egg, the cells’ membranes fuse and the egg pulls the sperm – and it’s genetic contents – inside.
Once fused, the two gametes form one zygote that migrates to the uterus and adheres to the uterine wall, allowing development to continue. However, many fertilized eggs never make it this far, and pass out of the woman’s body during menstruation. Clearly, conception is a chancy process that requires numerous factors to line up perfectly. Given the odds against us – from before we were even born – each one of us really is lucky just to be alive.