How long does it take the body to...

Part one Also in this week's column:

• How long does it take the body to...(Part two)
• Do our ears grow longer with age?
• Do we still remove the appendix as often as we used to?

Part One: How long does it take the body to...

It takes time for everything, including what happens in the human body.

• Fingerprints form six to eight weeks before birth.
• Fingernails grow about four times faster than toenails - about .02 of an inch (0.05cm) per week.
• If a child below the age of 12 has their finger tips and nails severed above the first crease of the first joint, they can regenerate. Regeneration takes about eleven weeks. Adults do not have this ability.
• At the time of birth, the human female possesses 400,000 egg cells in both ovaries. Of these, only about 480 may ovulate during her entire reproductive life. And of these, only five per cent or so will be fertilised. On average, it takes 72 seconds for a mature egg to be pushed out of the ovary. The fertilised egg remains within the oviduct for about three days before it enters the uterus.
• In the testes of the normal human male, a thousand sperm cells are produced every second. It takes about two months to manufacture a fully mature sperm cell. After ejaculation, the sperm swim for the egg cell at the speed of 15 cm (5.91 inches) per hour. This is the equivalent of a human swimmer covering about twelve metres per second. Sperm reach the fertilisation site in about 50 minutes and remain alive for roughly two days.
• The amniotic fluid that surrounds the embryo and fetus during development is anything but a stagnant pool. While over 98 per cent of it is water, between one and two per cent is made up of substances such as fetal hair, skin cells, enzymes, urea, glucose, hormones, and lipids. It is constantly and completely replaced about every three hours.
• In the fetal brain, nerve cells develop at an average rate of more than 250,000 per minute. At birth, a child's brain contains close to a trillion nerve cells. After birth, this rate of neuron growth slows down dramatically.
• Taste buds are among the earliest sense organs to appear in the fetus. By the third trimester of pregnancy, fetal taste buds are responsive to chemicals in the amniotic fluid. The life-span of a taste bud cell is about 10.5 days.
• Twins are born, on average, 19 days earlier than singletons. Their larger combined size stretches their mother's uterine muscles, causing earlier contractions which push the twins out (there’s an old joke: A mother-to-be carrying twins doesn't give birth for 60 years! When doctors check her out with ultrasound, they find two little old men inside saying over and over to each other: "After you").
• A lactating mother produces about three pints of milk every day. Even so, if milk is not continuously removed from a mother's breasts, the ability to continue secreting milk is lost within one or two weeks. However, if the mother continues to have her breasts stimulated, milk production can continue for several years. Milk begins to flow within 30 seconds after an infant begins to suckle. During this time, nerve signals move from the breast through the spinal cord and then to the brain. The brain then secretes the hormone oxytocin. Oxytocin travels through the bloodstream back to the breasts where it causes milk to be released.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au