CURIOSITY: How does your hair know when to stop growing
Did you ever wonder why the hair on your head just seems to keep growing and growing? Yet the hair on your legs, armpits, and pubic region seems to have limit to how long it can grow? It’s kind of weird when you think about, but also very convenient. Can you imagine what we’d all look like if all the hair on our bodies just kept on growing (and we stopped shaving)? Maybe that’s where Wookies came from!
Your hair grows in cycles
Each hair on your body goes through the same cycle of growing, and then falling out. The the part of the cycle that deals with hair growing is called the anagen phase. During the anagen phase blood flow starts to feed oxygen to stem cells, which in turn start to divide and produce keratinocytes, which expands to form a hair. As the cells die, a chemical called keratin is released which keeps the strands of hair together. As long as a hair is in the anagen phase it can grow up to 1,25 centimetres a month.
When the hair goes into the next phase, called catagen, which lasts roughly two weeks. During the phase, the blood flow to the hair stops, and therefore no new keratinocytes can be produced. The hair then dies completely, shrinks, and is pushed to the surface.
The last phase is the telogen phase, and here the hair follicle remains dormant for anywhere from one to four months. After this period the hair that has been close to the surface all this time, is released and the hair falls out. Then the whole process starts all over again with the anagen phase. Anywhere from fifty to one hundred hairs are shed each day from a healthy scalp. So don’t freak out the next time your brush your hair and you realise the brush is full. It’s probably perfectly normal.
The hair on your arms, legs, armpits and pubic regions grow for about thirty to forty-five days, whereas the hair on your head can grow anywhere from two to six years. That’s why some people have ridiculously, but lovely, long hair.
Your hair knows when to stop growing due to stem cells
Why exactly your hair has a certain expiration date isn’t too clear yet. Scientists are still trying to get to the root of the problem of how your hair knows when to stop growing. The current generally accepted theory is that hairs get their instructions from the stem cells in the skin. The same stem cells most likely that activate the hair growth in the first place.
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