There is hair on the human body and on animals that seems to know when it is long enough and stops growing. Strangely enough, if you shave that hair it starts growing again. How does this happen? Please Register or Log in to view the hidden image!
?? My hair has never stopped growing. Some people's hair stops growing (or actually slows down) during malnutrition or disease, but that's not the norm.
I don't think it actually stops growing. I think each hair eventually falls out, to be replaced by new one. The rate of this would establish the average length of hairs in one region or another.
I would assume the growth moves from the case of the cell , so the longer the hair the harder it become to push and additional layer at the base , so if you shave it should grow faster up.
Hairs do not stop growing--except as we get older and some of them simply die. Remember that a hair grows from its root, so the only strong, live part is right where it comes out of the skin. The rest of it is dead tissue which has no blood supply and cannot repair itself. These dead tissues are subject to all kinds of destructive forces, including shearing, tearing, stretching, abrasion, temperature changes and chemicals in the natural environment as well as artificial ones in our cleaners and cosmetics. The hairs are somewhat protected closer to the skin, but the ends are more vulnerable to the environment. And since the ends are the oldest part of the hair, they have been subject to more destructive forces. So the ends are shorn, torn, stretched, and abraded off, as well as simply falling/breaking off due to structural weakening from temperature changes and chemicals. If you want your hair to grow as long as possible, braid it to keep all the individual hairs in contact with each other. In unity there is strength, so as one thick braid it will be much more able to resist the forces of the natural and unnatural environment. And if you closely examine someone's braided hair, you'll find that there's a lot less of it at the end than on top of their head. Even braided, very few people can get their hair much longer than waist-length. The destructive forces are still there, just a little weaker, so they take longer to do their damage. Dogs, cats, horses, all of the domestic animals we think of as "long-haired" have the same problem. Some varieties of these species may have significantly stronger individual hairs than ours, but they are still subject to the forces of the natural and unnatural environment. People who show these animals go to a lot of trouble to protect their hair so they look beautiful in the show ring. I don't know if a person in a coma will end up with very long hair, because for the people who take care of comatose patients, hair is not one of their most important responsibilities. Since they're lying on their back most of the time, their hair probably suffers great abuse. If somebody thought of it, perhaps they could braid it and set a world record. Nonetheless, even that hair will eventually reach a length at which the few dangers in the hospital environment will succeed in breaking it off. Your body hair, eyebrows, pubic hair, etc., are not nearly as thick and strong as the hairs on your head (and beard, for some men) so it breaks off much more quickly. The technology of clothing was invented approximately 70K years ago. (We know this because DNA analysis tells us that this is when body lice, which live only on humans, speciated from head lice, which live on many other mammals.) Ever since then, there has been no evolutionary advantage to having thick body hair, and in fact it's probably a disadvantage in some circumstances. So we are probably not as hairy as our distant ancestors.
Each hair follicle goes through phases. In the growth phase (years), the follicle produces keratin, the hair grows. The duration of this phase and the rate of growth determines the maximum length of the hair. In the dormant phase (months), the follicle stops making keratin, and the hair does not grow. When the next growth phase starts, the old hair is shed from the follicle, and a new hair starts from scratch.
It doesn't. The follicle (which is a group of cells) 'knows', in some sense, the length of time it has been active; ie it changes in some way such that the growth phase lasts for a limited time. I don't know the cellular mechanism involved. Perhaps (wild guess time) there is a limited supply of some chemical that is consumed during the growth phase and replenished during the dormant phase. Or vice versa.
