How does hair know how long to grow?
January 10, 2008 8:16 PM Subscribe
How does hair know how long to grow?
What is it that lets hair on your head (if you are lucky enough to have it) keep growing while the hair on your body stops growing at a certain length?
What is it that lets hair on your head (if you are lucky enough to have it) keep growing while the hair on your body stops growing at a certain length?
Old Straight Dope column on this. Upshot: nobody knows, but of course, that was in 1980.
posted by Chrysostom at 8:42 PM on January 10, 2008
posted by Chrysostom at 8:42 PM on January 10, 2008
I always assumed hairs grow for a set length of time and then fall out. After all, if you shave body hair, or your short-haired pet, the hair grows back. I doubt that the hair follicle 'knows' how long the hair is. The only mechanism I could see for this is a programmed growth rate and lifetime for each hair. Short hairs grow more slowly and/or fall out sooner. Programming courtesy of your genes.
posted by DarkForest at 9:10 PM on January 10, 2008 [1 favorite]
posted by DarkForest at 9:10 PM on January 10, 2008 [1 favorite]
It's definitely genetic. I used to have hair down to my thighs, and I've known some who were able to grow theirs even longer than that. When I had thigh-length hair, people would always ask how I'd gotten it to grow that long.
posted by cmgonzalez at 9:13 PM on January 10, 2008
posted by cmgonzalez at 9:13 PM on January 10, 2008
Well its all an average, but in general each individual hair follicle lives for about seven years and hair grows about half and inch a month.
posted by stormygrey at 9:22 PM on January 10, 2008
posted by stormygrey at 9:22 PM on January 10, 2008
I would say it's genetic too. But a brief Google search and pubmed perusal don't reveal any obvious investigation as to what genes in particular affect hair length. However, it might be useful to see search for information about hypertrichosis lanuginosa or the human hairless gene and the like for possible genes that influence hair growth. Otherwise I would say that DarkForest describes the factors of hair growth genes would have to influence for different hair lengths to appear.
posted by Mister Cheese at 9:24 PM on January 10, 2008
posted by Mister Cheese at 9:24 PM on January 10, 2008
I'm a member of DarkForest's school of thought. That makes the most sense to me.
posted by HotPatatta at 10:52 PM on January 10, 2008
posted by HotPatatta at 10:52 PM on January 10, 2008
It's genetic, yes, but the detail is interesting. As I remember from my first year molecular biology lectures (was it really only a few months ago?!):
Hair distribution is genetic, controlled as various protein/hormone producing genes switch on and off during embryogeneis. These in turn determine which genes are "switched on" in the various follicles which, in their own turn, determine what type of hair grows and how sensitive the follicles are to androgens and other hormones. Even at the gastrula stage - literally, just a few hundred cells - it can be determined what cell types will end up where in the body. So, you end up with 3 very slightly different follicle types spread round the body.
Firstly, as an embryo, all the follicles produce the fine baby hair called lanugo. Even at this stage, though, hairs on the head tend to be longer - that's controlled by genes switched on earlier in the development process. AFAIK, this hair growth is driven by hormones from the mother's body.
Once the baby is born it stops getting those hormones. This is what causes the baby hair to fall out, until the body ramps up its own hormone production and the hair grows again. This time, because of the increased / different hormones, the different follicle types produce two different types of hair - one type produces terminal hair on the head, and the remaining two types produce vellus hair on the body.
At puberty, hormone production changes / ramps up again. This time, the the two different vellus-producing follicle typesreact differently. Follicle cells in in the pubic area, underarms and, in males, the face, react to the increased production of hormones and start growing thicker / darker / longer / curlier hair. The remaining follicles on the body do the same too, but not to the same extent - they produce hair that is thicker / darker / longer / curlier, but only slightly.
And there you have it. There's a genetic basis to how long your hair will grow, but it's also affected to a large degree by hormones - which, in turn, are affected by other things e.g. nutrition, etc. Case in point: one of the effects of extreme anorexia is loss of body hair, which is replaced by "baby hair" - lanugo. In that case, the body is so whacked out and starved that it starts shutting down production of non essential hormones like androgens & oestrogens in its final attempts to stay alive.
posted by Pinback at 11:46 PM on January 10, 2008 [2 favorites]
Hair distribution is genetic, controlled as various protein/hormone producing genes switch on and off during embryogeneis. These in turn determine which genes are "switched on" in the various follicles which, in their own turn, determine what type of hair grows and how sensitive the follicles are to androgens and other hormones. Even at the gastrula stage - literally, just a few hundred cells - it can be determined what cell types will end up where in the body. So, you end up with 3 very slightly different follicle types spread round the body.
Firstly, as an embryo, all the follicles produce the fine baby hair called lanugo. Even at this stage, though, hairs on the head tend to be longer - that's controlled by genes switched on earlier in the development process. AFAIK, this hair growth is driven by hormones from the mother's body.
Once the baby is born it stops getting those hormones. This is what causes the baby hair to fall out, until the body ramps up its own hormone production and the hair grows again. This time, because of the increased / different hormones, the different follicle types produce two different types of hair - one type produces terminal hair on the head, and the remaining two types produce vellus hair on the body.
At puberty, hormone production changes / ramps up again. This time, the the two different vellus-producing follicle typesreact differently. Follicle cells in in the pubic area, underarms and, in males, the face, react to the increased production of hormones and start growing thicker / darker / longer / curlier hair. The remaining follicles on the body do the same too, but not to the same extent - they produce hair that is thicker / darker / longer / curlier, but only slightly.
And there you have it. There's a genetic basis to how long your hair will grow, but it's also affected to a large degree by hormones - which, in turn, are affected by other things e.g. nutrition, etc. Case in point: one of the effects of extreme anorexia is loss of body hair, which is replaced by "baby hair" - lanugo. In that case, the body is so whacked out and starved that it starts shutting down production of non essential hormones like androgens & oestrogens in its final attempts to stay alive.
posted by Pinback at 11:46 PM on January 10, 2008 [2 favorites]
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posted by clanger at 8:23 PM on January 10, 2008