Science Questions

Do hairs know they've been cut?

Thu, 1st Aug 2013

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Question

Maki asked:

Hello,

 

I have a question about body hair.

 

When we remove unwanted body hair by plucking/shaving, we know that they grow back more or less to its natural length.

 

When we trim them, the hair also seem to grow back to its natural length, but this has mystified me for some time.

 

a) How does the hair know it's been cut without stress caused to the hair follicle, and therefore start growing again; and

 

b) How does it know when to stop growing when it reaches the intended length?

 

Many thanks,

 

Maki Aoyama

Answer

Ginny - So, the short answer is that your hair doesnít know that itís been cut. Some hairThis is a really common myth. People think that if they trim their head hair when they're trying to grow it, that itíll make it grow faster and people say that if shave your legs or your face, it makes it grow back faster, but thatís not actually the case. The only thing that affects how long your hair is and how fast it grows is the hair itself and your genetics. So, hair goes through 3 phases when itís growing. There's the anagen phase which is the growth phase and this is when it actually grows, when it gets longer. Now, on head hair, that can last between 2 and 6 years and how long that phase is, counts as how long your hair could grow to. So, some people seem to be really lucky and they can grow hair down to their waist whereas other people, no matter how hard they try, it will never get pass shoulder length.

Chris - Not their eyelashes though, surely.

Ginny - No, not their eyelashes. Now, thatís a point because body hair has a much shorter anagen phase. Itís only a couple of months and thatís why it never gets as long as head hair. I think there are few exceptions of weird people whoíve managed to grow their underarm hair ridiculously long in the Guinness Book of Records, but with a couple of weird exceptions, it doesnít get as long as the hair on your head because this anagen phase only last a couple of months.

Now after that phase, what happens is the hair is cut off from the blood supply so the follicle sort of dies and it enters a dormant stage and then a bit of time after that, itíll fall out as a new hair begins to grow. So, what you're seeing is you're trimming say, your leg hair while itís still in the anagen phase - it's going to continue growing. But it can only ever get to a certain length because that anagen phase will end and it will fall out.

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Dr Chris answered this one previously ...


http://www.thenakedscientists.com/forum/index.php?topic=15392.msg181158#msg181158 RD, Fri, 19th Jul 2013

But before you shave your leg hair, most of it is not growing, when you shave it, most of it starts growing again, it seems. It's true that it only appears more thicker because now the whole length will be made of the thicker visible section of the hair befere it got shaved, because the thinner tip has gone lost. Anto, Tue, 21st Jan 2014

Ok but they certainly don't fall all at the same time, because we never notice them any different... Marcelo, Sat, 8th Feb 2014

Marcelo: true, that's what I was saying, in fact we are not dogs, if we lost most of our hair all at once, we would be left, for a short period of time, with sparser and/or much shorter leftover hair :). The complete replacement of old hair with new one is a very long process. Only very little hair falls out every few days, a minimal percentage over the total hair. That's why it looks like that before we shave our body hair, only a minimal part of it is still growing and most of it is resting due to reaching their length limit, but they are not dead, rather still active for the remainder of their cycle. Yes our hair and especially head hair are a demostration that shaving and cutting can't prolong the actual life of each follicle, as a part of the fall off every but as far as them not knowing they are being cut or shaved, one might observe that if it was true and only the hair which is already growing grew back when shaved, then only a small portion of our hair would grow back at first, and it would take at least a year or more to grow all hair back and to shed the resting shortened one. Instead we see that almost all of our hair grows back when we shave, leading to the conclusion that hair might know it's length, hence cutting might wake up dormant or resting follicle stimulating them to grow back until their set length is reached. Of course if the hair has little time left to grow back it will fall down much shorter, but hair live more than they grow, apparently :). Anto, Thu, 13th Feb 2014

But see, what I don't get is this.


Look at a short-haired dog. Their hair is basically all the same length. So it grows and then stops, until it falls out and grows again. But if you shave off all of a dogs hair, all of the hair starts growing in, even the hairs that had stopped growing. So how does it know it's been cut and needs to start growing?  You don't see places where there's no hairs growing, it all starts growing out again.

Likewise with human hair. Take a man's beard. Some guys have beards that reach their terminal lengths at a very short stage. So if the hairs are 6" long and have all stopped growing, if he shaves it off, they'll all start growing again.

So that seem to really refute the idea that a hair doesn't know it's been cut off.Look at a short-haired dog. Their hair is basically all the same length. So it grows and then stops, until it falls out and grows again. But if you shave off all of a dogs hair, all of the hair starts growing in, even the hairs that had stopped growing. So how does it know it's been cut and needs to start growing?  You don't see places where there's no hairs growing, it all starts growing out again.

Likewise with human hair. Take a man's beard. Some guys have beards that reach their terminal lengths at a very short stage. So if the hairs are 6" long and have all stopped growing, if he shaves it off, they'll all start growing again.

So that seem to really refute the idea that a hair doesn't know it's been cut off. july1962, Thu, 27th Mar 2014

hi july1962. Well I guess it also depends on how much time it takes for that dog to grow all of his hair back. It's possible that you just have to wait for his shedding period. That said, even though the hair shaft is a dead structure, it is a protective and sensorial organ which when touched or moved seems to communicate with the body via the follicle, broadcasting all kind of sensations. So it's entirely possible that our body has more than a few clues to understand how long it is, if it has been shortened, cut or whatever and to make it grow back in the case. Indeed it seems that most of our hair is not growing, being it a full length, yet if you shave a patch of, say your arm hair, it all grows back, yeah, exactly like in the example you made with men whose beard only grow to a short length and stays that way only to grow back if shaved. Moreso if the hair stopped at a length and it gets cut at the base, then the hair continues then to grow at the relative thickness to the base of the hair, so when it has got to that same length again, it now has the tip being as thick as the base. The hair then won't taper again until it will be shed off. That's why it seems like shaving makes it grow back thicker and darker, but it's an illusion and it's not permanent at all. Mind that a part of our hair is at the end of its cycle and is going to be shed however much we shave or cut it, though, which has no effect. That is why, however your hair is not at its full length you keep shedding it howevery you cut it (assuming you do, you could be a girl with hair up to your back and beyond ;), it was an example, goes without saying). The oldest hair is going to fall of anyway. So any single hair of our it's at a different point of its life. Anto, Mon, 31st Mar 2014

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