Why does some hair never stop growing?

Chris - It's a very good question. Why is it that your hair grows on your head can, in some people's cases, can reach their waists whereas eyelashes conveniently remain only a few millimetres long. If you had eyelashes that reached your waist that would make seeing quite difficult, I would imagine. Similarly pubic hair under you arms, for example. Why does that stay short and curly and the drop out before it gets really long whereas the hair on your head can become very long?

The answer is it's all down to genes and when you're developing as an embryo your body develops as a series of segments. Written into those segments is a genetic pattern that tells that bit of the body where it is in the body and anything that develops on that segment inherits that genetic pattern which dictates to it how it should grow and develop. If you look at how hairs work - hairs have three phases to their life cycle. They have what's called an anagen phase and this is where they grow. The hair follicle has a number of stem cells that are very, very active and they pump out keratin which is the hair chemical. Keratin forms a big polymer which is a filament for hair which you see. After the anagen phase, which can last anything from days - in the case of an eyelash that's about 2-3 weeks, to a head hair which can be three or four years. That determines how long the hair grows for its ultimate length. Then the hair goes into what's called a catagen phase. That's where the follicle switches off and the hair falls out.

Then there's a third phase which is called a telogen phase when the follicle rests. It then resets the system and the whole thing starts again. So the hair length is down to how long the hair grows for, the anagen phase, and that is determined by your genes. Basically the genes that are programmed into the bit of the body that's got the hair in it. Kat - I was thinking if you had some weird genetic mutation you could have pubic hair that grew down to your ankles!

Chris - Yeah, I suppose if you similarly transplanted head hair to your pubic region or vice versa you would get hair that had that behaviour because it had pre-programmed into it that way of growing. Kat - Freaky!

Chris - There's a company in America called Allergan. They're the company that brought you Botox. They've also got a drug for glaucoma which is this eye pressure problem where you have too much pressure inside your eye. This can damage your optic nerve. There's a drug called Lumigan which can be used to treat that. The generic name is bimatoprost for this. What they found is one of the side effects is it makes your eyelashes grow long in some people. They're actually applying to the FDA (that's the drug administration group in America) for permission to market this as an eyelash lengthener. The slight downside is that it also makes your eyes get darker and it also makes your eyelids get darker. The effect can be permanent. Not only will you have luscious lashes, you will also potentially have darker eyes and darker eyelids. If you don't use the same amount on both eyes the effect can end up being non-symmetrical. Kat - So you'd look like a panda.

Chris - You'd end up looking like a sort of David Bowie effect. It could be a bit dodgy.