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I'm sure many of you have experienced this - you scratch your back or brush your hand over your arm and find a ridiculously long thin white hair, sometimes as long as 3 or 4 inches. I know a few people who get these quite frequently, and anecdotal evidence seems to suggest they almost grow almost overnight. A quick Google search for "single long white hair" throws up plenty of references to them, and various beauty advice, but I'd like to know the biology behind them.

  • What triggers abnormal body hair growth?
  • What causes these hairs to grow so much longer than other "normal" body hairs in physical proximity?
  • Why are they so thin and white compared to other body hair?

My primary hypothesis is that the hair cell grows rapidly in an uncontrolled manner, similar to how cancer might, and that the unusual appearance is due to the cells being starved of nutrients. I may be completely wrong, though.

I'm mainly interested in humans here, but if more in-depth research is available on animals that'd be interesting too.

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I can add that this has happened to me. I actually hadn't ever heard of it happening to someone, and I thought maybe somehow someone else's hair became embedded in my skin! I seem to recall it being very long as well--three or four inches in my arm that just showed up one day. –  Jeff Axelrod Mar 30 '13 at 2:16

1 Answer 1

Since it's been so long, I guess a rushed speculative answer might be at least an idea.

DNA gets damaged randomly all the time, and repair mechanisms are in place to fix it. When the damage is too large or of a very complex kind, permanent mutations can develop, and cause disorders such as cells proliferating without control - i.e. tumours.

I could imagine that the same mechanism could result in cells which overproduce keratin (hair) without melanin (pigment), if the associated genes are affected. Considering that trichocytes and melanocytes, the two cell populations at the base of the hair responsible for production, are among the most rapidly proliferating cell types in humans, an increased risk for mutation would not surprise me. I was able to find a few papers on keratin-associated gene clusters by a quick search, but they all seemed to be related to production of fragile hair rather than excessive length.

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Melanoma is not the most common type of skin cancer. Squamous cell carcinoma and basal cell carcinoma are much more common, though with a much better prognosis. –  Rory M May 29 '13 at 12:58
    
Silly me, thanks for the hint, goes to show the frequency with which a condition is mentioned (or seems to be) doesn't always correlate with its importance. –  Armatus May 29 '13 at 14:08

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