Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hair growing on the body to different lengths

  1. Mar 29, 2013 #1

    Anna Blanksch

    User Avatar
    Gold Member

    The hair on my head grows longer and longer while other hairs, like arm hair, only grow to a much shorter length. Why do certain hairs on the body grow only to certain lengths? How do the cells know when to stop? Thank you! →
  2. jcsd
  3. Mar 29, 2013 #2
    Genes code for everything in your body
  4. Mar 29, 2013 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The hairs have a different growth/rest cycles, the hair on the scalp has an growth phase of years, while other hair only months. Proteins are very important in regulating the length of the cycle and must thus be differentially regulated in the different cell types.

  5. Mar 29, 2013 #4
    I asked this question a number of times but only recently got a satisfactory answer. When you cut/shave it grows right back to it's previous length where it seems to stop again. As hairs are dead and don't communicate their length back to the cell I wondered how this was the case too.

    The very last barber that cut me, my neigbour as it happens had quite an enlightening answer. She told me the hair doesn't stop growing but rather falls out after a certain time. :)
  6. Mar 30, 2013 #5
    The hair on your head and say the ones on your pubic area and arm pits are not under the same hormonal control. Testosterone is toxic to the head hair, hence those people with testosterone secreting tumors will have early baldness, but still have hairy pits and pubis. Similarly why kids can grow long head hair before puberty.
  7. Mar 30, 2013 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    This isn't true, your genes are not a blueprint that code for everything (there isn't a set of genes that code for how to build a hand for instance). Your body is an emergent feature.
  8. Apr 3, 2013 #7
    But then again, that statement appears without any context and is too vague to convey any clear meaning.
  9. Apr 3, 2013 #8
    Here is an entire book length article on the growth of hair. The growth of hair comes in a cycle with different parts.

    http://www.horizonti.net/clanki/the%20biology%20of%20hair%20growth.pdf [Broken]

    Anagen length generally determines hair length. The longer the anagen, the longer the hair. However, no one seems to explicitly explain why the hair can’t grow longer in subsequent anagens.

    There is a section of the cycle called the exogen. While the growth of hair slows down in some stages, there is another stage where the hair comes loose. So I think the hair grows during the anogen, stops growing, and eventually comes out during the exogen.
    Last edited by a moderator: May 6, 2017
  10. Apr 4, 2013 #9


    User Avatar

    Staff: Mentor

  11. Apr 5, 2013 #10
    What I want to know is what causes my curly hair to start curling in the opposite direction as it grows.
  12. Apr 5, 2013 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm not sure what you mean. Deviating from a straight line is what defines it as curly. So once it gets long enough it will be pointing back the way it came.
  13. Apr 5, 2013 #12
    The difference between straight and curly hair lies in the arrangement of the glycine amino acids in the hair collagen. It's all genetic not hormonal related
  14. Apr 6, 2013 #13
    I mean why it will grow in a spiral clockwise from the root down and then switch to counterclockwise?
  15. Apr 6, 2013 #14


    User Avatar

    Staff: Mentor

    Do they?

    They can form a spiral both ways and switch between cw and ccw any time.
  16. Apr 6, 2013 #15
    Maybe that part is environmental or hormonal. The time of switching may be very sensitive to the environment, even though the curliness itself is hereditary. The collogen contains these polymers that curl, which are probably genetic. The particular direction (cw or ccw) may be epigenetic or environmental.

    In physics, this would be called a spontaneous symmetry breaking. Thermodynamically, the hair has to curl one way or the other. However, the direction of the curl is so to environmental factors that over long time scales the direction is effectively random. Like the earths magnetic field, sort of.

    This is just my hypothesis. I meant only to suggest a possible lead. Please don't ding me!
  17. Apr 6, 2013 #16
    Yes I was thinking it would have to do with the left handed chirality (please correct my terminology if its wrong) of the molecules of all living organisms, which are racemic when produced in a lab; for that reason I thought they would always curl the same direction?

    I wondered if it had to do with a nutritional variance. I was malnourished throughout my teen years until I was about 21. At that age I had greater access to food and I gained a lot of weight. But then my metabolism increased and my weight dropped back to what it was before. Also, I think I am still growing heightwise, I am two inches taller than I was when I was measured two years ago at age 21 if I'm not mistaken, which is odd especially for a female...
  18. Apr 10, 2013 #17
    Why do certain hairs on the body grow only to certain lengths? How do

    As cited by Daven Hiskey in his article Why Does Hair Only Grow to a Certain Length?, "hair growth is completely controlled by what’s going on under the surface of your skin, within your hair follicles, and that genetics and hormones are the primary things determining hair growth length."

    Source: D. Hiskey (2012). Why Does Hair Only Grow to a Certain Length? Retrieved http://www.todayifoundout.com/index.php/2012/05/why-does-hair-only-grow-to-a-certain-length/, March 10, 2013.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook