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When compared to hair that grows forever after we cut it, why do eyebrows stop growing? What genes measure current length of eyebrow to ensure it stops growing after a specific length?

user438383
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Mohammad Fajar
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    no gene measures the length, all hair grows for a period of time then goes dormant, eyebrow hair just grows for a much shorter period of time and grows slower. even head hair stops growing after a few (2-7) years. https://en.wikipedia.org/wiki/Human_hair_growth for more detail on the genetics You can start here https://www.ncbi.nlm.nih.gov/pubmed/5173315 – John Apr 18 '19 at 13:56
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    If we cut it in the middle and in the base, why it regrow to the same length after all. I mean If we cut it in the base then it's need more time to regrow than if we cut it in the middle. But how gene determined this? – Mohammad Fajar Apr 25 '19 at 08:17
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    short answer, It doesn't – John Apr 25 '19 at 12:32
  • An interesting question, but backwards. Almost all hair seems to grow only to a certain length (or for a certain length of time), then stop. Human head hair seems to be one of the few exceptions. The only others I can think of are wool sheep and horse manes. As for human head hair stopping growth after a time, why am I still paying to have it cut every couple of months? – jamesqf Aug 08 '20 at 16:41
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    Does this answer your question? Why does hair grow after trimming but remains at a constant length after a while? – tyersome Sep 08 '20 at 03:03
  • My eyebrows used to be 1 cm, and after 35 they could easily get to 6 cm, they grow for a specific time and are renewed. – bandybabboon Sep 03 '21 at 04:37
  • @jamesqf - Stop getting it cut and see if it grows indefinitely. Some kinds of sheep will grow wool until it kills them if not shorn. – anongoodnurse Sep 06 '21 at 15:35
  • @anongoodnurse: I've actually tried that :-) But I have very fine hair (oops, ambiguious language there, I mean the individual hairs are much thinner than most people's) so it only grows to about shoulder length before the hairs break. Especially considering that everything that sticks out from under a motorcycle helmet gets seriously windblown. OTOH, I have one or two hairs in each eyebrow that seem to grow indefinitely... – jamesqf Sep 07 '21 at 16:43
  • @jamesqf - "I have one or two hairs in each eyebrow that seem to grow indefinitely..." Lol! Ok, you won that one! My hair, when young, made it down to my upper thighs, then stopped. Apparently my follicles had a long growth cycle. Decades later, it stops between my shoulder blades. Strange. Maybe my follicles are tired. :) – anongoodnurse Sep 07 '21 at 16:49

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Activation of the mammalian gene Lhx2 leads to increased hair growth. Lhx2 is a transcription factor which regulates hair formation.

"...length of the hair is determined by the duration of the growth phase; for example, the growth phase for scalp hair can proceed for a number of years, while the growth phase for eyebrows last a few months."

Other genes that have been associated with hair disorders include: FOXN1, ALX4, HOXC13, MBTPS2, VDR, CDH3, HR and ANTXR.

Andrew
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    I'm afraid I don't understand this answer. People who shave their eyebrows regrow them (over a few months). But your answer suggests that after an eyebrow hair begins its life, it grows for a few months, and then never ever grows again. Or the eyebrow hair will once again re-start growth once it has been shaved? How would the hair root know if the hair was cut? – cowlinator Jan 06 '21 at 03:15
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    @cowlinator - When I was a brand new doc, it was customary to shave either side of a laceration that went through the eyebrow before suturing it. But very soon after, the procedure changed, because 1. the rate of infection was no different between shaved and unshaved repairs, and 2. in some people, the shaved eyebrows never grew back. Can't tell you why, though. – anongoodnurse Sep 07 '21 at 16:54