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?
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4$\begingroup$ 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. en.wikipedia.org/wiki/Human_hair_growth for more detail on the genetics You can start here ncbi.nlm.nih.gov/pubmed/5173315 $\endgroup$– JohnCommented Apr 18, 2019 at 13:56
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2$\begingroup$ 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? $\endgroup$– Mohammad FajarCommented Apr 25, 2019 at 8:17
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3$\begingroup$ short answer, It doesn't $\endgroup$– JohnCommented Apr 25, 2019 at 12:32
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$\begingroup$ 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? $\endgroup$– jamesqfCommented Aug 8, 2020 at 16:41
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4$\begingroup$ Does this answer your question? Why does hair grow after trimming but remains at a constant length after a while? $\endgroup$– tyersomeCommented Sep 8, 2020 at 3:03
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1 Answer
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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.
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2$\begingroup$ 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? $\endgroup$ Commented Jan 6, 2021 at 3:15
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1$\begingroup$ @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. $\endgroup$ Commented Sep 7, 2021 at 16:54