How Does Some Hair “know” When To Stop Growing And Some Just Keeps Growing Infinitely?
i read an article about a man who cut his 3 inch eyebrows for charity which lead me to this question: how does some hair know when to stop growing & some just grows non-stop? the hair that stops at certain lengths are body hair (arms, legs chest & back) eyebrows, armpits & eyelashes. the hair that grows continuously are head (scalp) & facial (beard/moustache). an even bigger question is how does the hair that stops growing know when it’s been cut. like if i were to cut my eyebrows right now it would grow back, but they’ve been the same length my entire life. how does my body know my eyebrows have been cut? why don’t they grow continuously like the hair on my head & face? furthermore, what is the purpose of the scalp if hair can apparently grown anywhere on your body? just by observation i would guess it’s because the hair on your head grows more dense than everywhere else, & that is the scalp’s doing. now if that is the case, does this mean that hairy animals entire body is a scalp? lol, i know this sounds silly but i’ve been wanting to know this for a long time.
Tags: Does, Growing, Hair, Just, Keeps, Know, Some, Stop, When
February 23rd, 2010 at 3:53 am
I’m going to take a stab at answering this, but I can’t promise the answer is entirely correct.
I think that the length of our hair (both on the body & head) is determined by our genes, just like our hair color & texture. At one time during human evolutionary history, all of our hair was about the same length & thickness all over our bodies (like most mammals). But as we evolved, two things happened: we began to lose the need for full-body coverage, & the hair on our heads became a social display of sorts (like how male lions have manes). So I guess now our genes code for short, sparse body hair & thicker, longer head hair. But some people (like eyebrow dude) are just hairier than others.
February 23rd, 2010 at 4:48 am
good question, bad answer
i just wanna know who else answers this & what they say. thank you!
February 23rd, 2010 at 7:28 am
Hair grows in phases, each hair follicle begins, continues & stops/sheds growing hair. Some hair just has a longer growth period or grows faster in it’s growing phase. When it stops growing the hair shaft is shed then the cell begins growing a hair again. There is no means of sensing cut hair to trigger new growth, the hairs are simply on a continuous cycle so those in an active phase just reappear after shaving.
Follicles found on the scalp have a longer growth period than pubic or axillary hair follicles. So the hair on the scalp is shed very slowly compared to the other regions.
Scalp hair is ~1 cm/month or 0.34 to 0.36 mm/day throughout life. Growth phase is 3-6 years. Some 90% are in an active growth phase
at all times.
Eyelash/brow growth rate is ~ 0.16 mm/day.
Body hair on arms, legs, eyelashes, & eyebrows have a very short active growth phase of about 30-45 days.
Hair density over the body is the same as the scalp but the hair itself is very fine & is shed frequently to be kept short.
During evolution we began the process of increasing our brains size ans activity levels. This required a greater control of the internal temperature. So some think the development of eccrine thermo-regulating sweat glands led to the loss of hair density. Hair would block the fine tuning of our internal temperature possible with sweating & shivering. Thus sweating coevolved with a larger brain leaving body hair vestigial for thermoregulation.
The hair then comes under active sexual selection so the patterns & colors begin to shift.