Naked Science Forum

Life Sciences => Cells, Microbes & Viruses => Topic started by: thedoc on 27/10/2016 23:53:01

Title: How does a hair know how long it is?
Post by: thedoc on 27/10/2016 23:53:01

rob kamstra asked the Naked Scientists:
   How does a hair, which is dead, know how long it is? If, for instance a dog [or human] is shaved, that patch of hair will grow back to exactly the length of the surrounding hair and then stop growing. Hair all over  is at differing lengths. How does the hair follicle 'get the message' to stop producing new hair?
What do you think?

Title: Re: How does a hair know how long it is?
Post by: evan_au on 28/10/2016 10:47:04

Hair follicles have a regular growing and resting cycle. Each new cycle pushes out the old hair and starts a new hair.

The duration of the growth cycle is different for hair on your head, eyebrows, arms, underarms or eyebrows, and this affects the maximum length that the hair will reach.

Many mammals have hair growth cycles that are synchronized to the seasons, so they may grow a long white coat in winter, and a shorter grey coat in summer, with the old hair falling out in-between.

However, human hair growth cycles are not synchronized, and so the hair remains at a constant average length, all year.

See: https://en.wikipedia.org/wiki/Human_hair_growth#Growth_cycle