Luckily, not all of your hair grows as long as the hair on your head does: Eyelashes, pubic hair, leg hair, etc., grow at different rates. They all have their distinct features. But how does your leg hair know it should grow shorter than the hair on your head?
According to Kassel, the skin cells that produce head hair have a different genetic program than the skin cells on your legs or arms. Whether your hair will grow long or short, be black, blond, red, or brown is all controlled by the genes inside a skin cell.
The underlying program of hair growth
As an analogy, the factors that determine hair growth could be illustrated by imagining that each cell is part of a computer network, with an underlying genetic programming that triggers different software and programs in the cells depending on their location on your body. Some programs run all the time, like the body’s metabolism, while other programs are only activated periodically and run for a limited time, depending on their location — head, legs, etc.
The before-mentioned analogy of a computer network illustrates how it is possible for your body to know how long to grow hair on different parts of your body.
The skin cells on your head continually grow hair for several years. This allows the hair on your head to grow up to three meters in length.
Thanks to the right program triggering the hair growth on your arms and legs, arm hair and leg hair do not grow as long as the hair on your head.
If, however, a patch of skin from your head was transplanted to your arm or leg, the hair growing from that patch of skin would grow like the hair on your head.