Why and how do some hairs “know” when to stop growing (e.g. eyebrows, arms, eyelashes) while others (e.g. scalp, beard) doesn’t seem to?

Question

Is it that they don’t know or the “threshold” is so long that we don’t get to see it very often?

In: Biology

Answer 1

Every cell in your body has (mostly) the same DNA. However, not every cell looks the same.

Only small parts of the DNA are “activated”, and the method by which certain parts are activated and other parts are not is very complex. It happens though. So, muscle cells will have the “muscle” part of the DNA activated, while skin cells will have the “skin” part of the DNA activated.

Similarly, eyebrow hair follicles will have the “eyebrow hair follicle” part of the DNA activated (which keeps it relatively short as detailed in another answer), while head hair will have the “head hair” part of the DNA activated.

Some poor saps will have the “No hair for you on your head you loser” part of the DNA activated. I am one of those poor saps.