Question

Why does the hair on your arm or eyelashes (or the hair on a dog) stay short, while the hair on your head can grow very long?

Answer

Each hair on your body grows from its own individual hair follicle. Inside the follicle new hair cells form at the root of the hair shaft. As the cells form, they push older cells out of the follicle. As they are pushed out the cells die and become the hair we see.

A follicle will produce new cells for a certain period of time depending on where it is located on your body. This period is called the growth phase. Then it will stop for a period of time (the rest phase), and restart the growth phase again. When the hair follicle enters the rest phase, the hair shaft breaks. So the existing hair falls out and a new hair takes its place. Therefore, the length of time that the hair grows during the growth phase controls the maximum length of the hair.

The cells that make the hairs on your arms are programmed to stop growing every couple of months, so the hair on your arms stays short. The hair follicles on your head, on the other hand, are programmed to let hair grow for years at a time, so the hair can grow very long.

Animals that shed have hair follicles that synchronize their rest phase. So all of the follicles enter the rest phase at once. Therefore, all the hair falls off at one time. A dog that sheds will lose its hair in large clumps. Many animals can also switch the coloring agent in the hair follicle on and off. So in the summer the hair is pigmented brown with melanin (see How Sun Tans Work), but in the winter it is not, leaving the hair white in the winter.

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