The hair follicle, residing just below
the skin surface, is the essential growth structure. If a hair
follicle is destroyed, no new hairs can form. If hair is plucked
or cut but the follicle remains, new hair will grow. The total
number of hair follicles in an adult has been estimated at about five
million, of which about 100,000 are in the scalp. The number of
hair follicles decreases with age. As dividing cells at the
bottom of a hair follicle are pushed upward, they eventually die and
become the visible product we know as "hair". It is important to
emphasize that each hair we see above the skin is dead protein tissue;
the follicle, lying deeply within the skin, is the esssential growing
part of the hair.
In humans, each follicle grows hair in
cycles and the duration of the cycle is different in each part of the
body. In the scalp, for instance, each hair grows steadily and
continuously for 3 to 5 years; growth then stops and after 3 months
the hair is shed. After another 3 months of a resting phrase, a
new hair starts to grow from the same follicle. On the eyebrows,
however, the growing phrase is only about 10 weeks and thus the hairs
can never grow very long. Scalp hair grows about one-third
millimeter each (one centimeter per month, one inch in 2-3 months).
Since there are about 100,000 scalp hairs, this growth produces about
100 feet of practically solid protein each day (seven miles per year).
People who tend to grow long hair have long growth periods (6 - 8
years), but their hair does not grow any faster than others.