The growth of human hair

• Each human head carries roughly 100,000 hair follicles.
• Each follicle can grow many hairs over a lifetime: on average, each grows a new hair around twenty times.
• Not all these follicles are actively growing hairs at any one time. From the moment when it is first formed, each follicle undergoes repeated cycles of active growth and rest. The length of the cycle varies with the individual, and also with the part of the body on which the hair is growing.
• The hairs on an adult scalp do not grow in unison, as they do in an unborn baby. They are 'out of cycle' with each other. If this were not so, everyone would go temporarily bald from time to time.
• The growing and shedding of hair as a whole seems to happen at random, but for each hair follicle the process is precisely controlled. No one knows for certain, however, exactly how the body controls these cycles.
• Plucking a hair from a follicle brings forward the next period of hair growth in that follicle.

Electronmicrograph showing new hairs emerging from the hair follicles of the scalp

• Over the years, the number of follicles capable of growing hair declines naturally. The decline is especially noticeable on the top of the head. Some follicles increasingly produce only fine, short non-pigmented hairs that look more like vellus hairs than terminal hairs. In older women, this leads to a general thinning of hair. In men it tends to lead to common baldness. If you look at a bald scalp you will see these fine, poorly pigmented hairs.