Carolyn Johnson, age 8, of Peoria, I11., for her question:
HOW DOES HAIR GROW?
Hair has a number of special names. Fur, for example, is the soft, dense hair that you find covering the bodies of many animals including cats, rabbits and leopards. Fleece is the woolly, thick hair you find covering sheep, while bristles are the short, stiff hairs that grow on hogs. Quills are sharp, spiny hairs that grow on hedgehogs and porcupines.
Hair is a substance that grows out of the skins of mammals. It can grow only on certain parts of the body, such as in man, or it may cover most of the body.
Hair grows by forming new cells at the base of the root. As new cells form around the nourishing papilla, the old ones are pushed away and die. The new cells gradually force the rod of dead cells up out of the follicle. Thus, old cells from the root become part of the shaft.
Usually only one hair grows from each follicle. And hair continues to grow as long as the papilla provides nourishment for new cells.
The papilla, at the bottom of the hair follicle and the spot where
an artery enters to provide nourishment for the root, may remain active from weeks to years, depending on the mammal and the part of the body on which the hair grows.
A hair of the human scalp usually grows about one half inch each month for two to six years, although the period of hair growth is much longer for some people. The hair falls out when it stops growing, and a new hair usually replaces it.
The shorter hairs of the body reach their greatest length and are replaced within a much shorter period of time than hair on the head. An eyelash, for example, grows for only about three to five months before a new eyelash replaces it.
After an old hair falls out, the papilla again becomes active and a new hair appears. Many factors, including diet, age, general health and the condition of the skin, influence the activity of the papilla.
Climate and seasonal change also affect the production of hair. During hot weather, many mammals shed a large amount of hair. During cold months, many mammals tend to acquire a thicker covering of hair for extra warmth.
The color of hair is determined by melanin, a pigment deposited in the hair cells as they form in the root. As people grow older, pigment is no longer deposited in the newly formed hair cells of some people and the hair gradually becomes gray or white.
Baldness happens when the hair on a person's scalp is no longer replaced after it falls. The tendency to lose or retain hair is determined largely by inheritance.