David Grider, age 13, of DeKalb, I11., for his question:
HOW DOES HAIR GROW?
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, such as in a dog or cat.
Hair grows by forming new cells at the base of the root. As new cells form around the nourishing papilla, as the small projection at the root of the hair is called, 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 hair's 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 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 animals tend to acquire a thicker covering of hair for extra warmth.
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 to a great extent by heredity.
The color of hair is determined by melanin, a pigment that is deposited in the hair cells as they form in the root. As people grow older, the pigment is no longer deposited in the newly formed hair cells of some people and the hair gradually becomes white or gray.
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 and rabbits. Fleece is the woolly, thick hair that you find covering sheep, while bristles are the short, stiff hairs that grow on hogs. Quills are sharp, spiny hairs that you find growing on porcupines and hedgehogs.
Long, sensitive hairs on the lips and cheeks of some animals are called tactile hairs or whiskers. They help an animal feel its way in the dark.
Specialists can remove unwanted hair from the human body or face with a procedure called electrolysis. An electric needle is used to destroy the hair's papilla. No new hair can then grow from that spot.