Ben Thornton, age 12, of Cleveland, Ohio, for his question:
HOW DOES HAIR GROW?
Hair is a substance that grows out of the skins of mammals. It is made of the same kind of material that makes up the nails, claws and hoofs of mammals, the scales of reptiles and the feathers of birds.
Each hair is made up of a root and a shaft. The root of the hair is a soft, light colored bulb. The rest of the hair is the shaft. The root and a small section of the shaft lie below the surface of the skin in a sac called a follicle. At the bottom of the follicle, a projection called the papilla contains an artery that nourishes the root of the hair.
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 forming the root become part of the shaft.
Usually only one hair grows from each follicle. But in some animals, including the chinchilla, a number of hairs grow from a single follicle.
Hair continues to grow as long as the papilla provides nourishment for new cells. The papilla may remain active from weeks to years, depending on the species of 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 persons. The hair falls out when it stops growing and a new hair replaces it.
The shorter hairs of the body reach their greatest length and are replaced within a much shorter period. For example, the eyelash grows only about three to five months before a new eyelash replaces it.
After the old hair falls out, the papilla again becomes active and a new hair appears.
Many factors, including age, diet, general health and the condition of the skin, influence the activity of the papilla. Climate and seasonal changes also affect the production of hair. During cold weather, a thicker covering of hair tends to grow for extra warmth.
The color of hair comes from melanin, a pigment deposited in the hair cells as they form in the root. Melanin can produce hair colors ranging all the way from blond to black.
As people grow older, pigment is no longer deposited in the newly forming hair cells. The hair gradually becomes gray or white.
Baldness results when the hair on a person's scalp is no longer replaced after it falls out. This event happens more commonly in men than it does in women.
The tendency to lose or retain hair is determined largely by inheritance. But other factors, including scalp diseases, exposure to radiation and glandular disorders can also cause baldness. Doctors cannot cure hereditary baldness.