Human hairs grow at the rate of 0.35 mm a day, one-half inch per month. The rate of growth is slightly faster in summer months when the blood supply to the scalp is greater.
Each hair is encased in a follicle beneath the skin surface. It has an expanded end called the bulb, and from this point the hair grows. Layers of cells arc continually shed and gradually grow upwards in the form of a continuous strand of hair.
At the growing point is a special row of cells called melanocytes. These are responsible for giving the hair its characteristic color. Dark material called melanin is injected into the hair as it is being formed. Once this set of cells ceases to function, they never regain their power of pigmentation. But this plays no part in restricting further growth of the hair, which continues unabated for years.
Everyone is born with a fixed number of hair follicles. Each will subsequently grow a hair. If they are destroyed permanently for any reason, new ones never form. For this reason, follicle destruction through disease or accident becomes permanent.
The ability of the hair bulb to grow is phenomenal. It is one of the sturdiest and most vital organs of the entire body system. Even when continually plucked it will stoically continue to grow, as every woman knows so well!
Closely associated with each hair root is a small gland called a sebaceous gland. This produces an oily material called sebum. It gains access to the hair through a tiny canal called the sebaceous duct. The sebum covers the entire outer of the hair shaft. This gives the hair -11-5 characteristic sparkle and sheen when in good health. Microscopic scales cover the outer part of the shaft. When these aren’t brushed, it enhances the sheen if sebum is present. Dry, lusterless lacks normal sebum and has the lifeless appearance young women so dread. Hair grows continuously for certain periods, but it subsequently goes “resting phase.” Scalp hair grows where from two to six years before coming quiescent, but growth does vary. Some hair grows continuously for 25 years or more, accounting for the tresses of women who prefer it that way.
During the resting phase, hair drops out. In any single day it is usual for 20 to 100 hairs to come away from the scalp. This is perfectly normal even though it is a cause of major concern – for many people, especially the younger age groups. They fear prematureness. After a few months the hair recommences to grow again, and a new hair appears. At any given time, between 5-15 per cent of the total scalp hairs are in the resting stage.
The hairs of the eyebrows, eyelashes and body have a normal growing period of about six months before entering en resting phase. This is why they are shorter and rarely reach any extensive length.
It is well-recognized that the male hormone testosterone has a major effect on hair. It promotes the growth of hair on the face, under the arms, on the body pubic areas. But it has the opposite effect on scalp hair.
Often babies develop a bald patch on one side or other of the scalp. This may be due to the fact that the child sleeps for most of the time on one side. The constant rubbing against the pillow acts as an abrasive, and so shears off the hair from the scalp. Many mothers alternate the side baby sleeps on after feeds.
The nature of the hair seems to vary throughout life in many cases. Infants and children with straight hair may later develop wavy hair. Hair color is determined by the special cells in the hair root that impart the pigment. This often tends to darken as the years advance. It is common to see a fair-haired child later become much darker.
Hair normally grows at the rate of 0.35 mm a day, or roughly one-half inch a month. In the human head, by adulthood, there are about 100,000 hairs; 50 to 100 drop out each day, normally. But this means the hair bulbs are in the temporary “resting phases,” and they start to grow after a period of weeks or months.