Hair

The structure of hair is analogous to a plant bulb. The whole unit, with its immediate surrounding, is termed a follicle (see Figure 1). The plant bulb is equivalent to the bulbous hair germ, while the sprouting plant is equivalent to the hair. A plant bulb, which is dormant near the soil surface during the winter, begins to sprout in the spring. It moves deeper into the earth and then grows into a full plant, which lasts over the summer. In the fall, the plant shrivels and is detached. The bulb then goes into a resting phase and resprouts the next spring.

The three phases of the hair life-cycle are equivalent (see Figure 1) and are termed anagen (growing phase), catagen (transitional phase), and telogen (resting phase). The lustrous scalps of young adulthood have about 100,000 hairs, blondes having more and redheads less. This number declines in healthy individuals to forty to fifty thousand hairs between the ages of thirty and fifty, at which time the apparent bulk of the hair is 50 percent thinner.

Normally, about 90 percent of hair is in the anagen phase and 10 percent in the telogen phase. Hair is shed daily, with a loss of fifty to one hundred hairs—often found on a brush or pillow as club (the small knob on the root end) hairs.

The number of hair follicles is the same in men and women. No new ones develop after fetal life. The juvenile soft hair of childhood becomes the firmer and more lustrous terminal hair of adolescence.