Hair Composition and Growth
To start, the scalp has about 110,000 hair follicles, or the originating place from which hair sprouts. Hair grows about a half-inch a month, on average, and about 90 percent of the time, hair is growing. The other 10 percent is when the hair is going into, or is in, a resting phase.
There are three phases of hair growth. The “anagen” phase is when the hair grows in length. This period lasts between two to six years. The next phase, the “catagen” phase, is transitional. This period lasts about two to three weeks and prepares the hair for the next phase. The “telogen” phase is when the hair rests, older hairs are shed and replaced by new hairs.
Hair is composed of keratin, a tough protein. Keratin is produced in the hair bulb, at the bottom of the hair shaft, anchored under the skin by the follicle. In the hair bulb, cells divide and grow to build the hair shaft, the part that protrudes beyond the skin. Blood vessels in the hair bulb feed nutrients and hormones to the hair that modify growth and structure at different times of life. Pigment cells produce melanin for hair color, but with age, these cells die. Subsequently, there is a decrease in pigment and an increase in gray or white hair.