The Scalp and Hair Growth

The texture, thickness and diameter of your hair are all determined by tiny structures located in your scalp called hair follicles. The narrow corridor and shape of the follicle gives hair its shape. Straight follicles produce straight hair, and curved follicles produce curly hair. Hair diameter is determined by follicle diameter, and thickness is determined by the number of follicles per square inch of scalp.

Anatomy

At the base of the follicle lies the dermal papilla. The dermal papilla receives all the blood and nourishment needed to grow hair. It is here that cells divide and die to create the nonliving substance that we know as hair. Sebaceous glands are located in the follicle closer to the skin's surface. The glands coat the hair and scalp with a protective layer of sebum.

Process

Hair growth occurs when cells divide around and above the dermal papilla. As the cells divide, they contribute to the upward growth of the hair shaft. These cells create the three layers of the hair shaft: medulla, cortex and cuticle.

Phases

The growth cycle of hair is divided into three phases: anagen, catagen and telogen. Individual hairs enter into each phase at different times. Approximately 80 to 90 percent of hairs are actively growing during the anagen phase. On average, this phase lasts five years, during which hair grows at the rate of 1 to 2 cm per month. Eventually growth begins to slow, and hair enters the catagen phase. During this phase, the hair becomes disconnected from the structures in the follicle, and a new hair bulb begins to form underneath it. The telogen phase is a period of rest. The hair, severed from all connective structures, remains in the follicle only because of the bulb-shaped root. The hair can remained lodged in place for three weeks to three months, after which a new hair pushes it out or it is dislodged by shampooing or brushing.

Age

Hair undergoes several changes as you age. Aged cells are unable to reproduce as quickly as young cells, so hair growth slows down considerably. You may also notice a difference in the texture and strength of your hair. Perhaps the most noticeable change involves the appearance of gray hair. Gray hair starts to appear as individual follicles become unable to infuse growing hair with melanin, or pigment.

Hair Loss

Illness, medications, environmental factors, hormonal imbalances and heredity can slow or halt the hair-growth process, resulting in hair loss. The most common cause of hair loss, male and female pattern baldness, is passed down through the generations. In this type of hair loss, a byproduct of testosterone called dihydrotestosterone, or DHT, shrinks hair follicles. In the beginning stages, narrowed follicles continue to produce hair. The hair becomes finer, weaker and eventually disappears as the follicles continue to narrow.