A typical hair consists of the shaft, which is the part you can see, and the root, which resides in a hair follicle. The hair shaft is non-living tissue, and only matrix cells in a region at the root, surrounding what is known as the dermal papilla, grow and divide, gradually pushing the elongating hair outward. The numbers of hairs on a person’s head varies by hair type and race, but a typical range is 100,000 – 150,000 hairs on the scalp. Each day, an average of 50-100 hairs fall out of hair follicles as part of the normal growth process. Hair that is lost is continually replaced by new hair, which grows in a cycle. When the hair growth cycle is disrupted, and more hairs are lost than regenerate, thinning hair and possible baldness result.
The hair growth cycle includes three phases, known as the anagen, catagen, and telogen phase. The anagen phase is the longest, lasting between two and eight years. During anagen phase, the hair grows actively from matrix cells in the bulb, where the dermal papilla is nourished by blood vessels. The next phase of the growth cycle is the catagen phase. This is a short, two to four week transitional period in which the hair stops growing, and the lower portion of the hair follicle retracts up to the level of the arrector pili muscle. The dermal papilla is pulled until it breaks away and regresses, detaching the upper portion of the follicle from the blood supply. At this point, the hair stops growing. The last phase of the growth cycle, the telogen or resting phase, lasts two to four months while the dermal papilla is in a resting state. Following the telogen phase, the hair follicle reattaches to the dermal papilla and the follicle re-enters the anagen phase. As a new hair begins to grow, the old hair is pushed out of the follicle.
Alopecia areata is the number one cause of alopecia, or hair loss, in men. As suggested by its more technical name, androgenetic alopecia, the condition results from certain hormones, which are collectively known as androgens, and genes that influence how hair responds to androgens. The primary cause of hair loss in women is also a form of pattern baldness, called female pattern hair loss. Most men and women are genetically predisposed to developing pattern hair loss because they inherit genes for the trait. This form of hair loss typically occurs between the ages of 20 and 45 in the majority of patients, but can occur at a very young age and may continue throughout one’s life. Male pattern baldness generally starts with a receding hairline and a thinning crown, and can range from partial loss to a horseshoe-shaped area of hair remaining only at the sides and back of the head.