Balding and Hair Loss
Anatomy And Physiology (Structure And Function)
Each hair originates in a deep pouch-like structure in the epidermis (hair follicle) which penetrates the dermis. A hair root extends down into the hair follicle and widens into an indented bulb at its base. Extending into the indentation is the papilla (center of hair growth), which contains capillaries and nerves that supply hair. Newly dividing cells at the base of the hair multiply, forcing cells above them upward. As cells move upward, they gradually die and harden into a hair shaft. A hair shaft has two layers: cuticle and cortex. The cuticle (outer layer) consists of flat, colorless, overlapping cells. The cortex (inner layer) contains pigment and keratin, a tough protein. The cortex forms the bulk of a hair shaft. Coarse hair (e.g., scalp hair) contains an additional inner core (medulla). Hair is lubricated by sebaceous glands located in hair follicles. Illness or stress can lessen pigment secretion and cause hair shafts to whiten. Age-related whitening is genetically determined. Hair color is determined by pigment and air spaces in the cortex and medulla. Hair color and texture are inherited characteristics. Human scalp hairs generally shed every 2 to 4 years; body hairs shed more frequently.22
In the scalp, a hair growth cycle has three main phases: anagen, catagen, and telogen. The anagen phase typically lasts 3 to 5 years. On a healthy scalp, hair numbers approximately 100,000 and 90% of the follicles are continually in the anagen phase of hair growth. The catagen phase follows the end of the growth period when a follicle begins to become dormant. The telogen phase is a dormant or resting period lasting 3 to 4 months. When the dormant phase ends, an old hair falls out. A hair follicle then returns to the anagen phase and a new hair begins to grow.23
An average rate of hair growth is about half an inch per month depending upon hair follicles and age of an individual. On average, 50 to 60 scalp hairs are lost daily in a normal hair growth cycle and new hairs begin to grow from these follicles. Hair loss begins when less new hair begin the re-growth stage.
Etiology And Mechanisms Of Action
In male pattern baldness, scalp hair in affected areas becomes shorter, finer, and less pigmented with successive growth cycles. Androgenic alopecia is thought to be associated with the presence of dihydrotestosterone (DHT), a metabolite of testosterone. Eunuchs have low levels of testosterone and do not lose scalp hair; and men with a genetic deficiency of 5-alpha-reductase (the enzyme that converts testosterone to DHT) do not have male pattern baldness.24