Baldness, or alopecia, is a condition characterized by hair loss that arises from a complex interplay of biological processes. Understanding these mechanisms is key.
The Normal Hair Growth Cycle
Hair growth follows a cycle involving distinct phases: anagen, catagen, and telogen. The anagen phase is the active growth period, during which hair follicles produce new hair. This phase is the longest, typically lasting between two and seven years, and determines the maximum hair length.
Following anagen, hair enters the catagen phase, a short transitional period lasting about 10 days to three weeks. During this stage, the hair follicle shrinks, and growth ceases as the hair detaches from its blood supply. The telogen phase is a resting period of approximately three to four months, where hair remains in the follicle but is not actively growing. At the end of telogen, the old hair is shed, and a new anagen hair begins to grow, pushing out the resting hair. Each hair follicle operates independently, preventing sudden, widespread shedding.
The Role of Genetics and Hormones
A significant factor in hair loss, particularly in androgenetic alopecia (pattern baldness), involves both genetic predisposition and hormonal influences. This condition is the most common form of hair loss and affects both men and women. It is characterized by the progressive miniaturization of hair follicles, where they shrink over time and produce finer, shorter, and lighter hairs until they stop producing hair.
The primary hormone implicated is dihydrotestosterone (DHT), a potent form of testosterone. In individuals with genetic sensitivity, DHT binds to androgen receptors in susceptible scalp hair follicles. This binding shortens the anagen (growth) phase, leading to gradual shrinking of the follicles. The exact mechanism is still being researched, but it reduces hair’s growth potential, resulting in thinning and baldness.
Immune System and Inflammatory Responses
Hair loss can also result from the immune system mistakenly attacking healthy hair follicles or from chronic inflammation around the follicles. In autoimmune conditions like alopecia areata, the body’s immune system mistakenly attacks hair follicles. This can lead to sudden, patchy hair loss on the scalp or other parts of the body.
While the triggers are not fully understood, the attack primarily affects hair follicles in their active growth phase. In contrast, scarring alopecias involve inflammation that causes permanent damage to the hair follicles. This inflammation destroys stem cells and oil glands essential for hair regrowth. Once these stem cells are destroyed and replaced by scar tissue, the hair follicle cannot regrow hair, leading to permanent bald spots.
Physical and Environmental Triggers
External factors and physiological stressors can disrupt the hair growth cycle, leading to various forms of hair loss. Telogen effluvium occurs when a sudden or severe physical or psychological stressor prematurely pushes many hairs into the resting phase. This can result from events such as major surgery, illness, childbirth, emotional stress, or nutritional deficiencies, leading to widespread hair shedding two to four months after the triggering event.
Physical damage to hair follicles can also cause hair loss, as seen in traction alopecia. This condition results from prolonged or repetitive tension on the hair, often due to tight hairstyles like braids, ponytails, or extensions. Continuous pulling can loosen the hair shaft from its follicle, and if severe or prolonged, can permanently damage the follicle, preventing regrowth. Harsh chemical treatments, such as perms, dyes, or relaxers, can damage hair shaft and follicles, leading to breakage or chemical burns that impair growth.