Hair Growth
Hair grows in a cycle of three stages, starting with the anagen phase, where cells produce normally at a rate of 6 inches per year and transitioning into the catagen phase two to six years later. During catagen, the hair follicle begins to shrink and separate from the papilla, its nutrient supplier, and hair growth slows to a halt. Three weeks later, the telogen phase commences, which involves the shedding of more than 50 hairs a day due to the insecure attachment of hair roots to the follicle. After a period of three months in telogen, the follicle begins to absorb nutrients again, returns to anagen and resumes the cycle.
Pattern Baldness
All mammals go through the hair cycle. Like most bodily functions, the cycle takes longer to restart and lasts for shorter durations with old age. For some people, the thinning and graying of the scalp is a gradual process. For others, follicles stop reproducing in specific, advanced patterns. This form of hair loss is referred to as androgenic alopecia and mostly affects males. For whatever reason, assumed to be inherited from the mother's side of the family, hair loss occurs in patterns and at an earlier age. Even mice and some other animals experience pattern baldness. If you have the genes, it is just a normal part of hair loss.
Telogen Effluvium
Hair loss also can occur because hair prematurely enters the telogen phase simultaneously due to a fever, drug use, malnutrition, emotional distress, hormone irregularity or trauma. As a result, hairs shed more frequently and for a period of one to three months. However, when the stressful event ends, after enough time has passed to recover, hair returns to the anagen phase again. Unlike pattern baldness, telogen effluvium is generally not permanent or genetic.
Other Causes
Aside from age, genes and shock, hair loss can occur because of scarring, due to a skin disease or accident, because of medical treatment like chemo, which kills cells or depletes protein reserves, or because of chemical bleaching and constant pulling of the hair. Even random occurrences where the immune system attacks hair cells--as is the case with alopecia areata--can cause hair loss. Fortunately, aside from pattern baldness and scarring, undoing whatever caused the hair loss will more than likely permit the growth cycle to resume.
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