An interesting mechanism that appears to contribute to age-related hair loss was recently identified. It is unusual in that cells of one tissue structure are changing to cells of another as a result of age-related damage:
Scientists have uncovered a new mechanism behind hair loss: When stem cells in hair follicles are damaged by age, they turn themselves into skin. Over time, this happens to more and more stem cells, causing hair follicles to shrink and eventually disappear. This is the first time such a switch has been associated with aging in any tissue. Stem cells - precursor cells that can give rise to specialized cells like skin and hair - regenerate throughout the life of an organism and are located all over the body. But unlike stem cells in the blood or intestinal lining, hair follicle stem cells regenerate on a cyclical basis. Their active growth phase is followed by a dormant phase, in which they stop producing hair. These discrete on-off periods make hair follicle stem cells a useful model for studying stem cell regulation - and hair loss.
To figure out why hair thins in old age, researchers looked at hair follicle stem cell growth cycles in live animals - a daunting task - and found that age-related DNA damage triggers the destruction of a protein called Collagen 17A1. That in turn triggers the transformation of stem cells into epidermal keratinocytes. In their new state, the damaged stem cells slough off easily from the skin's surface. "When damaged cells deplete that niche of collagen 17A1, they alter their own signaling environment. It is interesting that these damaged cells change their fate rather than committing suicide through apoptosis (programmed cell death) or stopping cell division through senescence."
To see whether their results carried over to people, the researchers analyzed hair follicles in scalps from women aged 22 to 70. They found that follicles in people over 55 were smaller, with lower levels of Collagen 17A1. "We assume that ... aging processes and mechanisms similar to those in the mice explain the human age-associated hair thinning and hair loss." Stem cell depletion is unlikely to be the only factor behind the condition, however.