It’s known that stem cells, the key players in regenerative processes in the body, play a key role in continually making new hair. This role created interest in studying hair follicle stem cells to better understand androgenetic alopecia (AGA), or male pattern baldness, the most frequent type of hair loss among men. Naturally, the hair follicle stem cells were the prime suspects for causing AGA. However, earlier this month a study by George Cotsarelis at the University of Pennsylvania School of Medicine and colleagues published in The Journal of Clinical Investigation (Garza et al., 2011) revealed that patients with AGA actually had had a normal amount of follicle stem cells in their scalps. Surprisingly, it was found that different progenitor cell populations, suspected to be derived from the hair follicle stem cells, were in fact the ones playing the key roles in causing AGA. (Progenitor cells are like stem cells in that they can differentiate into different cell types, but progenitors’ fates are more limited and they can replicate only a restricted number of times.) By better understanding the exact cell types involved, it may help researchers devise better therapies for treating AGA.
A Hair’s Life-Cycle: In order to understand AGA and the newly discovered key role of these progenitor cells in it, it’s helpful to first review the normal life of a hair. In the skin, every hair sits inside a hair follicle, a little cavity that goes down through the dermis layer and has connected sebaceous glands (which lubricate the hair by secreting an oily substance) an arrector pili (a small bundle of muscles that can make the hair stand on end) (see Hair Follicle figure). Each hair carries out its own life-cycle. The first lifecycle phase is called anagen, a growing period that about 85 percent of the hairs on a person’s head are in at any given time. During anagen, which can last two to six years for one hair, the hair grows at the rate of about five inches a year. After anagen, the hair enters catagen, a transitional one- to two week-long stage when the hair follicle and root both shrink. The hair then enters the last stage, telogen, which is a resting phase that lasts about five to six weeks, during which time the old hair does not grow. At the end of telogen the hair follicle re-enters anagen, the growth phase, and often a new hair will push the old one out, starting the growth cycle over again (Furdon & Clark, 2003; Garza et al., 2011).
Androgenetic Alopecia: Normally, the new hair will grow similar to how the last one did. However, with AGA this isn’t the case. In AGA, hair follicles get smaller over time, and consequently make smaller and smaller, eventually microscopic, hairs. How is this caused? It’s not that well understood; it’s known that testosterone is necessary for this miniaturization (as inhibiting testosterone conversion to its active form can delay AGA progression), but not much else is known about what causes AGA (Garza et al., 2011).
But even if it’s not known what happens to cause AGA, researchers have done a lot of work to figure out what stem cells are normally active in the hair follicle. Within a hair follicle, there are stem cells that reside in an area called the hair follicle “bulge,” which is a small compartment located where the outer root sheath meets the arrector pili muscle (see Hair Follicle figure). The stem cells in the bulge are multipotent epithelial stem cells, and can become, or differentiate into, all the epithelial cell types in the follicle (including hair follicles, epidermis, and sebaceous glands) (Oshima et al., 2001). They’re intimately involved in the hair follicle lifecycle. Given this, it shouldn’t come as a surprise that if these stem cells are destroyed, so is the hair follicle (Ohyama et al., 2006; Ohyama 2007).