It's known that stem cells, the key players in regenerative processes in the body, play a major role in continually making new hair. Naturally, this role has caused researchers to suspect that hair stem cells are key players in causing androgenetic alopecia (AGA), or male pattern baldness, the most frequent type of hair loss among men. However, in early 2011 a group of researchers showed, surprisingly, that patients with AGA actually had a normal amount of hair stem cells in their scalps. Instead, the team found that different "progenitor cells" may actually be the cells whose fates are tied to AGA. This better understanding of the exact cell types involved may ultimately help researchers devise better therapies for treating male pattern baldness.

Adrogenetic Alopecia In order to understand how stem cells or "progenitor cells" may be involved in AGA, it's important to know how hair usually grows, and how this process is changed in AGA. At any given time, about 85 percent of the hair follicles on a person's head are in a growing phase. (Hair follicles are little cavities that go down through the dermis skin layer, and each cavity is home to a single hair. See the figure for details.) This growth phase can last two to six years for any given hair, during which time the hair grows about five inches every year (of course, there's some variation from person to person). After this growing period, the hair follicle and root shrink for one to two weeks, and during the following five to six weeks the hair stops growing. At the end of this whole process, the hair follicle re-enters the growing phase, sometimes continuing to lengthen the original hair and sometimes with a newly growing hair pushing the old one out, starting the growth cycle all over again.