The Stem Cells Of The Hair Follicle Make Hair
-Bruce A. Morgan, PhD.
Associate Professor of Dermatology, Harvard Medical School
Recent work from two labs has shed light on the properties of the stem cells of the follicular bulge. In addition to their role as the source of the progenitors that regenerate the epithelial portion of the hair follicle during the hair cycle, these cells were thought by some to more generally serve as the stem cells for the epidermis and other components of the pilosebaceous unit. Both groups studied the lineage of these cells in undisturbed mouse skin using different transgenic models. The Morgan group used cre recombinase under control of the Sonic hedgehog (Shh) gene to label both cells in the epidermis that express Shh as well as all of their descendents. They showed that the follicle is essentially a lineage unit- the cells of the epidermal placode make the follicle and do not require contribution from adjacent epidermal cells. Furthermore, the stem cells of the follicular bulge were labeled by this method, demonstrating that they arise in situ within the follicle and do not colonize the stem cell niche from an extra-follicular site. Over the life of the mouse, the lineage label in these cells and their progeny was observed in the follicular epithelium, but not in the epidermis, demonstrating that the stem cells of the follicular bulge are not the stem cells of the epidermis. The Cotsarelis group played a central role in identifying the stem cells of the follicular bulge and had identified keratin 15 as a gene preferentially expressed in these cells and adjacent follicular epithelium. They used an inducible form of cre recombunase driven by keratin 15 to more specifically label follicular stem cells in a similar manner. Again, although contribution to the regenerating follicle was readily demonstrated, contribution to the epidermis was not. Both groups confirmed previous observations that these follicular cells could be induced to alter their fate and contribute to the epidermis by wounding, but this was not observed in the absence of trauma. The demonstration that follicular and epidermal stem cells are distinct and independent populations suggests that therapeutic strategies can be devised which are targeted specifically to the stem cells of the hair or epidermis respectively.
Levy V, Lindon C, Harfe BD, Morgan BA.
Distinct stem cell populations regenerate the follicle and interfollicular epidermis.
Dev Cell. 2005 Dec;9(6):855-61
Ito M, Liu Y, Yang Z, Nguyen J, Liang F, Morris RJ
Cotsarelis G. Stem cells in the hair follicle bulge contribute to wound repair but not to homeostasis of the epidermis.
Nat Med. 2005 Dec;11(12):1351-4