028 Hair Follicle Stem Cells: An Overview.

Robert M. Lavker1 and Tung-Tien Sun2. Departments of Dermatology, 1University of Pennsylvania School of Medicine, Philadelphia, PA and 2New York University School of Medicine, New York, NY.

Hair follicle stem cells are important because they play a central role in the control of the hair growth cycle, help in the maintenance of the epidermis, and represent a major target of tumor initiation. One of the most accepted means to identify keratinocyte stem cells takes advantage of the fact that they are normally rarely cycling in vivo, and can be detected experimentally as the “label-retaining cells” (LRCs). When this approach was used to localize the rarely cycling cells of the hair follicle, all of the follicular LRCs were exclusively confined to the bulge – the part of the outer root sheath marking the lowest point of the upper, permanent portion of the follicle. Furthermore, in studies of mouse and human skin, the great majority of LRCs have been observed in the bulge region of the hair follicle, with very few, if any in the epidermis. Cells within the follicular bulge satisfy almost all of the criteria for keratinocyte stem cells. Bulge keratinocytes have a higher proliferative capacity than other regions of the follicular epithelium as well as the epidermis; have a primitive ultrastructure; are in contact with a specialized smooth muscle; are physically well protected; and are believed to be a major target of chemical carcinogens. Most recently, it has been demonstrated that the progeny of bulge cells can emigrate into the epidermis in normal mouse skin. These data suggest that the bulge represents a major repository of skin keratinocyte stem cells that may be bipotent as they can give rise to not only the hair follicle, but also to the epidermis.