Susan Kingsley, MBA, PhD (Susan Kingsley, a professional medical writer and President of BioPharma Solutions in Vancouver, BC, Canada, is an independent consultant to the ISHRS and assisted the Forum in covering several keynote talks at the ISHRS Annual Meeting in Vancouver.)
 
Based on Dr. George Cotsarelis's "Hair Follicle Stem Cells," presented Thursday, August 12, 2004, at the 12th Annual Meeting of the ISHRS in Vancouver, British Columbia.
 
Hair Transplant Forum International. Volume 15, Number 2. March/April 2005.

George Cotsarelis, MD, of the University of Pennsylvania School of Medicine, shared his research on hair follicle stem cells with delegates attending the 12th Annual Meeting of the ISHRS, held in Vancouver, BC, in August 2004, Dr. Cotsarelis set the scene by describing hematopoietic stem cells, the well-characterized common ancestors of all mature blood cells. These self-renewing, multipotent, permanent, and highly proliferative cells are clinically applied in the treatment of diseases such as cancer and congenital immunodeficiencies.
 
However, Dr. Cotsarelis stressed that hematopoietic stem cells are not the only self-renewing, multipotent, persistent cells with high proliferative potential. Epithelial stem cells, including those in the epidermis and hair follicle, also have these characteristics and their manipulation has wide ranging applications, including tissue transplantation and new approaches to hair loss. Summarizing the history of hair follicle research, Dr. Cotsarelis described experiments during the 1980s and 1990s that found stem cells present in the hair follicle in the form of keratinocytes. These keratinocytes had a high in vitro proliferative potential or were long-lived, slowly cycling "label-retaining cells" (LRCs) in animal models.
 
Although the hair follicle contains rapidly proliferating and differentiating cells, its growth (anagen) is interrupted by periods of regression (catagen) and rest (telogen), with subsequent regeneration. After conducting LRC studies on human scalp grafted to immunodeficient (SCID) mice, Dr. Cotsarelis and his colleagues determined that the stem cells responsible for this cyclical regeneration reside in an area of the follicle called the hair follicle "bulge." Further analysis of these bulge stem cells found them to be generally slowly-cycling, long-lived cells that preferentially proliferate at the onset of anagen. They also selectively expressed cytokeratin 15 (K 15) throughout all stages of the hair cycle.
 
Dr. Cotsarelis then described the results of recent studies that isolated and characterized bulge stem cells from adult mouse skin. These experiments followed the fate of the isolated cells and demonstrated that they do actually repopulate the hair follicle epithelium. In the first set of studies, the scientists used a K 15 promoter in combination with a fusion protein (CrePR 1: consisting of Cre recombinase and a truncated progesterone receptor) to design transgenic mice (K 15-CrePR 1). These mice actively expressed CrePR 1 in the bulge cells, but the recombinase was activated only following topical treatment with RU486 (a ligand for PR 1). The K 15-CrePR 1 mice were crossed with another strain of mouse, called R26R. R26R mice express LacZ after activation by Cre, which allows for genetic tagging of the bulge cells and their progeny. The hybrids were given RU486 for 5 days, after which the follicles spontaneously entered anagen. LacZ-positive cells were found throughout all epithelial cell types of the new hair follicle, indicating the multipotency and high proliferative potential of the hair follicle stem cells.
 
The scientists then used a different type of transgenic mouse, the K 15-EGFP mouse, which expressed EGFP specifically in the bulge. Once extracted from the mouse skin, the EGFP-containing cells fluoresced and could be sorted. Fluorescent cells were again found to have a high in vitro proliferative potential. "But," asked Dr. Cotsarelis, "do these stem cells maintain the ability to make new hair follicles?"
 
He and his colleagues addressed this question by conducting experiments known as recombination assays. They isolated fresh genetically-tagged bulge cells from adult mice and combined these with mouse neonatal dermal cells. This combination of cells was either directly injected into immunodeficient (SCID) mouse skin or inserted into a chicken trachea, then implanted into the SCID mouse. After 4 weeks of growth, new hair follicles derived from stem cells were detected. "Clearly, isolated bulge stem cells have the potential to make new follicles, as well as regenerate existing ones," Dr. Cotsarelis concluded.
 
Dr. Cotsarelis continued his presentation by describing the molecular signals behind bulge stem cell survival. Using microarrays (gene-based computer chips), he and his colleagues compared global gene expression patterns between isolated hair follicle bulge stem cells and non-bulge basal keratinocytes. They found that over 150 genes were differentially expressed in the cells from the two sources, with 100 genes from the bulge stem cells comparatively up-regulated (enriched expression) and 58 down-regulated (lower expression). Both the up-regulated and down-regulated genes included various growth factors, receptors, and transcription factors. The gene expression patterns were also consistent with the concept that the hair follicle is immune-privileged since numerous histocompatibility genes were down-regulated in the hair follicle stem cells. The scientists were able to identify a set of at least 12 up-regulated genes that may be responsible for maintaining bulge cells in a quiescent and undifferentiated state, and additional genes potentially involved in hair growth.
 
"These data demonstrate that bulge stem cells proliferate and repopulate the lower hair follicle at anagen onset during normal hair growth," Dr. Cotsarelis concluded. This new understanding of hair follicle stem cells leads us one step closer to developing new approaches to treating hair loss by means of altering hair follicle cycling (e.g., preventing catagen, triggering anagen onset, preventing exogen) or by generating new hair follicles (neogenesis). Although the procedure is many years away from regulatory approval by the U.S. Food and Drug Administration (FDA), autologous transplants for new hair follicle generation are most likely to be pursued initially. However, allogeneic transplants may be possible, with one anecdotal case reported in which a male hair follicle transplanted into the arm of a woman grew as normal hair, with a mixture of male and female cells.

Note: Additional details on some of these studies can be found in Morris R. J. et al. Capturing and profiling adult hair follicle stem cells. Nature Biotech. 2004; 22:411-417.