In Vitro Differences Between Keratinocyte Stem Cells and Transit-Amplifying Cells of the Human Hair Follicle

Epithelial stem cells within the human hair follicle are critical for hair development, hair cycling, wound healing, and tumorigenesis. We and others have previously shown that the hair follicle bulge area contains keratinocyte stem cells, whereas the hair matrix represents the proliferating and differentiating transit-amplifying (TA) cell compartment. In order to better characterize the phenotypic differences between human keratinocyte stem cells and their daughter TA cells, we compared the in vitro properties of cell adhesion, cell migration, clonogenicity, and in vitro life span. Epithelial outgrowths from the hair matrix appeared within 2 d of explant, whereas stem cell outgrowths appeared between 7 and 10 d after explant. Both populations form colonies; however, stem cells from telogen follicles formed more total colonies, and more colonies greater than 3 mm. Upon subculture, stem cells formed colonies until passage 6 and terminally differentiated at passage 7, whereas TA cells only formed colonies until passage 2. Stem cells express more β1 integrin and adhere more rapidly to collagen IV. Most strikingly, TA cells showed a 7-fold greater mobility on migration assays than stem cells (0.704 vs 0.102 μm per min). These results help define the human hair follicle stem cell and TA cell phenotypes and correlate with the in vivo properties of these compartments.