Where Are the Skin Epithelial Stem Cells Located?
On initial histological evaluation of mammalian skin, there is no obvious morphologically distinct region, or niche, of the basal layer where stem cells might be located. It has been known from the 1970s that the epidermis is organized into columns of maturing cell layers ~10 cells wide (see Fig. 1B). It was initially hypothesized that the entire basal layer consisted of stem cells, then later that the Langerhans cells were stem cells. Later studies suggested that stem cells might comprise 2-7% of basal layer cells. One method of retrospectively demonstrating the presence of stem cells in epidermal cultures is to label the population of cells and then use them to reconstitute epidermal tissue in vivo. Thus, when epidermal cultures expressing the β-galactosidase reporter gene were grafted onto a mouse, the reconstituted skin exhibited clonal columns of β-galactosidase-expressing epidermal cells in the host animal (Fig. 1B). The size of the columns over the 12-week study period suggested
