Continuously renewing tissues are maintained by division of cells in the proliferative population to replace cell in the differentiation compartment that are sloughed into the environment. This mechanism of balancing the ra of cell division with the rate of cell loss is essential for tissue homeostasis and must be maintained for life. In such tissues, the proliferative population consists of a hierarchy of dividing cells maintained by a small subpopulation of stem cells. These stem cells are self-renewing, such that each stem cell division produces on stem cell to maintain the population and one daughter cell committed to differentiation. The daughter cells divide to maintain the tissue balance, but they are limited to a finite number of cell divisions before they differentiate an are ultimately sloughed into the environment. Only the stem cells remain for the lifetime of the tissue. In stratified squamous epithelia, stem cells reside in the basal layer, and it has been generally accepted that they have the same characteristics found for stem cells in other tissues: They divide less frequently than the rest of the proliferative cell population; they repopulate the epithelia after damage; they have the ability to form large colonies in vitro; and they represent a small percentage of the population. Previously, using tritiated thymidine, we were able to label a subpopulation of slowly-cycling basal cells in mouse epithelia that we called label-retaining cell (LRCs), which showed many of the characteristics of stem cells. Although this tagging method has been use for the last decade to identify stem cells in several epithelia, to date no one has devised a method that selects only the stem cells, without including some cells from the surrounding basal cell population. Selecting the specific population of epidermal stem cells has become important as gene therapies, which require long term expression of recombinant genes, are developed to counteract the effect of mutated genes that occur in genetically inherited epidermal diseases. Thus, in this study, we propose to separate-viable epidermal LRCs from the basal cell population by selecting cells that preferentially adhere to integrin ligands, that are small in size, and that express specific cell surface antigens. Furthermore, we propose to characterize these selected cells by determining that such cells can reform an epidermis in vitro and in vivo, and by demonstrating that they show long-term expression of a transfected recombinant gene. To make future selection easier, we propose to develop monoclonal antibodies that can differentiate these selected cells from the surrounding basal cells.
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