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The cutaneous epithelium is a continually renewing tissue maintained in a dynamic equilibrium of proliferation in the basal layer and loss through terminal differentiation from the suprabasal layers. This process is orchestrated with great elegance by a hierarchy of stem cells, transient amplifying cells, and terminally differentiating cells. These populations of cells work together to maintain lifelong tissue function and to bring about tissue repair. This chapter focuses on the role of stem cells and their identification in the epidermis.
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Proliferation in the cutaneous epithelium begins with the stem cells.1,2 Stem cells in this regard lack many characteristics of terminal differentiation, and have an intrinsically high proliferative potential relative to the other proliferating cells, but are generally capable of lifelong proliferation.3 Upon division, a stem cell produces off one daughter that remains a stem cell, and one daughter that goes on to produce a series of transit amplifying cells that serve to magnify or amplify the stem cell's division resulting in the production of many differentiated cells with minimal input from the stem cell. This hierarchical system that usually involves decreasing proliferative potential is illustrated in Fig. 45-1. Stem cells typically interact with their surroundings in a supportive, protective niche.1
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Stem cells may be studied by the presence or absence of proteins on their surface that distinguish them from other proliferative cells.2 Such proteins may be internal, or more desirably, proteins on the cell surface that render the cells selective by various methods such as ...