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19 Epidermal Stem Cells

Fiona M. Watt


The epidermis of mammals forms the outer covering of the skin and comprises both the interfollicular epidermis and the adnexal structures, such as the hairs and sebaceous glands (Odland 1991). The major cell type in the epidermis is an epithelial cell called a keratinocyte. Interfollicular epidermis is made up of multiple layers of keratinocytes. The basal layer of cells, attached to the underlying basement membrane, contains keratinocytes that are capable of dividing, and cells that leave the basal layer undergo a process of terminal differentiation as they move toward the surface of the skin. The end point of this pathway is an anucleate cell, called a squame, which is filled with insoluble, transglutaminase-crosslinked protein and provides an effective barrier between the environment and the underlying living layers of the skin. The basal layer of interfollicular keratinocytes is continuous with the basal layer of keratinocytes that form the hair follicles and sebaceous glands; once again, the end point of terminal differentiation is a dead, highly specialized cell, forming the hair shaft or the lipid-filled sebocytes.

If stem cells are defined as cells with the capacity for unlimited self-renewal and also the ability to generate daughter cells that undergo terminal differentiation (Hall and Watt 1989; Watt 1998; Watt and Hogan 2000), then the epidermis is one of the tissues in which a stem cell compartment must be present. Throughout adult life there is a requirement for the production of new interfollicular keratinocytes to replace the squames that are continually being shed from...

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