Epidermal Cells Form a Multilayered Waterproof Barrier
The epidermis suffers more direct, frequent, and damaging encounters with the external world than any other tissue in the body. Its need for repair and renewal is central to its organization.
The epidermis is a multilayered (stratified) epithelium composed largely of keratinocytes (so named because their characteristic differentiated activity is the synthesis of intermediate filament proteins called keratins, which give the epidermis its toughness) (Figure 22-2). These cells change their appearance from one layer to the next. Those in the innermost layer, attached to an underlying basal lamina, are termed basal cells, and it is usually only these that divide. Above the basal cells are several layers of larger prickle cells (Figure 22-3), whose numerous desmosomes—each a site of anchorage for thick tufts of keratin filaments—are just visible in the light microscope as tiny prickles around the cell surface (hence the name). Beyond the prickle cells lies the thin, darkly staining granular cell layer (see Figure 22-2). It is at this level that the cells are sealed together to form a waterproof barrier, fulfilling the most fundamentally important of all the functions of the epidermis. Mice that fail to form this barrier because of a genetic defect die from rapid fluid loss soon after birth, even though their skin appears normal in other respects.
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The granular layer, with its barrier to the movement of water and solutes, marks the boundary between the inner, metabolically active strata and the outermost layer of the epidermis, consisting of dead cells whose intracellular organelles have disappeared. These outermost cells are reduced to flattened scales, or squames, filled with densely packed keratin. The plasma membranes of both the squames and the outer granular cells are reinforced on their cytoplasmic surface by a thin (12 nm), tough, cross-linked layer of proteins, including a cytoplasmic protein called involucrin. The squames themselves are normally so compressed and thin that their boundaries are hard to make out in the light microscope, but soaking in sodium hydroxide solution (or a warm bath tub) makes them swell slightly, and their outlines can then be seen (see Figure 22-2).
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