The layers of the epidermis. (Adapted with permission from Montagna W and Parakkal PF. The Structure and Function of Skin [3rd ed.]. New York: Academic, 1974.)
protein keratin surrounded by hydrophobic intercellular lipids, a remarkably effective barrier to many topically applied agents. The differentiation of ker-atinocytes in the basal layer from proliferative cells to highly differentiated nondividing cells in the stratum corneum is tightly regulated by a variety of intrinsic and extrinsic factors, including cytokines and calcium. The epidermis also contains melanocytes, which synthesize the photoprotective pigment melanin, and Langerhans cells, the dendritic antigen-presenting cells that compose the farthest outpost of the body's immune system.
The dermis provides a base for the epidermis and contains fibroblasts that elaborate proteins, such as collagens and elastin, which are crucial for the skin's structural integrity. In addition, mast cells, enriched in a variety of proinflammatory substances, play an important role in tissue remodeling, wound healing, and fibrosis.
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