The skin consists of two main compartments, the epi-dermis, a stratified squamous epithelium, and the un-derlying dermis, a richly vascularized tissue embedded in a connective tissue matrix (Fig. 41.1). The epidermis consists of multiple layers of keratinocytes, which dif-ferentiate into the outermost layer, the stratum corneum.
This layer contains the hydrophilic structural protein keratin surrounded by hydrophobic intercellu-lar lipids, a remarkably effective barrier to many topi-cally 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 com-pose 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 im-portant role in tissue remodeling, wound healing, and fibrosis.