epidermis /ep′idur″mis/ [Gk, epi + derma, skin] , the superficial avascular layers of the skin, made up of an outer dead, cornified part and a deeper living, cellular part. Each layer is named for its unique function, texture, or position. The deepest layer is the stratum basale. It anchors the more superficial layers to the underlying tissues, and it provides new cells to replace those lost by abrasion from the outermost layer. The cells of each layer migrate upward as they mature. Above the stratum basale lies the stratum spinosum. As the cells migrate to the next layer, the stratum granulosum, they become flat, lying parallel with the surface of the skin. Over this layer, such as in the thick skin of the palms of the hands and soles of the feet, lies a clear, thin band of homogenous tissue called the stratum lucidum. The outermost layer, the stratum corneum, is composed of scaly, squamous plaques of dead cells that contain keratin, a waterproofing protein that hardens over several days. This horny layer is thick over areas of the body subject to abrasion, such as the palms of the hands, and thin over other more protected areas. Altogether these layers are between 0.5 and 1.1 mm in thickness. Also called cuticle. See also skin. −epidermal, epidermoid, adj.
