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Skin surface. Coloured scanning electron micrograph (SEM) of a section through the surface of the skin. This image shows the uppermost layer, the stratum corneum (cornified layer). It is formed from flattened, dead skin cells. These are continuously shed and replaced by cells from the living epidermal layers below. The upper part of the image shows the topmost layers detaching (desquamation). The skin provides a first line of defence against infections. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)