Skin. Coloured scanning electron micrograph (SEM) of skin from the tip of a human thumb. The top layer of the skin, the stratum corneum, is composed of dead keratinised cells that are continuously sloughed off and replaced with new cells from below. The cell surface is covered in fold-like extension of cytoplasm (microplicae) providing a larger surface area. Magnification: x400 when printed 10 centimetres tall.