Keratinocyte skin cells. Coloured scanning electron micrograph (SEM) of a section through keratinocyte skin cells from a human finger, showing the nuclei (round). These epithelial cells produce keratin in the skin's outermost layers. As they are used to seed new skin growth, they are often cultured to produce skin grafts or for testing cosmetics and drugs. Magnification: x3300 when printed 10 centimetres wide.