Dividing skin cancer cells, coloured scanning electron micrograph (SEM). Cultured melanoma cell showing the numerous blebs and microvilli characteristic of cancer cells. Skin cancer arises from the skin's melanocytes, the cells that produce the pigment melanin which gives skin its colour. Melanoma is an aggressive cancer that often spreads (metastasises) to other tissues of the body. The long cell processes seen here are thought to help the cell to move. The main cause of melanoma is exposure to ultraviolet radiation in sunlight. Magnification: x4000 when printed at 10 centimetres wide. Specimen courtesy of Greg Towers, UCL.