Skin cancer cells. Light micrograph of melanoma cancer cells grown in a culture. This cancer arises from the skin's melanocytes, the cells that produce the pigment (melanin) that give skin its colour. Melanoma is an aggressive cancer that often spreads (metastasises) to other tissues of the body. The long cell process seen here help the cells to move. Once the cancer has spread the prognosis is poor. The main cause of melanoma is exposure to ultraviolet radiation in sunlight.