Skin cancer. Light micrograph (LM) of a section through skin with a basal cell carcinoma, a common skin tumour that arises from the basal cells of the epidermis. Although the cancer behaves malignantly by invading the underlying dermis it almost never metastasises (spreads). This is the most common form of skin cancer among fair-skinned people. It mostly affects the elderly, although in recent years the average age of onset has steadily decreased. It usually appears on the face or scalp and is thought to be caused by the ultraviolet radiation in sunlight. Cells within the tumour (purple) are haphazardly arranged. The skin surface is at top (red). The lesions are commonly known as rodent ulcers. Magnification: x 250 when printed at 10 centimetres tall. Human tissue