Lung cancer. Coloured scanning electron micrograph (SEM) of a section through a metastatic adenocarcinoma (green) in lung tissue. Adenocarcinomas are malignant (cancerous) tumours that arise from the lining (epithelium) of the lung. Pneumocytes (alveolar cells, yellow), erythrocytes (red blood cells, red) and macrophages (white blood cells, whitish) can be seen on the walls of the alveoli (spaces). Magnification: x800 when printed at 10 centimetres across.