Carcinoid tumour. Coloured transmission electron micrograph of a cancer cell from a carcinoid (neuroendocrine) tumour of the human gut. The large oval is the cell nucleus. The red patch inside it is the nucleolus, often prominent in cancer cells. The small red granules in the cytoplasm are packed with the hormone serotonin. Carcinoid tumours are the most common tumours of the small intestine. They arise from sparse hormone-secreting cells the gut wall and can spread cancer around the body with fatal results. High levels of serotonin, the hormone made by the cells, causes symptoms like flushing, wheezing & diarrhoea. Magnification ×3000 at 6×4.5cm size. Cover credit: SCIENCE PHOTO LIBRARY.