Colon cancer. Light micrograph(LM) of a section through a carcinoid tumour of the colon (large intestine). The groups of carcinoid cells (purple) have a typical endocrine appearance with small round cells containing small round nuclei. Carcinoid tumours are slow growing malignant (cancerous) tumours of the neuroendocrine system, the interface between the endocrine (hormonal) and nervous systems. They arise from cells in the epithelial lining and can occur throughout the gut. They are most common in people over the age of 60. Magnification: x150 when printed at 10 centimetres wide. Human tissue.