Bowel cancer. Light micrograph of a section through a carcinoid tumour in the small bowel. The tumour has invaded the submucosa of the bowel, with the surface mucosa itself becoming disrupted and reduced in height. A carcinoid or neuroendocrine tumour is locally invasive but can spread (metastasise). The tissue type is derived from gut neuroendocrine cells often arranged in clumps resembling small glands (seen here). Carcinoid tumor cells secrete one or more of the hormones normally produced by the neuroendocrine cells, and commonly serotonin is secreted in excess. Often asymptomatic, carcinoid tumours are usually treated by surgical resection depending on size, location and frequency. Magnification: x7, when printed 10 centimetres wide.