The diffuse endocrine-paracrine system of the gut in health and disease: ultrastructural features

At least 16 types of endocrine-paracrine cells have been identified ultrastructurally in the gastrointestinal mucosa. The production of hormones and local messengers such as 5-hydroxytryptamine, gastrin, cholecystokinin, somatostatin, secretin, gastric inhibitory peptide (GIP), enteroglucagon (glicentin, GLI), motilin, neurotensin, substance P and the enkephalins, by these cells, has been established. Progress has also been made in cytological studies of gut and pancreatic endocrine tumours. Argentaffin EC cell carcinoids, gastrinomas (of several ultrastructurally different varieties of gastrin cells), L-cell tumours and D-cell tumours are among those cytologically and functionally defined in the gut. Functionally undefined tumours include the so-called non-argentaffin carcinoids arising in various parts of the gut, some of which have been characterised cytologically as gastric ECL cell tumours and gastroduodenal P-D1-cell tumours. Gastrinomas, vipomas and rare argentaffin carcinoids are among gut-related pancreatic endocrine tumours. Non-functional paragangliomas, usually with some neuromatous component, occur in the duodenal wall. Extrapancreatic vipomas display ultrastructural features of ganglioneuroblastomas with peptidergic granules.