enterochromaffin cell

**Location and histology** - Scattered throughout the GI epithelium, most abundantly in the small intestine (especially ileum), appendix, and rectum. Also found in the stomach and in bronchial mucosa. - Part of the diffuse neuroendocrine system (APUD/Kulchitsky cells). They stain with silver stains (argentaffin/argyrophil) and are chromogranin A and synaptophysin positive. Their granules are neuroendocrine dense-core.

**Physiology** - Synthesize serotonin: tryptophan → (tryptophan hydroxylase, TPH1; rate-limiting) → 5-hydroxytryptophan (5-HTP) → (aromatic L-amino acid decarboxylase, requires B6) → serotonin (5-HT). Serotonin is degraded by MAO to 5-HIAA, which is excreted in urine. - Triggers of release: mechanical stretch/luminal pressure, luminal irritants and toxins, and vagal stimulation. - Serotonin acts on enteric neurons and vagal afferents (5-HT3 receptors → peristaltic reflex, secretion, nausea/vomiting; 5-HT4 → prokinetic effect). - Platelets take up serotonin via SERT but do not synthesize it. - Related but distinct: ECL (enterochromaffin-like) cells in the gastric fundus/body secrete histamine in response to gastrin, which stimulates parietal cells (H2 receptors). ECL cells are the origin of type I and II gastric carcinoids, seen with chronic atrophic gastritis/pernicious anemia and Zollinger-Ellison syndrome (MEN1), which cause hypergastrinemia.

**Clinical significance** 1. **Carcinoid (neuroendocrine) tumors** - Arise from EC cells. The most common sites are the small bowel (ileum) and appendix. The lung is also a site. - Histology: nests of uniform cells with "salt-and-pepper" chromatin. - Most are clinically silent. They are often found incidentally, for example in an appendectomy specimen. They can also cause intussusception or obstruction. - **Carcinoid syndrome** occurs when serotonin and other vasoactive substances reach the systemic circulation. This usually means liver metastases, or a bronchial/ovarian primary that bypasses first-pass metabolism. GI tumors without liver mets do not cause the syndrome, because the liver metabolizes serotonin. - Classic features: cutaneous flushing, secretory diarrhea, wheezing/bronchospasm, and right-sided valvular fibrosis (tricuspid regurgitation and pulmonic stenosis). Left-sided lesions are spared because the lungs inactivate serotonin (MAO). Niacin deficiency (pellagra) can occur because tryptophan is diverted to serotonin. - Diagnosis: elevated 24-hour urinary 5-HIAA, plus chromogranin A. Imaging includes CT/MRI and somatostatin receptor imaging (octreotide/DOTATATE PET). - Treatment: surgical resection. Octreotide/lanreotide (somatostatin analogs) control symptoms. Telotristat (a TPH inhibitor) is also used for diarrhea. Also know the "rule of thirds" and that serotonin antagonists (cyproheptadine) can be used for diarrhea. 2. **Pharmacology** - Ondansetron, granisetron: 5-HT3 antagonists block EC-derived serotonin signaling in chemotherapy-induced nausea. Chemo damages EC cells, which release serotonin and activate vagal afferents. - Alosetron: 5-HT3 antagonist for severe diarrhea-predominant IBS in women. Risk of ischemic colitis. - Tegaserod, prucalopride: 5-HT4 agonists that improve constipation and motility. 3. **Other** - Gut serotonin excess is implicated in diarrhea from infections such as cholera toxin, which stimulates EC cell release. Postinfectious IBS may involve EC cell hyperplasia.

**High-yield pearls** - ~90% of body serotonin is in the gut (EC cells). The rest is in the CNS (raphe nuclei) and platelets. - EC cells produce serotonin. ECL cells produce histamine. Do not confuse them. - Carcinoid syndrome requires liver metastasis, except for non-GI primaries (e.g., lung, ovary). - Elevated urinary 5-HIAA is the key diagnostic test. Foods rich in serotonin (bananas, walnuts, pineapple, avocado) and some drugs can cause false positives.