Enterochromaffin cell

EC cells are small polygonal cells located in the crypts between intestinal villi. They are discriminated from other cells of the gastrointestinal epithelial crypts by the presence of basally located granulations that contain serotonin and other peptides. Ultrastructurally, these granules are reported to vary in size and shape and are considered pleomorphic. Most EC cells communicate with the lumen of intestinal crypts through apical microvilli (protrusions) and are referred to as "open". A proportion of EC cells do not protrude into the crypt lumen and are subsequently referred to as 'closed'. EC cells typically extend down to the basal lamina with cytoplasmic extensions known to pass through the connective tissue and neighbouring glands. Tissue beneath EC cells typically contains abundant fenestrated capillaries, lymph vessels and small unmyelinated nerve fibres. Secreted serotonin can either be taken up into residing vessels (transported in the blood by platelets) or act on nerve synaptic terminals. The proportion of high-density cell populations varies between species attributed to differences in dietary requirements and physiological characteristics. Enterochromaffin-like cells Enterochromaffin-like cells (ECL cells) are a population of cells that are found in the gastric glands of the stomach luminal epithelium and secrete histamine. In response to gastrin released by neighbouring G-cells, secreted histamine from ECL cells acts on parietal cells to stimulate the release of gastric acid. The presence of ECL cells is critical in regulation of endocrine-induced gastric acid secretion. ECL cells histologically appear similar to EC cells and are hence named as such. They are however a different cell type and do not possess any serotonin synthesizing mechanisms. ==Development==

In developing chick embryos, EC cells have been found in biopsies of developing GIT tissue before the migration of neural crest cells. Whilst EC cells have neuroendocrine properties and are similar chemically and histologically to cells of the adrenal medulla they are not derivatives of the neural crest and do not share a similar cell progenitor. EC cells are believed to be derived from endodermal origins and are descended from the stem cells that form other epithelial cell types of the gastrointestinal lumen. == Function ==

The primary function of EC cells is to synthesise and secrete serotonin for modulation of gastrointestinal neurons. Serotonin, also named 5-hydroxytryptamine (5HT), can be classified as a hormone, neurotransmitter and a mitogen. It is primarily known for its role in the central nervous system but plays an important role in the periphery, with the largest endogenous pool of serotonin residing in the gut (90% of endogenous store). In the ENS, serotonin is an essential modulator of sensory transduction and mucus secretion. Release of serotonin from EC cells can be triggered by a multitude of stimuli, particularly luminal distension, parasympathetic innervation or changes in osmotic concentrations in intestinal contents. Serotonin synthesis The synthesis of 5-HT, in EC cells, is catalyzed by the enzyme tryptophan hydroxylase 1 (TpH1) from the amino acid L-tryptophan. The reaction proceeds in two stages with an initial rate limiting step involving the conversion of L-tryptophan to 5-hydroxytryptophan (5-HTP). Following conversion to 5-HTP, the non-rate limiting L-amino acid decarboxylase converts 5-HTP to 5-HT by decarboxylation. Following synthesis, 5-HT is then stored in vesicles by vesicular monoamine transporter 1 close to the basal margin of the cell for eventual secretion. The vesicles pass from the basal margin into the surrounding lamina propria for interaction with nearby nerve synapses, lymph and blood vessels. The serotonin synthesised by EC cells is predominately exocytosed from the basal border, but is also known to be apically secreted into the lumen of the gut and can be present in faecal samples. Secreted 5-HT acts on different receptor subtypes found localised in cells in the gastrointestinal epithelium, smooth muscle and connective tissue with responsiveness dependent on the concentration of the secreted hormone. ==Clinical significance==

Irritable bowel syndrome Irritable bowel syndrome (IBS) is a diverse condition associated with chronic bowel discomfort and abdominal pain that ranges in severity between patients. Abnormal concentrations of serotonin have been associated with IBS, predominantly increased concentrations intensifying gastrointestinal motility and mucosal secretions from the gut mucosa. Severe IBS often manifests as either chronic constipation or chronic diarrhoea, and abnormal EC cell populations have been correlated with both conditions. In patients suffering post-infectious IBS, rectal biopsies have shown a dramatic increase in populations of EC cells associated with diarrhoeal symptoms. Likewise, reduced populations of EC cells in patients suffering chronic constipation have been observed, indicating a lack of 5-HT, and therefore decreased GI motility and secretion. Ongoing research indicates that abnormal EC cell populations, and therefore 5-HT signalling, may significantly contribute to gastrointestinal dysfunction. Treatment using 5-HT-receptor agonists for patients with functional constipation have shown some effectiveness in achieving normal GI functionality. Carcinoid syndrome Carcinoid syndrome is a rare condition characterized by an abnormal increase in circulating biologically active hormones, largely serotonin, with early symptoms involving diarrhea, abdominal cramping and episodic flushing. ==History==

The name 'enterochromaffin' comes from the Greek word "enteron" (ἔντερον), in relation to intestines, and "chromaffin" as a grouping of the words chromium and affinity, as they can be visualised by staining with chromium salts. Similarly named, chromaffin cells (of the adrenal medulla) share this characteristic and are histologically similar to EC cells. Their embryological origins, however, are quite different, nor do they possess similar functions. == See also ==