Enterochromaffin (EC) cells (also known as Kulchitsky cells) are a type of enteroendocrine cell, and neuroendocrine cell. They reside alongside the epithelium lining the lumen of the digestive tract and play a crucial role in gastrointestinal regulation, particularly intestinal motility and secretion. They were discovered by Nikolai Kulchitsky.
EC cells modulate neuron signalling in the enteric nervous system (ENS) via the secretion of the neurotransmitter serotonin and other peptides. As enteric afferent and efferent nerves do not protrude into the intestinal lumen, EC cells act as a form of sensory transduction. Serotonin in the ENS acts in synergy with other digestive hormones to regulate sensory and motor gastrointestinal reflexes. EC cells respond to both chemical and neurological stimuli. They are also reactive to mechanosensation, which is the case in the peristaltic reflex of the gut, and can be stimulated by a bolus moving through the bowel. Upon activation, EC cells release serotonin to act upon serotonin receptors on ENS neurons. Dependent on concentration, serotonin can then modulate peristaltic contraction and secretion through activation of smooth muscle and glands, respectively.
Pulmonary neuroendocrine cells in the respiratory tract are known as bronchial Kulchitsky cells.
Structure
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.
Distribution
EC cells are found aggregated in specific locations throughout the gastrointestinal tract, predominantly in the small intestine, colon and appendix. The proportion of high-density cell populations varies between species attributed to differences in dietary requirements and physiological characteristics.
Enterochromaffin-like cells
Enterochromaffin-like (ECL) cells are a population of cells that are found in the gastric pits 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.