The gastric mucosa contains at least seven distinct types of endocrine cells (D [somastostatin] and D1 [unknown secretion] cells, G [gastrin] cells, P and X
[unknown secretion] cells, enterochromaffin [serotonin] cells, and enterochromaffin-like [ECL] [histamine] cells), which constitute about 2% of all of the mucosal cells. ECL cells represent a group of hista-mine-storing argyrophil cells that are present in the gastric corpus and fundus (oxyntic mucosa) and rarely in the antral mucosa.44 ECL cells differ from gastric enterochromaffin cells.45 Relatively little is known about ECL cell function beyond its modulation of acid secretion via histamine secretion: gastrin drives the ECL cell to secrete histamine, which then activates the histamine receptor of the adjacent parietal cells, leading to acid secretion.46 Gastric NETs are separated into three distinct groups on the basis of both clinical and histologic characteristics. Type I is associated with chronic atrophic gastritis with or without pernicious anemia (Figure 18-4). Gastric atrophy results in hypergastrinemia because the neutral gastric pH leads to stimulation of the antral G cells to produce gastrin.112 ECL cells are exquisitely sensitive to the trophic action of gastrin and thus undergo hyperplastic growth, eventually leading to dysplastic and neoplastic proliferation (ECLomas)
over an average 15-year period. In patients with extensive chronic atrophic gastritis type A, fundic ECL hyperproliferation has been noted in up to half of all cases and development of ECLomas in about 10 to 30% of these patients. The most important predisposing factors appear to be the serum gastrin level and the duration of gastric atrophy.47 However, what constitutes a dangerous degree of gastrin exposure (duration and plasma gastrin level) appears to be variable and has not yet been determined.46 Type II tumors develop in patients with MEN type I and Zollinger-Ellison syndrome. Like type I tumors, these gastric NETs are thought to arise from ECL cells in patients with hypergastrinemia owing to the gastrinoma (instead of chronic gastric atrophy in type I). Recent reports, however, suggest that such gastric carcinoids can occur in MEN type I patients without hypergastrinemia.44 Type III tumors are sporadic, arising in normal-appearing mucosa, and thus are independent of the trophic stimulus of gastrin.1 The latter are more aggressive.
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