Cellular Classification
There is no histologic difference between carcinoids arising in various sites
or between metastasizing and nonmetastasizing lesions. Carcinoid tumors are neuroendocrine tumors composed of uniform, round, or polygonal cells. Immunohistochemistry reveals the presence of neuron-specific enolase (NSE) and chromogranin. Electron microscopy shows neurosecretory granules. Morphologic features suggesting malignancy (cellular pleomorphism, hyperchromatic nuclei, necrosis, high mitotic activity) can occasionally be seen. Such cases are designated as atypical carcinoids and usually have an aggressive clinical course. Carcinoids are
classified by their embryologic relationship to the foregut (the anterior part
of the alimentary canal, from the mouth to the intestine or to the entrance of
the bile duct), midgut (the middle part of the alimentary canal, from the
stomach or entrance of the bile duct to or including the large intestine), or
hindgut (the posterior part of the alimentary canal, including the rectum and
sometimes the large intestine), which is correlated with clinical behavior and
the secretion or nonsecretion of various neuroendocrine peptides. Proximal
carcinoids may secrete histamine-like peptides causing a pink flush and
bronchoconstriction. Peptide secretions of midgut carcinoids cause a cyanotic
(purplish) flush, diarrhea, and hypotension. Hindgut carcinoids usually do not
secrete syndrome-producing peptides.
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