Histopathology

TCs are composed of round and/or spindle cells that grow in small nests, trabec-ulae, large sheets, glands and/or rosettes. The cytoplasm is pink and the nuclei have finally stippled or dense chromatin. Nucleoli and mitoses are

Fig. 3.32 Atypical carcinoid, polypoid and focally haemorrhagic, arising from the epiglottis.

sparse to absent (less than 2 mitoses/10 HPFs). Necrosis and pleomorphism are not seen. The stroma is highly vascular and often focally fibrotic or hyalinized.

Rarely, carcinoids, either typical or atypical, may be oncocytic or oncocytoid. The distinction depends on the presence (oncocytic) or absence (oncocytoid) of mitochondria on ultrastructural examination. A few may contain mucin and, exceptionally, even amyloid.

Immunohistochemistry

TCs are positive for cytokeratin, epithelial membrane antigen (EMA), carcinoem-bryonic antigen (CEA), synaptophysin, chromogranin, neuron specific enolase (NSE), and protein gene product 9.5. They are also variably positive for a variety of peptides, including serotonin, cal-citonin, bombesin and somatostatin.

Electron microscopy

TCs contain abundant membrane-bound, electron-dense neurosecretory granules varying in size from 90-230 nm {2762}. Cellular junctional complexes are observed as well as numerous mitochondria if the TC is of the oncocytic type.

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