Gastroenteropancreatic neuroendocrine tumors. A histochemical and immunohistochemical study of epithelial (keratin proteins, carcinoembryonic antigen) and neuroendocrine (neuron-specific enolase, bombesin and chromogranin) markers in foregut, midgut, and hindgut tumors.

Thirty-four gastroenteropancreatic (GEP) neuroendocrine tumors were evaluated for expression of epithelial (keratin, carcinoembryonic antigen [CEA] and neuroendocrine (neuron-specific enolase, chromogranin, bombesin) markers, and results were correlated with histologic patterns and histochemical staining. Tumors of mixed pattern (insular or trabecular with glandular areas) predominated. CEA localization corresponded to staining for mucin, with polarized apical or lumenal staining in glandular areas. Four trabecular midgut carcinoids, however, revealed diffuse cytoplasmic staining for CEA. Staining for keratin proteins was present in 68% of tumors. Bombesin immunoreactivity was demonstrated in 60% of GEP neuroendocrine tumors, indicating that bombesin positive metastatic tumors may not be predominantly of pulmonary origin, as previously suggested. Chromogranin was a sensitive marker for identifying normal gastrointestinal neuroendocrine cells that were not demonstrated by staining for neuron-specific enolase. Chromogranin was present in most neuroendocrine tumors, but was absent from three of five rectal carcinoids in keeping with the distinctive profile of hormonal and silver staining in these tumors. All GEP neuroendocrine neoplasms expressed both neuroendocrine and epithelial markers, supporting their derivation from endodermal epithelium.