The endodermal origin of digestive and respiratory tract APUD cells. Histopathologic evidence and a review of the literature.

Twenty-seven small cell carcinomas of the lung and three tumors of the large intestine with combined adenocarcinomatous and small cell and/or anaplastic carcinoid-type histologic features were studied by light and electron microscopy. It was shown that the small cells have morphologic characteristics of APUD cells. Also presented are the histologic features of a carcinoma of the lung with large cell undifferentiated carcinoma, adenocarcinoma, squamous cell carcinoma, and giant cell carcinoma areas in the primary site and in several metastatic foci. Two of the renal metastases showed small cell carcinoma. The combined tumors and the numerous other similar neoplasms described in the literature and reviewed here suggest an endodermal origin for digestive and respiratory tract APUD cells based on the hypothesis that cancer is a clonal proliferation, and mucous and squamous cell differentiation is an endodermal rather than neural crest characteristic. The ultrastructural features of tumors of cells of known neural crest origin, including a medullary carcinoma of the thyroid, three carotid body tumors, a pheochromocytoma, and two cutaneous melanomas were compared with those of other APUD cell tumors including small cell carcinomas of the lung, two bronchial carcinoids, a carcinoid of the appendix, and a carcinoid of the kidney. Cells of the latter group sometimes possessed cytoplasmic tonofibrils, round compact masses of cytoplasmic microfilaments, and ductal lumina. These features were lacking in the former group and may signify a different embryologic origin. The histologic, histopathologic, and embryologic evidence regarding the origin of digestive and respiratory tract APUD cells is reviewed, showing that the former are, and the latter probably are, of endodermal and not neuroectodermal origin.