The precise localization of carcinoembryonic antigen (CEA) and nonspecific cross-reacting 50-kDa antigen (NCA 50) in normal colon mucosa and colon adenocarcinoma was investigated by using an indirect immunoperoxidase electron microscopic technique with specific monoclonal antibodies. In normal adult colon both antigens were localized to microvesicles and filaments of the “fuzzy coat” on the apical surface of the epithelial cells. In addition, NCA 50 was found in the narrow spaces between adjoining microvilli. Mature columnar cells at the free luminal surface contained most of the antigen positive material. CEA and NCA 50 were also detected as intracellular components of goblet cells.

In multilayered tumor glands, the cell surface expression of the antigens was dependent on the position of the tumor cell in the gland. The neoplastic cells showed either a predominant apical labeling or a positive staining of almost the entire cell surface. Some of the neoplastic cells contained CEA in so-called “intracellular lumina.” In contrast to normal colon epithelial cells most tumor cells synthesized NCA 50 actively.

In normal colonic mucosa, unlike in cancerous tissue, CEA and NCA 50 appear to be released via vesicles formed from the microvillous membrane of mature columnar cells. These results are consistent with the hypothesis that CEA and NCA play a role in the nonspecific defense against micro-organisms in the large intestine.

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This work was supported by a grant from the Swedish Cancer Society.

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