Antigen uptake has been shown to occur in the teleost intestine, but so far, limited information is available on the distribution and nature of cells involved in the process, and M cells, known for their antigen-sampling abilities in mammals, have not been identified. Here, different intestinal segments from salmonid fish were exposed to gold-BSA to identify antigen-sampling cells. Sections from exposed intestine were examined by light and electron microscopy. Uptake of gold-BSA was restricted to very few dendritic-like cells and to a limited number of epithelial cells located in the mucosal folds in the second segment of the mid-intestine. Gold-positive epithelial cells displayed diverging and electron-dense microvilli with channels intruding into the cytoplasm. A lectin binding experiment demonstrated the presence of cells with mammalian M-cell characteristics in the identical regions. As the identified epithelial cells shared some morphological similarities with immature mammalian M cells, this phenotype may represent evolutionary early antigen-sampling enterocytes.
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