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Microscopy of the absorptive cells and gut-associated lymphoid tissue of the flounder Platichthys flesus

A light and electron microscopic study of the enterocytes and gut-associated lymphoid tissue (G ALT) of the flounder Platichthys flesus showed regional differences in the structure of the epithelial cells. Epithelial villi decreased in height and spatial complexity, but increased in overall thickness in the posterior intestine. There were no specialized cells comparable with mammalian M cells. Periodic acid Schiff (PAS) activity increased in the enterocytes and goblet cells in the anterior intestine where alkaline phosphatase activity was most pronounced in enterocytes. Adjacent cells were joined by tight junctions. Gut-associated lymphoid tissue was composed of isolated leucocytes in the epithelium and lamina propria with no accumulations. Numbers of intra-epithelial leucocytes were not statistically different in the various regions of the intestine. Five leucocyte types were distinguished among the gut-associated lymphoid tissue although lymphocytes and granulocytes were the most common. Granulocytes were positive for the Romanowsky stains with PAS/Alcian blue positive granules. Blast cells similar to large lymphocytes were unevenly distributed through the different regions of the intestine. Plasma cells were more numerous in the epithelium than in the lamina propria. Macrophages were rarely detected.