The dome epithelium of Peyer's patches from different parts of the intestine of four dogs was examined by scanning and transmission electron-microscopy and by alkaline phosphatase histochemistry on glycol-methacrylate embedded sections. M cells were scattered among more numerous enterocytes in duodenal and jejunal Peyer's patches, but constituted the major cellular component of dome epithelia of the ileal Peyer's patches. Alkaline phosphatase histochemistry demonstrated low enzyme activity in the brush border of M cells, as compared to enterocytes, in the duodenum and jejunum, allowing identification of M cells at the light microscopic level. Alkaline phosphatase activity was too low in ileal enterocytes to permit visualization of M cells. The presence of intrafollicular invaginations of dome epithelium is a consistent finding in duodenal Peyer's patches of the dog and these invaginations were characterized by few M cells, many intraepithelial lymphocytes and strong alkaline phosphatase activity.
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