RESUMEN
Microfold cells [M cells] act as gateways to the mucosal immune system. The aim of the study was to perform a detailed ultrastructural study on M cells and microvillus enterocytes in the follicle-associated epithelium that covers the dome region of the ileal Peyer's patches in the albino rat to understand the relationship between the two types of cells, the origin of M cells, and their possible mechanism in sampling of the luminal antigens. The follicle-associated epithelium over the Peyer's patches in 10 adult albino rats was studied carefully under transmission electron microscope with a special attention to M cells and microvillus enterocytes. M cells were characterized by pale cytoplasm and either paucity or lack of apical microvilli beside intracytoplasmic pockets harboring migrating lymphocytes. In the microvilli-lacking areas, however, bundles of microvillus filaments occasionally remained in the apical cytoplasm and small vesicles and tubules were found at the apical poles of M cells. Microvillus cells had apical brush borders composed of long, regular, and densely packed microvilli; however, a wide range of diversity of the apical surface morphology was noted. Sometimes, transitional epithelial cells from microvillus to M cells were observed as columnar cells, which had microvilli of moderate height in their apical border with appearance of bare areas, in addition to other cells, which had a microfolded profile that bore few stout and short microvilli. Pale cytoplasmic extensions of dendritic morphology were seen extending among the epithelial cells covering the Peyer's patches. The M cells were differentiated from the intestinal enterocytes by distinctive ultrastructural features. The observed transitional epithelial cells might indicate that the M cells probably originated from the microvillus enterocytes. The M cells transport the antigens from the intestinal lumen to the lymphocytes in the intraepithelial pockets for induction of mucosal-associated immune response