Structural features of and cholesterol distribution in M-cell membranes in guinea pig, rat, and mouse Peyer's patches.

Evidence suggests that M cells, which are antigen-sampling epithelial cells that overlie the domes of Peyer's patches, have an apical plasma membrane that differs from that of absorptive cells. We examined the structural features of rat, mouse, and guinea pig M-cell apical membranes and compared them with those of dome and villus absorptive cell apical membranes using electron microscopy of thin sections and freeze-fracture replicas. We also determined the distribution of morphologically detectable cholesterol in M-cell plasma and intracellular membranes in dome epithelium exposed to the polyene antibiotic, filipin. The areal density of P-face intramembrane particles was significantly less on M-cell microvilli than on microvilli of dome absorptive cells and villus absorptive cells. Areal densities of E-face intramembrane particles were similar on M-cell and absorptive-cell microvilli. M-cell apical plasma membranes were rich in cholesterol, displaying numerous filipin-induced membrane lesions in thin sections and freeze-fracture replicas. In contrast, apical membrane endocytic pits and coated vesicles in M cells failed to show filipin-induced membrane lesions. These findings suggest that, compared with the apical membranes of absorptive cells, those of M cells have a low protein to lipid ratio and an abundance of morphologically detectable cholesterol except in domains involved in endocytosis. Additionally, a subpopulation of dome epithelial cells displayed distinctive tight junctions with high strand counts and unusual depth. Although the cell type associated with these modified tight junctions could not be identified with certainty, their interspecies frequency paralleled the interspecies frequency of M cells in dome epithelium. These expanded tight junctions may result from physical tension caused by epithelial shape changes induced by intraepithelial lymphoid trafficking or, alternatively, may help buttress M cells that have attenuated cytoplasmic processes due to the presence of central hollows.