The follicle-associated epithelium (FAE) in the rabbit caecal lymphoid patch is characterized by the presence of membranous (M) cells, which are believed to be functionally equivalent to those present at other sites of gut-associated lymphoid tissue (GALT). Caecal patch M cells display distinctive features compared with those of other GALT sites, despite similar general morphology and expression of the M cell marker vimentin, suggesting marked heterogeneity in the apical surface of M cells at discrete GALT sites. Electron microscopy reveals that rabbit caecal patch M cells differ from those in the small intestinal Peyer's patch FAE: the former have a prominent aspect within the epithelium and possess microvilli which are longer than those of adjacent enterocytes. Many of the M cells in peripheral regions of the caecal patch FAE are not associated with leucocytes and may thus represent an immature M cell population. The M cells are also histochemically distinct from adjacent enterocytes and from Peyer's patch M cells, showing greater expression of brush-border alkaline phosphatase activity and affinity for certain lectins (peanut and wheat germ agglutinins, Bandeiraea simplicifolia agglutinin II). The differences in the brush-border morphology and glycocalyx structure between M cells at different GALT sites may affect their function at these sites by influencing the interaction of luminal antigens and microorganisms with the M cell surface. The present data also support the hypothesis that M cells arise directly from differentiation of crypt stem cells and not from the transformation of existing fully differentiated enterocytes.

M-cell surface glycoconjugate expression was investigated by applying a panel of lectins to whole fixed mouse Peyer's and caecal patches. While the majority of lectins failed to identify mouse M-cells, the lectin Euonymus europaeus differentially stained the surface of M-cells in both mouse Peyer's and caecal patches, and the lectins Ulex europaeus II and Bandeiraea simplicifolia I isolectin B4 identified M-cells in the Peyer's and caecal patch follicle associated epithelium, respectively. These three mouse M-cell markers failed to identify rat and rabbit Peyer's patch M-cells, although both Euonymus europaeus and Ulex europaeus II differentially stained M-cells in the periphery of rabbit caecal patch domes. These site and species related variations in M-cell surface glycoconjugate expression may reflect the local microorganism populations and will have important implications if orally delivered vaccines and drugs are to be targeted to M-cells via their surface glycoconjugates.