EPITHELIAL M-CELLS IN THE RABBIT CECAL LYMPHOID PATCH DISPLAY DISTINCTIVE SURFACE CHARACTERISTICS

The follicle-associated epithelium (FAE) in the rabbit caecal lymphoid patch is characterised 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 d isplay distinctive features compared with those of other GALT sites, d espite similar general morphology and expression of the M cell marker vimentin, suggesting marked heterogeneity in the apical surface of M c ells at discrete GALT sites. Electron microscopy reveals that rabbit c aecal patch M cells differ from those in the small intestinal Peyer's patch FAE: the former have a prominent aspect within the epithelium an d possess microvilli which are longer than those of adjacent enterocyt es. 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 e xpression of brush-border alkaline phosphatase activity and affinity f or certain lectins (peanut and wheat germ agglutinins, Bandeiraea simp licifolia agglutinin II), The differences in the brush-border morpholo gy and glycocalyx structure between M cells at different GALT sites ma y affect their function at these sites by influencing the interaction of luminal antigens and microorganisms with the M cell surface. The pr esent data also support the hypothesis that M cells arise directly fro m differentiation of crypt stem cells and not from the transformation of existing fully differentiated enterocytes.