ULTRASTRUCTURE AND PROTEIN-TRANSPORT OF M-CELLS IN THE RABBIT CECAL PATCH

Background: The gut-associated lymphoid tissue of the rabbit cecum includes a single lymphoid patch located close to the ileocecal orifice. Vimentin immunoreactivity, which can now be regarded as a marker for M cells in rabbits, has identified a subpopulation of epithelial cells as M cells in the domes of this patch, The aim of the present study was to demonstrate that these M cells are capable of antigen transport and to characterize their ultrastructure. Methods: M cells of the rabbit cecal lymphoid patch were studied by scanning, thin section, and freeze-fracture electron microscopy, The transcytosis across these M cells was investigated using horseradish peroxidase as a soluble tracer protein. Results: The M cells were concentrated at the flanks of the domes and had long, thick, branched microvilli, a well-developed terminal web, and a deep invagination of their apical membrane, Numerous small vesicles lay beneath the terminal web in close vicinity to the base of the invagination. These vesicles transported the luminally applied horseradish peroxidase through the M cells, In contrast to adjacent enterocytes, the glycocalyx of M cells was thin, stub-like, and had very few glycocalyceal bodies. Bacteria adhered to the surface of M cells and were also found in the apical invagination. Conclusions: The M cells of the rabbit cecal lymphoid patch differ from those of Peyer's patches of the small intestine in their ultrastructure and route of antigen transport, These differences might be related to the situations resulting from differences in the microbial populations at these locations. (C) Wiley-Liss, Inc.