Three-dimensional (3D-) reconstruction of M cells in rabbit Peyer's patches: definition of the intraepithelial compartment of the follicle-associated epithelium

One of the major cell components of the rabbit follicle-associated epithelium is represented by the M cells. M cells are able to harbour variable amounts of immunocompetent cells inside peculiar invaginations of their basolateral cytoplasmic membrane, currently referred to as "pockets." This study provides a description of the exact spatial relationships between the M cells and the cells harboured in these so-called "pockets." Pieces of Peyer's patches, taken from the small intestine of an adult male rabbit, were treated as usual for conventional electron microscopy. Consecutive semithin and ultrathin sections were made through the entire thickness of the follicle-associated epithelium along planes parallel to the mucosal surface. Micrographs, taken from the ultrathin sections, were transposed into a software MacDraw Pro to obtain a computerized three-dimensional reconstruction. Three-dimensional reconstruction of the M cells showed that the "pockets" were not formed by mere invaginations of the cytoplasmic membrane, but that they resulted from the branching of the supranuclear portion of the M cell cytoplasm around the M cell-infiltrating lymphocytes. These intrusive cells could be found inside the "pockets" or lined up with one another, in vertical columns, bordering on the basal aspect of the M cells. The particular arrangement of the M cell apical cytoplasm created a labyrinth within the follicle-associated epithelium, which could be assumed as a real intraepithelial compartment expandable virtually throughout all the epithelium. The functional meaning of the intraepithelial compartment delimited by the M cells and its possible role is discussed.