3-DIMENSIONAL (3D-) RECONSTRUCTION OF M-CELLS IN RABBIT PEYERS-PATCHES - DEFINITION OF THE INTRAEPITHELIAL COMPARTMENT OF THE FOLLICLE-ASSOCIATED EPITHELIUM

One of the major cell components of the rabbit follicle-associated epi thelium is represented by the M cells. M cells are able to harbour var iable amounts of immunocompetent cells inside peculiar invaginations o f their basolateral cytoplasmic membrane, currently referred to as ''p ockets.'' This study provides a description of the exact spatial relat ionships between the M cells and the cells harboured in these so-calle d ''pockets.'' Pieces of Peyer's patches, taken from the small intesti ne of an adult male rabbit, were treated as usual for conventional ele ctron microscopy. Consecutive semithin and ultrathin sections were mad e through the entire thickness of the follicle-associated epithelium a long planes parallel to the mucosal surface. Micrographs, taken from t he ultrathin sections, were transposed into a software MacDraw Pro to obtain a computerized three-dimensional reconstruction. Three-dimensio nal reconstruction of the M cells showed that the ''pockets'' were not formed by mere invaginations of the cytoplasmic membrane, but that th ey resulted from the branching of the supranuclear portion of the M ce ll cytoplasm around the M cell-infiltrating lymphocytes. These intrusi ve cells could be found inside the ''pockets'' or lined up with one an other, in vertical columns, bordering on the basal aspect of the M cel ls. 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 thr oughout all the epithelium. The functional meaning of the intraepithel ial compartment delimited by the M cells and its possible role is disc ussed. In particular, the definition of a new intraepithelial compartm ent of the follicle-associated epithelium, in order to differentiate i t from the more generic intraepithelial compartment of the intestinal epithelium, is proposed. (C) 1995 Wiley-Liss, Inc.