'M' cells in the follicle-associated epithelium of the human colon.
The morphology of the epithelium overlying human colonic mucosal lymphoid follicles has been studied by electron microscopy. Epithelial crypts stop at the edge of the follicle and the centre is covered by a single layer of enterocytes. Numerous lymphocytes and, occasionally, other mononuclear cells, are associated with these enterocytes. The lymphocytes are situated in the intercellular spaces, invaginate the lateral cell wall and ultimately appear to be completely surrounded by the enterocyte. When the lymphocyte is near the centre of the enterocyte there is usually little change in enterocyte morphology, but when the lymphocyte is near the apical surface of the enterocyte, the latter closely resembles the membranous epithelial cells ('M' cells) of human small intestine. It is probable that, as with the 'M' cells in the small intestine, these colonic 'M' cells have an antigen sampling function. Their presence may explain the localization of the early lesions of some colonic inflammatory diseases, including Crohn's disease. They may also provide a portal of entry for some rectally-transmitted viral infections.