"M" cell distribution in follicle-associated epithelium of mouse Peyer's patch

The distribution of "M" cells within the follicle-associated epithelium (FAE) of mouse Peyer's patches has been estimated from a series of SEM micrographs. "M" cells are found generally to be distributed evenly throughout the higher regions of the FAE. The frequency with which "M" cells occur decreases in the lower regions of the FAE. No "M" cells are found in the follicle-associated crypts. "M" cell microvilli are longer than those of enterocytes found at the base of the follicle-associated crypts and shorter than those found on enterocytes in other parts of the FAE. No lymphoid cells occur within the epithelium of the follicle-associated crypt. Lymphoid cells in the higher regions of the FAE are associated closely with "M" cells. There is, in the lower regions of the FAE, a transitional zone where lymphoid cells occur both free within the epithelium and closely associated with "M" cells. It is suggested that an initial close association of lymphoid cells with young, fully differentiated enterocytes is responsible for the latter's subsequent transformation to "M" cells.