The development of M cells in Peyer's patches is restricted to specializeddome-associated crypts

It is controversial whether the membranous (M) cells of the Peyer's patches represent a separate cell Line or develop from enterocytes under the influ ence of lymphocytes on the domes. To answer this question, the crypts that produce the dome epithelial cells were studied and the distribution of M ce lls over the domes was determined in mice. The Ulex europaeus agglutinin wa s used to detect M cells in mouse Peyer's patches. Confocal microscopy with lectin-gold labeling on ultrathin sections, scanning electron microscopy, and laminin immuno-histochemistry were combined to characterize the cellula r composition and the structure of the dome-associated crypts and the dome epithelium. In addition, the sites of lymphocyte invasion into the dome epi thelium were studied after removal of the epithelium using scanning electro n microscopy, The domes of Peyer's patches were supplied with epithelial ce lls that derived from two types of crypt: specialized dome-associated crypt s and ordinary crypts differing not only in shape, size, and cellular compo sition but also in the presence of M cell, precursors. When epithelial cell s derived from ordinary crypts entered the domes, they formed converging ra dial strips devoid of M cells. In contrast to the M cells, the sites where lymphocytes invaded the dome epithelium were not arranged in radial strips, but randomly distributed over the domes. M cell development is restricted to specialized dome-associated crypts. Only dome epithelial cells that deri ve from these specialized crypts differentiate into M cells. It is conclude d that M cells represent a separate cell line that is induced in the dome-a ssociated crypts by still unknown, probably diffusible lymphoid factors.