It is controversial whether the membranous (M) cells of the Peyer's patches represent a separate cell line or develop from enterocytes under the influence of lymphocytes on the domes. To answer this question, the crypts that produce the dome epithelial cells were studied and the distribution of M cells over the domes was determined in mice. The Ulex europaeus agglutinin was 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 cellular composition and the structure of the dome-associated crypts and the dome epithelium. In addition, the sites of lymphocyte invasion into the dome epithelium were studied after removal of the epithelium using scanning electron microscopy. The domes of Peyer's patches were supplied with epithelial cells that derived from two types of crypt: specialized dome-associated crypts and ordinary crypts differing not only in shape, size, and cellular composition but also in the presence of M cell precursors. When epithelial cells derived from ordinary crypts entered the domes, they formed converging radial 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 derive from these specialized crypts differentiate into M cells. It is concluded that M cells represent a separate cell line that is induced in the dome-associated crypts by still unknown, probably diffusible lymphoid factors.

Membranous (M) cells are specialized epithelial cells of the Peyer's patches that sample antigens from the gut lumen, thereby enabling the host to respond immunologically. Recent studies suggest that this transport can be up-regulated within hours by de novo formation of M cells from enterocytes. To test this hypothesis, we used an in vivo model and induced the transcytosis of tracers in Peyer's patches by application of Streptococcus pneumoniae R36a into the gut lumen. Using cell-type-specific markers, we quantified M cells in the Peyer's patch domes, lymphocytes associated with M cells, and the transport rate for experimentally applied microbeads after 3 hours of exposure to R36a. The transport of latex microbeads was significantly increased by +131% in the R36a-treated patches as compared to buffer controls (P < 0.001). While in controls, each M cell was associated with 2.05 +/- 0.64 lymphocytes, a significant increase (+55.1%; P < 0.001) was determined in the R36a-treated patches. However, no statistical difference was detected in the percentage of M cells in the dome epithelia (46.0 +/- 4.6% versus 45.5 +/- 3.8%). It is concluded that bacteria-induced up-regulation of particle transport in Peyer's patch domes is due to an increased transport rate of the M cells, but not to a de novo formation of M cells. The data support the hypothesis that M cells represent a separate cell lineage that does not derive from enterocytes on the domes.

Generation of three-dimensional (3D) data sets from serial sections of tissues imaged by light microscopy (LM) allows identification of rare structures by morphology or fluorescent labeling. Here, we demonstrate a workflow for correlative LM and electron microscopy (EM) from 3D LM to 3D EM, using the same sectioned material for both methods consecutively. The new approach is easy to reproduce in routine EM laboratories and applicable to a wide range of organs and research questions.

Brush cells are specialised epithelial cells that are assumed to represent chemoreceptors of the digestive tract. They comprise a small population of the epithelial cells lining the intestine, possess a unique ultrastructure and, in many aspects, resemble the receptor cells of taste buds. To characterise glycoconjugates possibly involved in a sensory function, we investigated brush cells in the small intestine of three species using lectin histochemistry in confocal light and thin-section electron microscopy. Brush cells of rats were selectively labelled by the sialic acid-specific lectin Maackia amurensis agglutinin, those of guinea-pigs by the D-galactose-specific lectin Bandeiraea simplicifolia agglutinin, isolectin B 4 and those of mice by the L-fucose-specific lectin Ulex europaeus agglutinin lectin I. Lectin binding sites were consistently located in the glycocalyx of the apical membrane and in that of cytoplasmic vesicles. In vivo lectin labelling revealed that the glycoconjugates of the apical membrane are accessible under physiological conditions, that brush cells do not endocytose and that they probably possess a high membrane turnover rate. The results show that specialisations exist in the composition of glycoconjugates forming the glycocalyx of brush cells in all species investigated. The presence of brush cell-specific glycoconjugates would be in accordance with the current hypothesis of a receptive function of brush cells. Differences in the specific glycosylation patterns among rats, guinea-pigs and mice indicate that species-specific adaptations exist.