M cells in the rabbit palatine tonsil: the distribution, spatial arrangement and membrane subdomains as defined by confocal lectin histochemistry.

The crypt epithelium of the palatine tonsil contains M cells that play an important role in the uptake of luminal antigens to initiate immune responses. To study the close interaction of M cells, squamous epithelial cells and lymphocytes we used confocal laser scanning microscopy and the lectin from Ulex europaeus (UEA-I), which selectively labels rabbit tonsillar M cells. Confocal serial sections and synthetic section planes showed that the M cells comprise up to 35% of the epithelial cells in the tonsil crypt and completely engulf clusters of two to eight lymphocytes with their apical cytoplasm. These lymphocytes lie in a pocket of the M cell's basolateral membrane that invaginates from one of the lateral aspects and forms a tunnel-like opening. Therefore, the tonsillar M cells closely resemble the M cells of the small and large intestines in their spatial structure, and likewise maintain an intraepithelial compartment for the interaction of lymphocytes, macrophages and antigens. The UEA-I bound intensely to the apical membrane of the M cells and to transcytotic vesicles in the apical cytoplasm. The pocket membrane bound the UEA-I more avidly than the remaining basolateral membrane, suggesting that the basolateral membrane of M cells is subdivided into membrane domains with different compositions of glycoconjugates.