Ultrastructure of M cells in the conjunctival epithelium of the guinea pig

The part of the guinea pig conjunctival epithelium overlying a lymphoid follicle contains specialized M (membranous) cells. The M cells are located in the superficial epithelial layer and are interposed between the epithelial surface and invading intraepithelial lymphocytes. The morphology of the M cells is characterized by irregular microvilli on the epithelial surface and numerous vesicles and vacuoles concentrated in the apical cytoplasm opposite the lymphocytes. These morphological features indicate pronounced functions of uptake and transport of material from the epithelial surface to closely apposed lymphocytes. The present morphological findings are in accordance with previous reports stressing the function of M cells as rapid carriers of antigenic information from the epithelial surface to intraepithelial lymphocytes.