Morphological study of antigen-sampling structures in the rat large intestine.

Organized lymphoid tissue in the rat colon exists as clusters (colonic lymphoid patches) of intramucosal and submucosal follicles in the proximal, mid, and distal colon, interspersed by solitary follicles. The follicular lymphoid cells of colonic lymphoid patches are separated from the gut lumen by a highly specialized lymphoepithelium which lacks mature goblet cells. Cells of this epithelium are of two types: those characterized by an electron-dense cytoplasm, large numbers of apical vesicles and lysosomes, and prolonged extensions of the apical cytoplasm forming thin partitions between the gut lumen and underlying intercellular spaces; and cells with a less electron-dense cytoplasm, distorted mitochondria, and little endoplasmic reticulum. Both cell types bear normal microvilli and have numerous lateral membrane processes which penetrate large intercellular spaces. A ferritin-India ink label infused into the colonic lumen was preferentially adsorbed onto the surface of this follicle-associated epithelium. Indigenous colonic bacteria were observed penetrating the superficial cytoplasm of the electron-dense cells where they were enclosed in lysosomes and digested. An antigen-sampling role is proposed for the colonic lymphoid patch epithelium.