Peyer's patches: Morphologic studies

Abstract

The aggregates of lymphoid tissue associated with the intestine represent a large fraction of the total lymphoid tissue of rabbits (1) and presumably have an important immunological function. The importance of intestinal plasma cells in the synthesis of IgA is established, but the IgA synthesizing cells seem to be diffusely scattered through the lamina propria rather than existing in the follicle-like aggregates (2). It has been suggested that the aggregates are responsible for the local production of IgG and IgM antibodies against the bacterial and food antigen present in the gut, but there has been some difficulty in experimentally substantiating this concept (3, 4). Some investigators have proposed that the gut-associated lymphoid tissues have a central function in the ontogeny of the immune response. The lymphoid system of birds is considered to consist of two components. These are ( a) thymic-dependent lymphocytes, and ( b) a system of plasma cells and germinal centers dependent on the bursa of Fabricius (5). Cells under thymic control are thought to be responsible for cellular immunity (6), and bursal oriented tissues for immunoglobulin synthesis (7, 8). Lymphoepithelial differentiation in both the thymus and the bursa of Fabricius are believed to be governed by mesenchymal induction (9, 10), though this has been questioned (11). Morphologic and phylogenetic studies have suggested that the rabbit equivalent of the avian bursa is a lymphoepithelial structure in the intestinal tract (12–14). The development of the appendix and Peyer's patches are not dependent on the presence of the thymus (15, 16). Surgical removal of gut-associated lymphoid centers in rabbits has resulted in a condition resembling bursectomy and sublethal irradiation in birds (17, 18) suggesting that the Peyer's patch may be the rabbit equivalent of the avian bursa. The division of lymphoid organs into “central” or “peripheral” categories is generally based on a number of nonimmunologic criteria among which are morphology, the stage of development at birth, and the patterns of cellular repopulation. On immunologic grounds, central organs are considered to be essential in the genesis of immune reactivity, but not to participate directly in the response. There is a paucity of information on the morphology of Peyer's patches, and the evidence for their direct participation in the immune response is conflicting. The investigations reported in this paper were undertaken to provide information on the structure and ultrastructure, the histochemistry, and the cellular kinetics of rabbit Peyer's patches. We have included tissues from young and mature rabbits and rabbits which had received repeated injections of antigens in the hope of disclosing differences among the groups which might shed some light on the function of the organs. In view of the specialized role proposed for the overlying epithelium, we were particularly interested in characteristics which distinguish it from the general gut epithelium. The electron microscope has also permitted a closer inspection of the anatomical relationships between the cells.