SCANNING ELECTRON-MICROSCOPIC OBSERVATIONS OF THE IMMUNODEFENSIVE SYSTEMS WITH SPECIAL REFERENCE TO THE SURFACE-MORPHOLOGY OF THE NONLYMPHOID CELLS

This paper reviews scanning electron microscopic observations of cellular elements forming various lymphoid organs. The reticular cells in the secondary lymphoid organs are stellate, smooth-surfaced forms extending slender processes to comprise a three-dimensional network. The reticular fibers are usually covered by reticular cell processes, though they are naked in certain regions. Other types of reticular cells are observed in certain places: the "retothelial" type in the lymphatic sinus of the lymph nodes, and the "follicular dendritic" type in the germinal center of various lymphoid organs. The thymic epithelial cells are divided into two main types: stellate cells which form a three-dimensional meshwork throughout the thymus parenchyma; and large vacuolated cells located in the medulla. A continuous single layer consisting of the processes of the stellate epithelial cells separates the parenchyma from the connective tissues of the capsule, septa and vessels. The M cells in the epithelium of the gut-associated lymphoid tissues (GALTs) are cells with numerous irregular microprojections on the luminal surface. They often attach microorganisms to the luminal surface, reflecting their functions of antigen transport into the underlying lymphoid tissue. Lymphocytes of various shapes often cluster in the intercellular spaces under the M cells, a phenomenon believed to indicate direct stimulation of lymphocytes by certain transported substances. Macrophages are amoeboid cells independent of and unable to transform into reticular and endothelial cells, at variance with prerequisites of the reticulo-endothelial system concept. Multiple features of macrophages probably reflect the presence of the subpopulations as well as the phases of their activity. The interdigitating cells (IDCs) are clearly distinguished by macrophages since they possess characteristic knob-like cytoplasmic processes gearing with each other. The IDCs often embrace lymphocytes, suggesting that the former cells may either nurse or deliver certain immunological information to the latter. The endothelial cells in the postcapillary venules (high endothelial venules) in the secondary lymphoid organs (except the spleen) are dome-shaped and provide a special route for the circulating lymphocytes entering into the lymphatic parenchyma. The postcapillary venules in the thymus are composed of flattened epithelial cells and provide a specific site for lymphocytes "educated" in the thymus to pass into the general circulation.