Scanning electron microscopic observations of the immunodefensive systems with special reference to the surface morphology of the non-lymphoid 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.(ABSTRACT TRUNCATED AT 250 WORDS)