The fine structure of lymphoid tissue

Abstract

The fine structure of the lymphoid tissue is described. Six cell types can be identified and described with reasonable accuracy: reticular cells, macrophages, littoral cells, stem cells, lymphocytes, and plasma cells. The reticular cell resembles the fibroblast in having a pale elongated nucleus, branching cytoplasmic processes, a varying amount of ergastoplasm, and often an organelle-free portion at the cell border (ectoplasm), with delicate filaments. Marcophages may resemble reticular cells in shape or may be more rounded. The pale nucleus is elongated or kidney-shaped and contains a small nucleolus. The nucleus is often eccentric. In the cytoplasm there may be varying amounts of phagocytosed material and lysosomes. There are usually many smooth-surfaced vesicles, and the ergastoplasm is often confined to a more or less circumscribed area. Characteristic are the numerous fingerlike processes at the cell border, which may interdigitate with those of neighboring cells. Littoral cells resemble reticular cells and ordinary vascular endothelial cells. They show little or no phagocytosis under normal conditions, but may pick up certain substances, e.g., carbon, just like ordinary macrophages. The lymphocyte nucleus is more or less rounded, most dense in the small lymphocyte, and least densc in the lymphoblast. Lymphoblasts and medium lymphocytes have nucleoli. The nucleo-cytoplasmic ration of small lymphocytes is the greatest of any cell. The cytoplasm is pale and contains a moderatc number of free ribonucleoprotein granules (ribosomes), well-formed mitochondria, and a small Golgi region. In addition, particularly in the medium lymphocytes, the cytoplasm has bodies which probably correspond to the Gall bodies of light microscopy. Plasma cells are characterized by a well-developed ergastoplasm that fills the cytoplasm to its margins, a large and complex Golgi apparatus, fingerlike cytoplasmic processes, and a densc eccentric nucleus. Immature plasma cells have a prominent nucleolus. The variants of the reticular cell are discussed, as is the evidence for and against the existence of a primitive multipotent reticular cell. A cell referred to as a “stem cell” is described. It is found occasionally in normal lymphoid tissue but frequently soon after antigenic stimulation. Its significance and the origin of lymphocytes and plasma cells are discussed. Extracellular elements in the spleen consist of reticular fibers and basement membranes. The latter support the littoral cells of the sinusoid. The reticular fibers of light microscopy are made up of delicate collagen fibrils, an occasional elastic fiber, and an interfibrillar ground substance.