FIGURE 14.19 • Electron micrograph of a reticular cell. The body of a reticular cell and its processes (arrows) are evident. The arrangement of the reticular cells contains and isolates the collagen fibrils from exposure to the lymphocytes. Note the adjacent lymphocytes on the right. In the light microscope and using a silver staining method, these collagen fibrils are recognized as a reticular fiber. 12,600.
Lymphocytes in the superficial cortex are organized into nodules.
As elsewhere, the lymphatic nodules of the cortex are designated primary nodules if they consist chiefly of small lymphocytes and secondary nodules if they possess a germinal center. Lymphatic nodules are found in the outer part of the cortex, called the superficial (nodular) cortex (Plate 37, page 479). The portion of the cortex between the medulla and superficial cortex is free of nodules; it is called the deep cortex (paracortex). This region contains most of the T cells in the lymph node (Fig. 14.22a). Because of its dependence on the thymus, perinatal thymectomy in animals results in a poorly developed deep cortex. On the basis of this
FIGURE 14.20 • Diagram of a follicular dendritic cell. This cell, usually found in germinal centers, has multiple, thin, hairlike cytoplasmic processes that interdigitate between B lymphocytes. Antigen–antibody complexes adhere to the dendritic cytoplasmic processes by means of Fc receptors. Follicular dendritic cells are not antigen-presenting cells because they lack MHC II molecules.
observation, the deep cortex is also called the thymus-dependent cortex.
The medulla of the lymph node consists of the medullary cords and medullary sinuses.
The medulla, the inner part of the lymph node, consists of cords of lymphatic tissue separated by lymphatic sinuses called medullary sinuses. As described above, a network of reticular cells and fibers traverses the medullary cords and medullary sinuses and serves as the framework of the parenchyma. In addition to reticular cells, the medullary cords contain lymphocytes (mostly B lymphocytes), macrophages, dendritic, and plasma cells (Fig. 14.22b). The medullary sinuses converge near the hilum, where they drain into efferent lymphatic vessels.
Filtration of lymph in the lymph node occurs within a network of interconnected lymphatic channels called sinuses.
There are three types of lymphatic channels called sinuses in the lymph node. Just beneath the capsule of the lymph node is a sinus interposed between the capsule and the cortical lymphocytes called the subcapsular (cortical) sinus
FIGURE 14.21 • Schematic diagram of lymphocyte circulation within a lymph node. The green arrows indicate the circulation pathway of lymphocytes that enter the lymph node with the flow of lymph. Afferent lymphatic vessels carry lymph from the surrounding tissues and neighboring lymph nodes into the elaborate network of lymphatic sinuses. The wall of the sinuses allows lymph to percolate freely into the superficial and deep cortex, allowing lymphocytes to engage in immunosurveillance. The lymphocytes that enter the tissue next migrate back to the sinuses and leave the lymph node with the flow of the lymph. Lymphocytes that migrate to the lymph node from the blood (blue arrows) enter the deep cortex via high endothelial venules (HEVs) and also migrate to the superficial cortex. Here lymphocytes perform the same functions as lymphocytes that enter via lymphatic vessels. They also leave the lymph node by the efferent lymph vessels.