(Plate 38, page 481). Afferent lymphatic vessels drain lymph into this sinus. Trabecular sinuses that originate from the subcapsular sinuses extend through the cortex along the trabeculae and drain into medullary sinuses. Lymphocytes and macrophages or their processes readily pass back and forth between the lymphatic sinuses and the parenchyma of the node. The sinuses have a lining of endothelium that is continuous where it is directly adjacent to the connective tissue of the capsule or trabeculae but discontinuous where it faces the lymphatic parenchyma. Although a macrophage may reside in the lymphatic parenchyma, it often sends pseudopods (long cytoplasmic processes) into the sinus through these endothelial discontinuities. These pseu
FIGURE 14.22 • Distribution of T and B lymphocytes in the superficial cortex of the lymph node. a. Distribution of T lymphocytes in the lymph node of a marmoset monkey was visualized using an immunocytochemical method employing antibodies against CD3 protein, a specific marker for T lymphocytes. Tissue sections were initially treated with primary rabbit antihuman antibodies against a CD3 marker and later exposed to biotinylated secondary swine antirabbit antibodies. After incubation with avidin-biotin-peroxidase complex, the positive response was then visualized with diaminobenzidine (DAB) solution (brown-colored reaction). Cell nuclei were counterstained with hematoxylin. Note that the majority of T cells are distributed within the deep cortex (DP), a small number of T cells is present in the superficial cortex (SC), mainly around germinal centers (GC). b. Using the same immunoperoxidase DAB reaction described above, B cells were localized using primary monoclonal antibodies against human CD20 protein (specific marker for B lymphocytes). Subsequently, secondary rabbit antimouse antibodies were used to visualize location of B cells, which the accumulations are found within germinal centers (GC) of the superficial cortex (SC). Cap, capsule. 200. (Courtesy of Dr. Douglas F. Paulsen.) dopods monitor the lymph as it percolates through the sinus.
Lymphatic sinuses are not open spaces, as are blood sinuses. Particularly in the medulla, macrophage processes, along with the reticular fbers surrounded by reticular cell processes, span the lumen of the sinus and form a crisscrossing meshwork that retards the free fow of lymph and enhances its fltration. Antigenic material and transformed cells of metastatic cancer are trapped by this mechanical flter and then phagocytosed by macrophages. In metastatic cancer, the system can be overwhelmed by an excessive number of cancer cells fowing through the lymphatic sinuses; as a result, the cells may establish a new metastatic site in the lymph node.
Specialized high endothelial venules (HEVs) are the site of fluid absorption and the entrance for circulating lymphocytes into the lymph node.