This higher-magnification micrograph of a lymphatic nodule from figure above illustrates the germinal center (GC) containing medium and large lymphocytes. Germinal centers also contain plasma cells. Dividing lymphocytes are shown at slightly higher magnification in the inset (arrows), which corresponds to the area in the circle in this figure. The inset also reveals nuclei of the reticular cells (RC) that form the connective tissue stroma throughout the organ. The and their lighter-staining germinal centers characterize the outer cortex, a more dense mass of lymphocytes, which impart a distinct basophilia, characterize the deep cortex. In contrast to these areas, the medulla is characterized by narrow strands of anastomosing lymphatic tissue containing numerous lymphocytes, the medullary cords (MC), separated by light-appearing areas known as the medullary sinuses (MS). The medullary sinuses receive lymph from the trabecular sinuses and lymph filtered through the cortical tissue.
ovoid reticular cell has a large pale-staining nucleus, and its cytoplasm forms long processes that surround the reticular fibers. In H&E preparations, the reticular fibers and the surrounding cytoplasm are difficult to identify. Reticular cells are best seen in the sinuses, where they extend across the lymphatic space and are relatively unobscured by other cells.
A unique vessel, the high endothelial venule (HEV), is found in relation to the lymphatic nodules, particularly in the deep cortex. These vessels have an endothelium composed of tall cells between which lymphocytes migrate from the vessel lumen into the parenchyma.
KEY C, cortex Cap, capsule CS, cortical or subcapsular sinus GC, germinal center H, hilum LN, lymphatic nodule M, medulla MC, medullary cords MS, medullary sinus HEV, high endothelial venule RC, reticular cells T, trabecula TS, trabecular sinus arrows, dividing lymphocytes
Immunocompetent B cells that have been exposed to an antigen that they can recognize and bind migrate to a lymph node, where they undergo activation and begin a series of mitotic divisions that produce large numbers of immature lymphocytes. They proliferate further in superficial cortex into a clone of lymphocytes that differentiate into antibody-secreting plasma cells and memory cells. B cell proliferation and differentiation take place in germinal centers in the superficial cortex of the lymph node. T cell activation and differentiation take place in the deep cortex. Newly differentiated plasma cells migrate to the medulla, where they release antibodies into the lymph leaving the node. They may also leave the node, enter the blood vascular system at the thoracic duct, and travel to localized sites in the connective tissue where they may continue to produce antibodies.
Deep cortex, lymph node, human, H&E ×365.
This micrograph shows the lymph node deep cortex. As noted in the previous plate, it lies below the region containing the lymph nodules and consists of closely packed lymphocytes. A number of blood vessels can be seen in this region. Whereas typical small blood vessels such as capillaries (Cap) and480 venules are present, the more unusual postcapillary venule called high endothelial venule (HEV) is also found in this region. A small vessel that can be identified as a venule (Ven), based on lumen size and wall thickness, is seen at a point of transition to become a high endothelial venule (arrowheads). The endothelial cell nuclei at this point of juncture have become
Hilar region, lymph node, human, H&E ×250.