Plate 33 Lymph Node I

Lymph nodes are small, encapsulated lymphatic organs that are located in the path of the lymph vessels. They serve as filters of the lymph and as the principal site in which T and B lymphocytes undergo antigen-depenclent proliferation and differentiation into effector lymphocytes (plasma cells and T cells) and memory B cells and T cells. A low-magnification (X14) micrograph of a section through a human lymph node is shown on this page for orientation. The capsule appears as a thin connective tissue covering.

The parenchyma of the node is composed of a mass of lymphatic tissue, arranged as a cortex (C) that surrounds a less dense area, the medulla (M). The cortex is interrupted at the hilum of the organ (H), where there is a recognizable concavity. It is at this site that blood vessels enter and leave the lymph node; the efferent lymphatic vessels also leave the node at the hilum.

Afferent lymphatic vessels penetrate the capsule at multiple sites to empty into an en-dothelium-lined space, the cortical or subcapsular sinus. This sinus drains into the trabecular sinuses that extend through the cortex alongside the trabeculae and then supply the medullary sinuses. These, in turn, drain to the efferent lymphatics that leave the node at the hilum.

Figure 1, lymph node, human, H&E x120.

An area from the cortex is shown here at higher magnification. The capsule (Cap) is composed of dense connective tissue from which trabeculae (T) penetrate into the organ. Immediately below the capsule is the cortical or subcapsular sinus (CS), which receives lymph from the afferent lymphatic vessels after they penetrate the capsule. The cortical sinus is continuous with the trabecular sinuses (TS) that course along the trabeculae.

The cortex contains the lymphatic nodules (LN) and a deeper component that lacks nodules, known as the deep cortex. Whereas lymph nodules 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.

Figure 2, lymph node, human, H&E x400; inset x640.

This higher-magnification micrograph of a lymphatic nodule from Figure l 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 Figure 2. The inset also reveals nuclei of the reticular cells (RC) that form the connective tissue stroma throughout the organ. The ovoid reticular cell has a large pale-staining nu cleus, 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 unob-scured by other cells.

A unique vessel, the postcapillary venule (PCV), 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.

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