can be seen at the boundary between the epithelium and lamina propria. In contrast, the lower right side of the micrograph displays numerous lymphocytes that have invaded the epithelium. More striking is the presence of what appear as isolated islands of epithelial cells (Ep) within the periphery. The thin band of collagen (C) lying at the interface of the epithelium is so disrupted in this area that it appears as small fragments. In effect, the small portion of the nodule seen in the right side of the micrograph has literally grown into the epithelium with the consequent disappearance of the well-defined epithelial-connective tissue boundary.
KEY BC, basal cells C, collagen CF, collagen fibers Ep, islands of epithelial cells GC, germinal center L, lymphatic tissue Ly, lymphocytes MG, mucous secreting glands N, nodule S, submucosa SE, surface epithelium SSE, stratified squamous epithelium TC, tonsilar crypts
PLATE 37 • LYM PH NODE I Lymph nodes are small, encapsulated lymphatic organs that are located in the path of the lymph ves-sels. They serve as filters of the lymph and as the principal site in which T and B lymphocytes undergo antigen-dependent proliferation and differentiation into effector lymphocytes (plasma cells and T cells) and memory B cells and T cells. A low-magnification (×14) 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 endothelium-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. MM CC H
Cortex of lymph node, human, H&E ×120.
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
Lymphatic nodule, lymph node, human, H&E ×400; inset ×640.