FIGURE 14.14 • Photomicrograph of a lymph node. This photomicrograph shows the superficial cortex (SC), deep cortex (DC), and medulla (M) of the lymph node in a routine H&E preparation. The capsule (Cap) is composed of dense connective tissue from which trabeculae (T) penetrate into the organ. Below the capsule is the subcapsular sinus (SCS). It receives lymph from afferent lymphatic vessels that penetrate the capsule. The subcapsular sinus is continuous with the trabecular sinuses that course along the trabeculae. The superficial cortex contains the lymphatic nodules (LN). The deep cortex is nodule free. It consists of densely packed lymphocytes and contains the unique high endothelial venules (not visible at this magnification). The medulla consists of narrow strands of anastomosing lymphatic tissue called medullary cords (MC), which are separated by light-appearing spaces, the medullary sinuses (MS). The medullary sinuses receive lymph from the trabecular sinuses as well as lymph that has filtered through the cortical tissue. 140.
FIGURE 14.15 • Photomicrograph of a palatine tonsil. a. This low-magnification photomicrograph shows an H&E–stained palatine tonsil. The stratified squamous epithelium that forms the surface of the tonsil dips into the underlying connective tissue in numerous places, forming tonsillar crypts. 25. b. This higher-magnification photomicrograph of the rectangular area in a shows the stratified squamous epithelium (SSE) lining the tonsillar crypt. In the portion of the photomicrograph below the lumen of the crypt, SSE is well defined and separated by a connective tissue layer (CT) from the lymphatic nodule (LN). In the upper portion of the photomicrograph, the SSE is just barely recognized because of the heavy infiltration of lymphocytes; the epithelial cells are present, however, although they are difficult to identify. In effect, the lymphatic nodule has literally grown into the epithelium, distorting it and resulting in the disappearance of the more typical, well-defined epithelial–connective tissue boundary. 450.
With age, the amount of lymphatic tissue within the organ regresses and is diffcult to recognize.
As noted, diffuse lymphatic tissue and lymphatic nodules are named according to the region or organ in which they appear. In the alimentary canal, they are collectively referred to as gut-associated lymphatic tissue (GALT); in the bronchial tree, they are known as bronchus-associated lymphatic tissue (BALT). The term mucosa-associated lymphatic tissue (MALT) includes GALT and BALT. Diffuse lymphatic tissue and lymphatic nodules of MALT are present in many other regions of the body (e.g., female reproductive tract) where the mucosa is exposed to the external environment. All lymphatic nodules become enlarged as a consequence of encounters with antigen.
Lymph nodes are small encapsulated organs located along the pathway of lymphatic vessels.
Lymph nodes are small, bean-shaped, encapsulated lymphatic organs. They range in size from about 1 mm (barely visible with the unaided eye) to about 1 to 2 cm in their longest dimension. Lymph nodes are interposed along lymphatic vessels (Fig. 14.17) and serve as filters through which lymph percolates on its way to the blood vascular system. Although widely distributed throughout the body, they are concentrated in certain regions such as the axilla, groin, and mesenteries.