LAB NINE: LYMPHATIC SYSTEM

A. Lymph Nodules (esophagus, slide 1) Lymph nodules are unencapsulated lymphoid tissue located in several organs of the digestive and respiratory tracts, and all organs of the lymphatic system. They are composer of densely packed lymphocytes (mainly B-lymphocytes) that differentiate into plasma cells after appropriate antigenic stimulation. Secondary lymph nodules exhibit a pale staining germinal center, indicating that an antibody response to an antigen is occurring. Primary lymph nodules have not been activated, therefore, lack germinal centers. Peyer's patches are aggregates of unencapsulated nodules found in the lamina propria of the ileum.

B. Lymph Node (lymph node, slide 73) Lymph nodes are spherical or kidney-shaped organs that are distributed throughout the body. The node is enclosed by a connective tissue capsule from which large connective tissue trabeculae arise and subdivide the underlying parenchyma. Each lymph node is divided into an outer cortex, where most of the lymphocytes are located and the immune response is initiated, and an inner medulla, characterized by medullary cords of cells that are separated by medullary sinuses containing lymph, reticular cells, and macrophages. Internal support is provided by an elaborate network of reticular fibers. Lymph nodes are important in the body's defense against microorganisms and the spread of tumor cells. They act to filter all tissue fluid-derived lymph. Afferent lymphatic vessels cross the capsule of each node and pour lymph into the subscapular and medullary sinuses. After lymph has flowed through the cortex and medulla, it is collected by efferent lymphatic vessels located at the hilus of each node.

C. Spleen (slide 13) The spleen is the largest accumulation of lymphoid tissues in the body. It is surrounded by a capsule of dense connective tissue that sends out trabeculae, which penetrate and divide the underlying parenchyma, or splenic pulp. White pulp (stained red) is characterized by the presence of lymph nodules and central arteries (CA), or white pulp arteries. The lighter staining areas surrounding the white pulp constitutes the red pulp (the designation "white" and "red" pulp refers to the color of tissues in the fresh state). The red pulp is characterized by the presence splenic cords (Billroth's cords), that lie between sinusoids. The spleen, like other lymphoid organs, has an elaborate, supporting network of reticular fibers.

D. Thymus (slide 40) The thymus is surrounded by a connective tissue capsule that penetrates the parenchyma and divides it into lobules. Each lobule has a peripheral dark zone known as the cortex and a central light zone called the medulla. Because the cortex has a higher abundance of lymphocytes it stains a darker color. The medulla contains Hassall's corpuscles (HC), which are characteristic of this region. These structures are concentrically arranged, epithelial reticular cells (these cells also participate in the cortical blood-thymus barrier). The thymus attains its peak development during youth and after puberty the lymphatic parenchyma is slowly replaced by fat. T-cells exit the thymus during childhood. Later in life, the thymus is no longer important because the previously emigrated T-cells no longer need thymic augmentation.