Changes

Lymph nodes are round, ovoid or bean-shaped organs found widely distributed in the body. Important groups of lymph nodes occur in chains along the course of blood vessels in such regions as the axilla, groin and mesentery. Lymph nodes are the only lymphatic organs to have an afferent lymphatic supply. They are interposed in the lymphatic drainage of numerous regions of the body. The lymph flowing through the nodes is filtered and phagocytic activity occurs on the particulate matter carried in the lymph. Lymph nodes are important centers for the production of lymphocytes, plasma cells and antibodies. Lymphocytes and antibodies are added to the lymph, which leaves the node by way of the efferent lymph vessels. Lymph nodes are fixed to surrounding connective tissue by loosely arranged fibers that continue into the capsule of the node.

Scan these sections with low power to observe the large number of small darkly staining cells. Most of these cells are lymphocytes. On slide 35, study the largest node first.

*Efferent lymph vessels collect the lymph from the medullary sinuses.

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Slide 93, Lymph node (H&E) in the hilus of the lung. These nodes help filter dust, carbon particles, and other debris that get into the lungs.

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In slide 35 note that most of the lymphatic nodules contain germinal centers containing small and medium-sized lymphocytes, macrophages, and lymphoblasts (few in number).

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Lymphoblasts (large lymphocytes) are more easily recognized on slide 63, although fewer germinal centers are present on this slide. To identify lymphoblasts, look in the germinal centers for relatively large cells that have basophilic cytoplasm (not very distinct on our slides) and a large mostly euchromatic nucleus with one or two prominent nucleoli.

Four slides of thymic tissue are to be studied. Slides 55 and 61 are from young individuals. Slide 60 is from a nineteen-year-old, and slide 59 is an involuted thymus from a fifty-year-old person. Since the thymus responds to illness by undergoing stress involution, it should be realized that some sections might not be optimal for studying all features. Slide 55 from a ten-day old child best shows the normal features for a young thymus.

Observe the thin connective tissue capsule surrounding this organ and the septa (trabeculae) which project inward from the capsule. These strands of connective tissue subdivide the thymus into lobules. Blood vessels are contained within some of the larger septa.

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Slide 61, Thymus, (H&E) is from a seven-year-old child. Identify as many features as possible. Observe how different this thymus appears from the section of the younger thymus previously studied on Slide 55. Slightly more connective tissue and fat are present in the seven-year-old thymus and the thymic corpuscles are larger.

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Study slide 60, Thymus (H&E) from a nineteen-year-old. Note the reduced number of lobules that are widely separated by a large increase in the amount of fat. Identify the thymic corpuscles. Observe that arterioles, venules, and larger blood vessels are absent from the cortex of the lobules. The thick-walled capillaries that form the blood-thymus barrier are not readily identifiable on our slides.

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Study slide 59, Thymus (H&E) from a fifty-year-old individual. Compare this involuted thymus with the sections of thymus previously studied.

=== Palatine Tonsil ===

Observe that “one side,” the surface, is covered by stratified squamous epithelium. From the surface a total of 10 to 20 deeply invaginated epithelial pockets called crypts extend into the substance of the organ. Note that the epithelium lining the crypts is continuous with the surface epithelium. Only a few crypts will be present on any one slide.

The nodules originate in the lamina propria but they may extend into the submucosa disrupting the muscularis mucosae. Where the nodules project to the luminal surface of the ileum, villi are absent.

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The spleen is an organ which both forms and destroys blood cells. In the embryo the spleen is a fully hemopoietic organ generating all the kinds of blood cells. In late fetal life the spleen stops producing erythrocytes and granulocytes, but it continues to generate lymphocytes and monocytes. However, it retains full hemopoietic potentialities so that pathologically, in adulthood, the spleen may once again initiate the functions of red bone marrow. The spleen is highly active in the production of antibodies. Plasma cells are the main source of the antibodies. The spleen also serves as a specialized blood-filtering organ and it can serve for the temporary storage of platelets and erythrocytes.

Examine slide 18, Spleen (H&E) and identify the capsule and trabeculae

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The pulp cords consist of the tissue occupying the narrow areas between the venous sinuses. With high power, study the splenic sinuses and pulp cords. Note how much easier it is to identify the venous sinuses by using the PASH stained section of Spleen (slide 19, Spleen PASH).