Reading: RR&K, Chapter 13
SLIDE 21c lymph node (Wilder)
(RR&K p357, Fig.1,2; p359,Fig. 1,2,3,4)
First, examine the section with the reversed ocular. This slide is used to emphasize general organization of the lymph node, which is clearly delineated by the reticular fibers. Identify structures from the outside in. The capsule is continuous with the trabeculae that project into the stroma of the lymph node. Locate the cortex and the medulla. In the cortex, note the area of lesser concentration of reticular fibers known as germinal centers. Note also, however, that the germinal center itself is surrounded by prominent fibers. In the medulla, identify the medullary cords and medullary sinuses. Reticular fibers form a dense network within the medullary cords, sinuses, and in the walls of the sinuses.
Check list for SLIDE 21c:
SLIDE 21b lymph node, dog (H&E)
(RR&K p357, Fig.1,2; p359,Fig. 1,2,3,4)
Study the sections with the reversed ocular and at low magnification. Locate the capsule, cortex, medulla, and hilus.
The capsule is formed by dense irregular connective tissue. Adipose tissue, blood vessels and afferent lymphatic vessels may be present outside the capsule. At high power, examine the tissue in the capsule and the trabeculae which are continuous with it. The subcapsular (marginal) sinus is located just deep to the capsule. These sinuses are continuous with the trabecular sinuses that run adjacent to the trabeculae and become the medullary sinuses. Arteries and veins entering the node via the hilus are found within the trabeculae.
In the cortex, identify germinal centers in the outer cortex. Germinal centers are easily identified by the presence of the light region. The cap (corona) may appear to surround the light region due to the plane of sectioning. Note densely packed, small lymphocytes in the corona. The light centralized region of the germinal centers contain dividing and dying (apoptotic) B lymphocytes, reticular cells (dendritic cells), and macrophages containing ingested material. Do not distinguish between large and medium lymphocytes, but be aware of the different size classes of these cells. A diffuse lymphoid tissue forms the inner or deep cortex (thymus-dependent zone). This zone as well as the corona is composed predominantly of T lymphocytes. Why is the deep cortex thymus dependent? Study diffuse lymphatic tissue in the deep cortex. Note the small blood vessels with prominent cuboidal-like endothelium. These vessels are referred to as the post-capillary venules or high endothelial venules (HEV). In the medulla, identify medullary cords and medullary sinuses. The medullary cords contain large, medium, and small lymphocytes as well as plasma cells, fixed macrophages, and large blood vessels. The medullary sinuses are located next to the cords and are lined by flat endothelial cells. Select a sinus in which the cells are sparse and easy to identify. Identify stellate reticular cells, macrophages, and lymphocytes. Finally, examine the hilus. Efferent lymphatics and their valves are well preserved on this slide. Compare the walls of the lymph vessels with those of the large blood vessels found in the neighborhood of the hilus.
Check list for SLIDE 21b:
(RR&K p357, Fig.1,2; p359,Fig. 1,2,3,4; p343, Fig. 13.8)
Study this section of the lymph node from the monkey. Following the directions from the previous slide, examine the capsule, cortex, and medulla. Within the cortex, identify the germinal centers which contain mitotic and apoptotic lymphocytes as well as macrophages which are responsible for removing apoptotic cells. Identify those macrophages and reticular cells present in the sinuses. Where would you find plasma cells, fibroblasts, and endothelial cells? You should be able to distinguish between the medullary cords and sinuses. Where would you find post-capillary venules, and what is their function? Finally, trace the flow of blood and lymph through the node.
Check list for SLIDE 21a:
SLIDE 20a palatine tonsil, human (H&E)
(RR&K p355, Fig.1,2)
With the reversed ocular, identify the following structures: germinal centers with light central regions of B lymphocytes and outer dark coronas of T lymphocytes; stratified squamous epithelium of oral mucosa appearing as a thin, pink band along one side of the tonsil; and dense connective tissue on the opposite side of the tonsil. At higher power, identify a crypt and observe that the stratified squamous epithelium is infiltrated with lymphoid cells. Why? What histologic features are different between the tonsil and the lymph node?
Check list for SLIDE 20a: 
SLIDE 21p lymph node, metastasis (H&E)
(RR&K p357, Fig.1,2; p359,Fig. 1,2,3,4)
Metastases are tumor implants that have become discontinuous with the primary site of the tumor. All malignant, (not benign), tumors produce cells that are capable of penetrating blood vessels, lymphatic vessels and body cavities, providing an opportunity for the malignancy to spread. The presence of metastasis is unequivocal evidence of malignancy because benign tumors do not metastasize. Since the walls of lymph vessels are very thin, metastasis into the lymph system is quite common. Study the sections with the reversed ocular. Lymph nodes with metastatic cells are very large. Orient yourself and locate the capsule, germinal centers, medullary cords, sinuses, and hilus. Note large masses of darkly stained cells inside the lymph node. These are metastatic cells originating from a squamous cell carcinoma. Despite their ability to divide abnormally, these cells still have some properties of squamous cells in that they organize into an epithelium and become keratinized. Much of the eosinophilic circular structures are keratinized cancer cells. In which part of the lymph node would you expect to first find tumor cells? Examine the cancer cells at 40X magnification; compare them to the normal lymphatic tissue cells. These larger cancer cells will be actively dividing (observe mitotic figures) and will have abnormal nuclei. Note the presence of numerous plasma cells near the cancer cells. Other than plasma cells, what other lymphoid cell types would you expect to be see in close association with the cancer cells?
Check list for SLIDE 21p:
The lymphatic system is involved in removing excess fluid from the interstitial compartment, as well as monitoring for the presence of antigens in the body. This unit examines some lymphatic organs and their structure.