Reading: RR&K, Chapter 13
SLIDE 23c thymus, fetus (H&E)
(RR&K p369, Fig.1,2,3;)
A dense connective tissue capsule and septa divide the thymus into lobules. The thymus proper has a basophilic cortex and lighter stained medulla. At higher power, examine the heterogenous population of lymphocytes of differing sizes. Small lymphocytes have small nuclei with little cytoplasm that is not always visible. The medium and large lymphocytes have larger, round nuclei with a cytoplasm that is usually visible as a thin rim around the nucleus. The epithelioreticular cells forms the structural cellular support for the thymus. These cells adhere to one another and have a a free surface typical of epithelial cells. Can you identify their a free surface? Despite their name, these cells do not synthesize reticular fibers. They posses light staining nuclei, often ovoid, sometimes very large, and have cytoplasmic processes that may be visible. Observe the close arrangement of the endothelial cells with epithelioreticular cells of the thymus in the cortex. The close arrangement of these cells forms the blood thymus barrier which prevents foreign antigens from entering the cortex. Why is this barrier important? Within the lightly stained medulla, note the Hassall's corpuscles, which are usually round eosinophilic structures of various sizes. They may contain epithelioreticular cells in a concentric arrangement and some cellular debris. Try to identify blood vessels, large, medium and small lymphocytes, and epithelioreticular cells within the medulla of the thymus. Because of the loose packing of the lymphocytes, the epithelioreticular cells are fairly easy to identify.
Check list for SLIDE 23c:
SLIDE 23a thymus, adult (H&E)
(RR&K p369, Fig.1,2,3;)
First, observe the structure of the thymus at low power. Identify the dense connective tissue capsule and septa which divide the thymus into lobules. Locate the cortex and the medulla within each lobule. Within the cortex, identify the lymphocytes and epithelioreticular cells. Observe the structure of the blood thymus barrier. Within the medulla, identify Hassall's corpuscles, which are more evident in the adult than in the fetus (SLIDE 23c). Identify the lymphocytes and epithelioreticular cells. Locate the blood vessels in the septa and medulla and trace the path of lymphocyte movement through the thymus.
Check list for SLIDE 23a: 
SLIDE 22a spleen, monkey (H&E)
(RR&K p365, Fig.1,2,3; p367 Fig. 1,2,3)
With the reversed ocular, identify the capsule, red pulp (stained pink) and the white pulp (stained blue).
The capsule and trabeculae are made up of dense connective tissue. Observe myofibroblasts scattered between fibroblasts inside the capsule and within the connective tissue of trabeculae. Within the trabeculae identify trabecular arteries, that arise from the splenic artery and bring the blood to the red pulp. Accompanied trabecular veins drain blood from splenic sinusoids and converge into splenic vein.
The staining of the red pulp is due to the abundance of red blood cells. In the red pulp identify sinusoids and splenic cords. Some sinusoids contain blood, while others are empty. The sinusoidal lining cells appear in cross sectioned sinusoids as densely packed cuboidal cells. In a tangential section through the wall of a sinusoid, the long narrow shape of the lining cells can be observed. Many white blood cells are fixed while in the process of moving across the sinusoidal wall. Can you find any? The splenic cords appear as loose tissue with many red blood cells, reticular cells (large moderately dense staining nuclei), lymphocytes, some eosinophils, many neutrophils (these may reflect the presence of an inflammatory reaction), and occasional macrophages and plasma cells. What is the function of the macrophages?
The basophilic white pulp is composed of lymphoid tissue. It may appear as round nodules, often with a light staining central region (germinal center), or it may appear as elongate bands along longitudinally sectioned blood vessels (periarterial lymphatic sheath, PALS). The germinal centers are predominantly B lymphocytes, while T lymphocytes surround the nodule and make up the PALS. In the white pulp identify the central artery which is found within most splenic nodules. There are numerous dividing cells in the light region of germinal center. Between the red and white pulp on the periphery of the splenic nodule, identify the penicillar arteries which continue into the red pulp as the arterial capillaries.
Check list for SLIDE 22a:
SLIDE 22c spleen, human (H&E)
(RR&K p365, Fig.1,2,3; p367 Fig. 1,2,3)
With the reversed ocular, review the structures identified in SLIDE 22a. The capsule and trabeculae provide support for the vessels that enter and leave the spleen. In the red pulp identify sinusoids and splenic cords. The sinusoidal lining cells appear in cross sectioned sinusoids as densely packed cuboidal cells. In a tangential section through the wall of a sinusoid, the long narrow shape of the lining cells can be observed. The splenic cords appear as loose tissue with many red blood cells, reticular cells (large moderately dense staining nuclei), lymphocytes, some eosinophils, many neutrophils (these may reflect the presence of an inflammatory reaction), and occasional macrophages and plasma cells. Examine the relationship of the central and penicillar arteries and trabecular veins with the splenic sinusoids and cords. Trace the flow of blood (open and closed circulation) through the spleen. The basophilic white pulp is composed of lymphoid tissue with germinal centers of ongoing lymphocyte differentiation.
Check list for SLIDE 22c: 
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.