Lab Assignment
1.
The large circular structure indicated by the arrow is most likely a venule. The majority of cells in the lumen are empty looking, but the cell membrane is still intact and helps distinguish the border those cells. These cells are probably red blood cells that have been lysed during the fixation process. There are 2 cells darkly stained with hematoxylin in the lumen, which are probably some leukocytes. According to the few number of cells present in the lumen of the structure, its considerable size relative to the surrounding cells, presence of single layer of simple squamous cells lining the lumen, and lack of tunica media (smooth muscle), the structure can either be a capillary, a lymphatic vessel, or a venule. However, it can’t be a capillary because the size of the lumen is big enough to store multiple cells, while a capillary lumen is usually small enough to allow one or two cells to file through at a time. Also, it can’t be a lymphatic vessel because (1) there are too many cells present in the lumen, while lymphatic vessels usually transport mostly lymph with occasionally some cells (2) the cells are too densely packed to allow transport of lymph. Therefore, the indicated structure is most likely a venule. There are also two other pieces of evidence for this conclusion: (1) the presence of many red blood cells in the lumen, which is indicative to the venous family of blood vessels in histological slides (2) irregular shape of the lumen; notice this in the structure at the bottom right of the image, which also depicts some connective tissue overlying the simple squamous cells.
The region of the thymus indicated in the image is the cortex. The cells in this region are mostly small developing T lymphocytes that stained more basophilic than those in the medulla. Their nuclei are round and heterochromatic. There are also some type 2 epithelioreticular cells joined together by desmosomes that form the meshwork for the cortex. There are also some type 1 epithelioreticular cells that separates cortex and capsule as well as surround capillaries forming a blood-thymic barrier. The cytoplasm of these cells are not well stained, while some nuclei stained darkly with hematoxylin (T cells) and some are more euchromatic with visible nucleolus (ER cells). Also, cells in the cortex are more densely distributed relative to the medulla.

2.
The pinkish roundish structure indicated by the arrows are thymic corpuscles. Each of these corpuscle is composed of concentric layers of closely packed type 6 epithelioreticular (ER) cells, which appear to stain well with eosin. In large corpuscle, the center indicates keratinization and the shape is not clearly defined. Also, cells are more clearly defined with lighter stained nuclei than those in smaller corpuscles, which appear to be amorphous and homogeneously stained with eosin. The nuclei of type 6 ER cells appear to be euchromatic with clearly defined nucleolus.
These structures belong to the medullary region of the thymus. This region is less packed with larger immunocompetent T lymphocytes, which stain less basophilic and whose nuclei are heterochromatic as well as type 4 and 5 ER cells that are mostly euchromatic with visible nucleoli. Because of the dispersed distribution of cells, the medulla is the lighter region of the thymus while the cortex is the darker region. Besides type 4 ER cells, which associate with type 3 ER cells to form the corticomedullary junction and type 5 ER cells, which form medullary cytoreticulum. Another type of cells present is thymic interdigitating cells which presents self-antigen to T lymphocytes to induce apoptosis of self-reactive T lymphocytes.

3. Draw!

Please refer to question 1 and 2 for description of the cortical and medullary regions of the thymus, respectively. In addition, the cortex is also associated with trabeculae which carries blood vessels, lymphatic vessels, and nerves. Trabeculae don’t penetrate into the