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Lymphoid Tissue

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Slide 1: Low power survey - note the capsule and mesentary (not all nodes are within a mesentary), subcapsular sinus, cortex, medulla, and hilus.  

Slide 2: Low power image of a lymph node, reticular stain. Note again general morphology shown in slide. Reticular fibers and cells form the CT framework of a node.  

Slide 3: Reticular stain - higher power showing part of the capsule, subcapsular sinus and cortex. Cells have picked up some stain and are brownish.  

Slide 4: Lymph node medulla, high power. Reticular stain. Note the fibers that form part of the stroma. What else is part of the stroma? (Ans: reticular cells).  

Slide 5: Afferent lymphatic vessels adjacent to the capsule and the cortex of the node. One vessel, in lower portion of capsule, has an obliquely sectioned valve. The vessels open into the subcapsular sinus.  

Slide 6: Blood vessels and efferent lymphatic vessels seen at the hilus of a lymph node.  

Slide 7: Two post-capillary venules (the specialized ones with high endothelial cells) in the deep cortex of a lymph node (plastic embedded tissue). Here lymphocytes recirculate from where to where? (Answer: blood to node).  

Slide 8: Moderate magnification image of a lymph node demonstrating the capsule, subcapsular sinus, cortex with germinal centers (secondary nodules) and a bit of the medulla to the bottom right.  

Slide 9: High power Capsule, subcapsular sinus, reticular cells (with long eosinophilic cytoplasmic processes), phagocytes (with cytoplasmic inclusions) and lymphocytes (smaller cells). The reticular fibers associated with the reticular cell processes are not seen unless specially stained or in EM.  

Slide 10: The cortex of a node in an area without a germinal center (plastic embedded section). The lymphocytes of various sizes with their very dark (heterochromatic) nuclei are easily distinguishable from the very light (euchromatic) nuclei of the reticular cells. Cells with irregular nuclei of intermediate chromatin staining are most probably macrophages. Pink blobs are reticular cell processes and reticular fibers (which are indistinguishable from each other without special staining).  

Slide 11: Another high power of a lymph node cortex with a germinal center filling most of the field (note the mitotic cells). Compare the nuclear size and euchromatization of the lymphocytes in the germinal center with those in the non-germinal center cortex to the right edge of the field.  

Slide 12: The cortex is the "solid" appearing mass at the right edge of the slide. It grades into medulla which makes up most of the field and is composed of cords and sinuses. With SEM a flattened lining of the sinuses has been described. Find the reticular cells. Note the arteriole within one cord (it contains rbc's). Where is its blood going? (Answer: to capillaries and to post-capillary venules and then out of the node at the hilus).  

Slide 13: Low power of a spleen - note the capsule, hilus, trabeculae, red and white pulp.  

Slide 14: Low power of a spleen stained for reticular fibers. Note that in comparison to a lymph node that there are no obvious cortex & medulla or afferent lymphatics. Here, as in the lymph node, the stroma is composed of reticular cells and fibers. Note the deeper staining of the capsule and trabeculae which also contain type I collagen and some smooth muscle cells.  

Slide 15: Red pulp and trabeculae containing smooth muscle cells (rodent - PAS-H stained). The PAS+ cells are macrophages.  

Slide 16: Low power image of the spleen. Trabecular arteries and veins are easily found because their content of rbc's. Also note the regions of red and white pulp.  

Slide 17: Reticular stain of the spleen. Note the difference in the arrangement of the reticular fibers in the white and red pulp and the reticular fibers in the wall of the central artery and its branches. If you have a good imagination the "face" is the white pulp (complete with couple of extra right ears!) and the rest of the field is red pulp.  

Slide 18: White pulp (and central artery branches) and surrounding red pulp. Mallory stain. The aniline blue stained collagen is seen in the arterial walls and in the two deeply stained trabeculae.  

Slide 19: White pulp (containing a nodule or germinal center, central artery and arterial branches) and surrounding red pulp; toluidine blue and acid fuchsin stain. This tissue was fixed by perfusion, so blood vessels are devoid of blood cells and sinusoids are more empty than normal. Plastic embedded section.  

Slide 20: Red and white pulp of the spleen of a rodent given TiO2 (titanium dioxide). Macrophages (in the marginal zone and red pulp) have ingested TiO2. Note the unusually thick marginal zone. From a student microscope slide.  

Slide 21: Red pulp at high power. Note the splenic sinuses filled with red cell ghosts. Identify the nuclei and cytoplasm of sinus endothelial cells. Macrophages are identifiable because they contain brown hemosiderin pigment. From a student microscope slide.  

Slide 22: Cross section of a sinus in the center of the field with red and white cells moving across the wall. Can you locate at least 9 WBC (PMN's and lymphocytes) traversing the sinus wall? How about RBC? Can you find the 5 clearly identifiable endothelial nuclei? Find the cells in the cords that are good candidates for identification as macrophages. Plastic embedded section.  

Slide 23: Thymus of a 7 month infant, low power. Note the lobules, cortex and medulla. It is clear that the medulla is continuous, i.e., that the lobules are not completely separate.  

Slide 24: Thymus, human, plastic embedded section. Note the dense cortex and Hassel's corpuscles in the medulla.  

Slide 25: In this section of the cortex of plastic embedded thymus note the nuclei of epithelial reticular cells (light staining and large). How would this field appear if stained with a reticular fiber stain? [Answer: Since reticular fibers are synthesized by reticular cells (modified fibroblasts derived from mesoderm) and not by epithelial reticular cells (which are deprived from endoderm and do not synthesize reticular fibers) the field would appear unstained]. You may also be able to pick out some larger cells with granules in their cytoplasm; these are mast cells.  

Slide 26: Thymus, Hassal's corpuscle in the medulla.  

Slide 27: Thymus. Hassal's corpuscle and blood vessels near the cortico-medullary junction. Plastic embedded section.  

Slide 28: Thymus of an adult at a fairly low power. It is mostly fat; a small amount of cortex and medulla remain. Dilated blood vessels are not typical. Note how prominent the Hassl's corpuslces appear.  

Slide 29: Palatine (human) tonsil. One crypt and parts of 2 others which are obliquely sectioned are shown. Note the follicles and the surface stratified squamous epithelium.  

Slide 30: Epithelium of a palatine tonsil extending down into a crypt. Carefully examine the area where the stratified squamous epithelium is deeply infiltrated with lymphocytes; it almost looks as if the epithelium ends.  

Slide 31: Lingual tonsil (human). These are smaller than the palatine - usually only a few crypts.  

Slide 32: Pharyngeal, low power - note the nodules and the mixed glands (human). The typical respiratory epithelium of this image is seen on the next slide.  

Slide 33: High power of pseudostratified columnar epithelium typical of a pharyngeal tonsil.  

Slide 34: Tongue. Adjacent to this papilla near the right side of the image is a small area of diffuse lymphocytic infiltration around a gland duct (a small circular structure).  

Slide 35: Anus, diffuse lymphocytic infiltration just beneath the stratified squamous epithelium of the anal canal.  

Slide 36: Appendix, low power. Note the large areas crowded with lymphocytes. Find at least 3 well developed nodules.  

Slide 37: Ileum,low power. Note again areas of aggregated lymphocytes referred to as Peyer's patches. Did you open the ileum in Gross anatomy and see them? The patches are large enough to be seen without magnification.  

Slide 38: Colon, (appendix) mucosa showing simple columnar epithelium and connective tissue extensively infiltrated with lymphocytes.

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