- Be able to describe the organization and function of thymus, lymph nodes, spleen and tonsils.
- Be able to identify the regions rich in B and T lymphocytes in each organ and explain the cellular processes, relevant to immune functions, that are taking place in these regions.
- Know the homing patterns of B & T lymphocytes.
There is a continuous production and recirculation of lymphocytes in the body. Many of the lymphocytes you see in blood vessels, lymphatic vessels or in tissue have been exposed to antigen and are thus poised to respond to specific antigenic stimuli. The lymph nodes and the spleen facilitate immunological surveillance of the host. They are the sites of 1) antigen trapping, 2) homing of cells of the immune system that have been exposed to their triggering antigens, and 3) subsequent expansion and release of lymphocyte clones capable of further response to specific antigens. The exposure of lymphocytes to antigens occurs in solitary lymph nodes (follicles), in aggregates of lymph nodes present beneath mucous membranes, and in the spleen. The palatine tonsils and the Peyer’s patches in the ileum (not seen in this laboratory) consist of multiple aggregates of lymph nodes. T lymphocytes, which participate in cell-mediated immunity or modulate the immune responses of other cells, differentiate in the thymus during childhood. B lymphocytes, which have the capacity to become plasma cells and secrete immunoglobulins, differentiate in the bone marrow throughout life.
A. Lymph nodes (W pgs 218-227)
Slide 27 (lymph node, H&E) WebScope ImageScope
Slide 26_20x (mesentery, H&E) WebScope ImageScope
Slide 26_40x (same as above but showing just a lymph node) WebScope ImageScope
Slide 142 (lymph node, H&E) WebScope ImageScope
Slide 30 (mesentery, H&E) WebScope ImageScope
Slide 28 (lymph node, silver stain) WebScope ImageScope
Use slide 27 to study the overall organization of a lymph node and identify the cortex, medulla and hilus [ORIENTATION] . Some slides do not show a medulla or hilus, depending on the plane of section, especially, the hilus. The hilar region is rather small and will not be present in all slides. Using slide 27 and other slides, locate the capsule, lymphatic follicles or nodules (B cell-rich), the diffuse or deep cortical zone (T cell-rich), in addition to the medulla and the hilus. Recall that the deep cortex is the zone of antigen presentation, while T cell help occurs in the follicles. What is the distribution of B cells and T cells in the lymphatic tissue? (LY1).
Generally the sinuses show up best beneath the capsule (subcapsular sinus) in either slide 26 [example] [ORIENTATION] or slide 142 [example] [ORIENTATION] or in the medulla (medullary sinuses, see below). Make sure you have found a real sinus and not just a separation artifact. You should be able to see: 1) reticular cells spanning the sinus, 2) some endothelial cell nuclei defining the inner wall of the sinus (adjacent to the lymphatic follicles), and 3) circulating lymphocytes and macrophages in the lumen. What is a fiber associated reticular cell? (LY2) Afferent lymphatic vessels that penetrate the capsule are difficult to identify, but can be observed in slide 30 [example]. If you see venule-sized vessels adjacent to or within the capsule you can make the provisional identification of an afferent lymphatic vessel, especially if these vessels contain lymphocytes and show valves.
With the low power objective find primary and secondary follicles (follicles with germinal centers, best seen in slide 27) [example]. It is not always possible to distinguish primary and secondary follicles because a glancing section through a secondary follicle may miss the germinal center and thus mimic a primary follicle, and that sometimes the boundary between the follicles and the diffuse lymphatic cortex is indistinct. The germinal centers [example] [ORIENTATION] in these particular specimens are at an end stage; that is, blastogenesis has already occurred and most of the cells present in the center are simply reticular cells and a few macrophages. Recall that the germinal centers are the sites of T cell help, B lymphocyte proliferation and antigen-dependent differentiation of B cells into lymphoblasts. The lymphoblasts continue to differentiate into plasma cells and memory B cells. Where do the plasma cells in the lymph node go? (LY3) Make sure that you find the high endothelial venules (specialized cortical veins lined by high endothelium) especially visible in either slide 27 [example] or slide 142 [example]. How do cells travel from the blood to the lymph node? (LY4)
With a low power objective locate the medullary cords and sinuses in either slide 26 [example] or slide 142 [example]. Find endothelial lining cells, reticular cells, circulating lymphocytes and macrophages (that may appear foamy after having engulfed some RBCs). Review the blood supply to the node. In slide 27, a number of efferent lymphatic vessels [example] with valves can be found in the hilar connective tissue. Now look at slide 28 to review the role of reticular fibers in supporting the structure of the lymph node.
B. Mucosa-Associated Lymphoid Tissue
Slide 161 (gastroduodenal junction, H&E) WebScope ImageScope
Slide 157b (fundic stomach, H&E) WebScope ImageScope
Slide 175 (appendix, H&E) WebScope ImageScope
UCSF slide 261 (appendix, H&E) WebScope ImageScope
Slide 168 (jejunum, H&E) WebScope ImageScope
Solitary lymphatic nodules are present throughout the GI tract, respiratory tract and, to a lesser extent, the urinary tract. Look for solitary nodules in slide 161 (the gastro-duodenal junction). At low power, localize the duodenum by identifying mucous glands (Brunner’s glands) in the submucosa and, then, look for discrete basophilic nodules (follicles) [example] in the lamina propria or in the submucosa in the midst of mucous glands, some of which will have germinal centers. Solitary lymphatic nodules can be also seen in our slides of the stomach (slide #157 [example]) and the appendix (slide 175 [example] and UCSF 261 [example]). Many such nodules combined give rise to named structures, such as tonsils or "Peyer’s patches." The term, Peyer’s patches, specifically refers to aggregates of lymphatic nodules in the ileum, but often the term is used mistakenly to describe any aggregates of lymphatic nodules in other parts of the GI tract.
In addition to lymphoid aggregates and nodules present in the lamina propria and submucosa are intraepithelial lymphocytes which are T-cells that develop in the thymus and migrate into various epithelial tissues in the skin, GI, respiratory, and genitourinary tracts where they serve an important function as a first line of defense against pathogens. You can find intraepithelial lymphocytes in just about any of the slides that we've used to study these organ systems so far, but they are probably easiest to see in slide 168. To find them, look for small, dark, round nuclei [example] situated between the epithelial cells.
Using slide 138L, observe the numerous aggregates of lymphatic follicles with germinal centers and the stratified squamous epithelium that covers the oral surface of these aggregates. The epithelium also lines crypts that dip into underlying lymphatic nodules. The epithelium lining some crypts is almost obliterated by infiltrating lymphocytes. Some crypts will probably be cut in cross section so continuity with the oral cavity will not be apparent. At 10X, locate a follicle with a germinal center. Note small, lightly stained areas (they almost look like small holes) distributed throughout the center. If you can’t find them look at another germinal center. These lightly stained areas are macrophages [example] in the germinal center. Look at them with high power. You should see a large cell with a large, vesicular nucleus that contains at least one conspicuous nucleolus. You will have to work to see them, but once you’ve seen one you will be able to recognize them in most lymphatic tissue (e.g. slide 175 [example] and UCSF 261 [example]). The cytoplasm of these macrophages will have a variable appearance but will usually include two or three hyperchromatic spheres that are ingested B lymphocytes that failed to differentiate properly and underwent apoptosis.
In slide 138_20x, locate the dense, irregular connective tissue capsule [example] that separates the tonsil from the rest of the pharynx. In this section it does not completely enclose the tonsil. There are a few mucous glands and muscle fibers included in this section.
D. Spleen (W pgs 229-233, 11.18-11.21)
Slide 147B (spleen, human, H&E) WebScope ImageScope
Slide 148_20x (spleen, human, H&E) WebScope ImageScope
Slide 148_40x WebScope ImageScope
Slide 301 (spleen, monkey, H&E) WebScope ImageScope
Of the two human slides, #147 is better in that it is more compact and the content of erythrocytes is reduced. With low power try to distinguish between the red and white pulp. Think about why these terms are employed. While the white pulp has an obvious role in the immune response, don’t forget the role of the red pulp in potential antigen trapping by macrophages. Find the capsule [example] and trabeculae [example] (containing trabecular arteries and veins—you may find just one or the other). Much of the white pulp you will see in your section of slide 147 is composed of B-cell dependent follicles, showing germinal centers. Slide 148 contains little easily recognizable white pulp (somewhat abnormal) but is helpful in seeing the periarteriolar lymphatic sheath (PALS) [example] that surround central arteries (which range in size from small arteries to arterioles). Recall that the PALS contains mostly T lymphocytes and is homologous with the deep cortical zone of a lymph node.
Going back to slide 147, you'll see many instances where a PALS is expanded eccentrically to include a lymphatic follicle [example], usually with a germinal center; recall that these follicles are B-cell rich. With this expansion, the "central" artery is pushed off to one side and therefore not very "central." Many texts and atlases usually show this classic view of a the central artery and its PALS in cross section. However, you should realize that the PALS is a sheath that runs along the length of each central artery, and you can see this particularly well in slide 301 [example]. The marginal zone [example] [ORIENTATION] is a region surrounding the white pulp that is difficult to recognize unless the spleen is very well preserved and sectioned, or, as in slide 148, the red pulp is particularly obvious (here the red pulp is packed with erythrocytes). This region contains mostly reticular cells and antigen-presenting cells, as well as some lymphocytes that may play a role in antigen trapping or processing. Don't get too hung up on trying to definitively demarcate the marginal zone in these sections, but you should have a general idea of this transition zone between white pulp and red pulp and understand its function as a primary site of antigen presentation in the spleen.
In the red pulp, identify the splenic cords (tissue between the sinuses) and venous sinuses that are reasonably well preserved beneath the capsule in slide 147 [example] [ORIENTATION]. The cords and sinuses show up particularly well in slide 148 [example] [ORIENTATION]. The endothelial cell nuclei of the sinuses are characteristically plump and closely packed together. Note that the endothelial nuclei bulge toward the lumen of the sinuses --this may help you discern the cords from the sinuses.
With low power, look at slide 140 and distinguish between the cortex and the medulla [ORIENTATION]. Why is the thymic medulla lighter than the cortex? (LY5) Locate Hassall’s corpuscles [example] (which are aggregates of epithelial reticular cells) in the medulla. Try to find other epithelial reticular cells [example] [ORIENTATION] that are scattered throughout the cortex and medulla --the key to identifying them is their large, pale nuclei. Of course, most of the cells with small round nuclei packed into the cortex and medulla are T lymphocytes (when immature, they are called “thymocytes”). Also present are numerous macrophages [example] [ORIENTATION] which will appear eosinophilic with a foamy cytoplasm or they may even contain engulfed lymphocytes. What are the differences among the various reticular cells and fibers of the lymphoid organs? (LY6) How do lymphocytes leave the thymus? (LY7)
Look at slide 141 and observe the histology of thymic involution, which is basically a replacement of lymphatic tissue by fat. The Hassal's corpuscles [example] here are also quite large and a bit bizarre looking. The production of T cells by the thymus in the human adult is generally greatly reduced, but it can continue to produce T-cells even into adulthood.
Electron Micrograph Wall Charts
These micrographs should help you understand the organization of lymphoid organs; they are among the best available anywhere.
The nodule has a central germinal center and a peripheral corona of small lymphocytes. Find the subcapsular sinus and the reticular cells of the node. Identify several macrophages in the nodule.
Observe the loose arrangement of lymphoid tissue in the medulla of lymph nodes. The sinuses interconnect and their lumens are traversed by reticular cells, some of which can trap antigens and some of which are true phagocytes. Blood vessels are restricted to the medullary cords. Plasma cells occur abundantly in the cords. Review the function and origin of plasma cells. Note that the free, circulating lymphocytes are either small or medium size. Where are the lymphoblasts? (LY10).
Note that the thymus does not have lymph nodules. However, the small lymphocytes (thymocytes) are aggregated in the cortex, and the larger, epithelial reticular cells are concentrated in the center (medulla), giving the appearance of a “nodular” area. Remember Hassall’s corpuscles are found only in the thymus. (no Rhodin counterpart).
Note the very dense arrangement of lymphoid cells in the white pulp, and the loose arrangement of reticular cells, some lymphoid cells and extravasated erythrocytes in the pulp cords among the venous sinuses. Note also the marginal zone and recall that this is where most antigen presentation occurs in the spleen.
In this unique micrograph of the red pulp, one blood capillary is seen to have an open end, discharging formed elements of the blood into the extravascular space (pulp cords). Find the venous sinuses and observe the movement of erythrocytes across the wall. The presence of many macrophages in the interstitial space of the red pulp is obvious. WebScope (LY11) What are the functions of the red pulp? (LY12)
- The section shown is of: image
- lymph node
- gut-associated lymphoid tissue
- The asterisk is in: medium mag high mag
- splenic sinus
- splenic cord
- lymph node medullary sinus
- lymph node medullary cord
- thymic cortex
- thymic medulla
- In an individual with thymic aplasia (failure of the thymus to develop), which region of a lymph node shown would you expect to be attenuated?image
- ALL of the above
- NONE of the above
- The cell indicated is a/an:low mag high mag
- endothelial cell of a high endothelial venule
- rod cell of a splenic sinus
- epithelial reticular cell
- reticular cell
- dendritic cell
- plasma cell
- The electron micrograph shows an eosinophil traversing a vessel wall. The cell is MOST LIKELY:click here for image
- entering the deep cortex of a lymph node via a high endothelial venule.
- re-entering the bloodstream in a lymph node medullary sinus.
- entering the thymic medulla via an afferent lymph vessel.
- exiting the thymic medulla via intravasation into a medullary venule.
- entering splenic white pulp via an afferent lymph vessel.
- re-entering the bloodstream in a splenic sinus.
- The cell indicated is MOST LIKELY a/an:low mag high mag
- endothelial cell of a high endothelial venule
- rod cell of a splenic sinus
- epithelial reticular cell
- reticular cell
- dendritic cell
- plasma cell