HISTOLOGY REVIEW #7: LYMPHOID TISSUE

1. The stroma is made up of C.T. fibroblasts or connective tissue reticular cells and associated reticular fibers in all of the following EXCEPT:

1. Lymph nodes.
2. Thymus.
3. Bone marrow.
4. Spleen.
5. Peyer's Patches.

B. Embryologically, thymus is derived from endothelium, and so the stroma does not contain connective tissue. The thymic epithelial reticular cells have long cytoplasmic processes (as in the other lymphoid organs), but they contain intracellular keratin filaments, and adhere to each other through desmosomes. They do not manufacture the usual extracellular reticular fibers; instead, the reticulum is totally cellular.

2. Which lymphoid tissue or organ is NOT paired with the appropriate diagnostic feature?

1. Thymus : Hassall's corpuscle.
2. Spleen : Marginal zone.
3. Peyer's patches : Afferent lymphatic vessels.
4. Pharyngeal tonsil : Overlying epithelium is ciliated, pseudostratified columnar.
5. Lymph node : Medullary sinuses.

C. Diffuse lymphoid aggregations and nodules (both unencapsulated) do not have afferent lymph vessels. What is the "marginal zone" in the spleen? Why is the pharyngeal tonsil not covered with stratified squamous epithelium?

3. Concerning the thymus:

1. The cortical capillaries are discontinuous and lack a distinct basal lamina.
2. Numerous afferent lymphatics empty into the subcapsular sinus.
3. The reticular stroma stains with silver salts.
4. The Hassall's corpuscles are made up of concentrically arranged epithelial cells.
5. Most of the lymphocytes generated in the outer cortex ultimately leave the thymus and populate the T-cell-dependent regions of the peripheral lymphoid structures.

D is true. (E) is false because most of the lymphocytes generated in the thymic cortex never leave, but, instead, die there. Apparently, the process of immulogical specification is highly prone to errors, which, fortunately for us, are corrected before the faulty T-lymphocytes ever get released. NOTE that (B) is false! The thymus actually has NO afferent lymphatics.

4. Which of the following statements regarding lymphocyte circulation is INCORRECT?

1. Both T and B cells are found in the peripheral blood.
2. Lymphocytes exit lymph nodes primarily via efferent lymphatics at the hilus.
3. Blood-borne lymphocytes enter lymph nodes via high endothelial, post-capillary venules.
4. Lymphocytes populate the white pulp of the spleen by exiting small blood vessels in the marginal zone.
5. Lymphocytes enter the lingual tonsil via afferent lymphatics.

E. No afferent lymphatics in the lingual tonsil. Here's Wheater's explanation: Accumulations of lymphocytes are found throughout the digestive and respiratory tracts, in and below the lamina propria (i.e. just below the epithelium). This is Mucosa-Associated Lymphoid Tissue (MALT). The epithelium overlying these MALT aggregations is specialized for sampling the luminal contents for antigen, and is the equivalent of the afferent lymphatics of the lymph node. The lymphatics associated with MALT (and this includes the tonsils) are all efferent, passing to regional lymph nodes.

5. Which of the following lymphoid structures does NOT normally contain germinal centers?

1. Spleen.
2. Thymus.
3. Lymph node.
4. Tonsil.
5. Appendix.

B. Germinal centers are zones where B-lymphocytes are actively proliferating and differentiating into antibody-producing plasma cells. Plasma cells are found in peripheral tissues, and in lymph nodes and nodules. The thymus functions in T-cell maturation, and therefore is not a site where B cells proliferate; as a result, it doesn't contain germinal centers.

6. Accumulations of B lymphocytes can normally be found in all of the following areas EXCEPT:

1. Red bone marrow.
2. Germinal center in the palatine tonsil.
3. Primary nodule in Peyer's Patch.
4. Immediately surrounding the central artery in the white pulp of the spleen.
5. Medullary cords of a lymph node.

D. This area contains mostly T-lymphocytes. (A) is true because B cells (and all other lymphocytes) come from the bone marrow. (B) and (C) are correct because germinal centers contain primarily B cells. (E) is correct because medullary cords contain B cells and antibody-secreting plasma cells (these have come from the cortex of the lymph node, where they matured).

SOME GENERAL QUESTIONS

7. Describe an important structural difference between lymph nodes and lymph nodules.

Nodes have a capsule; nodules do not.

8. Describe an important functional distinction between the thymic cortex and the thymic medulla.

Cortical lymphocytes are protected within the blood-thymus barrier, because they are developing specific immunological reactivity. Those that develop a reactivity against the host are destroyed within the cortex. Only mature T-lymphocytes with the correct non-self reactivity will leave the protected environment of the thymic cortex; they then enter the general circulation via vessels within the thymic medulla.

9. Why do germinal centers appear as lighter-stained regions within a darker-stained lymphoid nodule?

The cells within the germinal center are proliferating; their nuclei are enlarged and their nuclear material is more spread out, so they stain less intensely than in quiescent lymphocytes.

MATCHING

1. Peyer's patch.
2. Lingual tonsil.
3. Palatine tonsil.
4. Spleen.
5. Appendix.

10. Partially encapsulated aggregation of lymphoid nodules associated with underlying salivary glands.

B. Small mucous salivary glands lie below the lingual tonsil, by the base of the tongue.

11. Unencapsulated aggregation of lymphoid nodules in ileum.

A. Peyer's patches do not have capsules.

12. Responsible for the destruction and phagocytosis of old and defective erythrocytes.

D. The blood sinuses of the spleen have a discontinuous endothelial lining. Erythrocytes have to squeeze through small openings to get in and out; older cells aren't flexible enough to do this, and get trapped and destroyed. Which spleen cells are responsible for the erythrocyte destruction (and recycling of heme)?

MICROSCOPIC IDENTIFICATIONS

13. A. Identify the organ. B. In general terms, what is this space?

1. Pharyngeal tonsil.
2. An airway. Specifically, the nasopharynx; the important point is that it's lined with a respiratory epithelium (pseudostratified ciliated columnar, in this case) rather than a stratified squamous epithelium.

14. A. Identify the cells. B. Identify the structure. C. Identify the structure.

1. Reticuloendothelial cells (of thymus).
2. Capillary.
3. Hassal's corpuscle.

15. A. Identify the organ. B. Identify the area. C. Identify the area. D. What is the predominant cell type in the area in (C)?

1. Thymus (it has many incomplete lobulations).
2. Medulla (a finger-like projection of medulla into cortex, NOT a germinal center).
3. Cortex.
4. T-lymphocytes.

16. Lymphoid organ of some sort. A. What are the dark dots? B. What is their principal biochemical component? C. How do you know that this micrograph could NOT have been made from the thymus?

1. Reticular fibers in cross-section.
2. Type III collagen.
3. The fibers in (A) are extracellular. The thymus does not have such extracellular fibers, however; they're only found in lymph nodes and spleen. The reticuloendothelial processes that make up the supporting structure of the thymus are cellular, not extracellular. The small, black arrows point to this cell's cytoplasmic extensions, which surround the newly-synthesized reticular fibers that it has made and assembled.

17. A. Identify the organ. B. What normally fills this space? C. Identify the region. D. What is the predominant cell type here? E. What is their principal product? F. How did the material in space 'B' get there (i.e. where was it, just before arriving there)? G. Where is it headed next?

1. Lymph node.
2. Lymph.
3. Germinal center.
4. B-lymphoblasts.
5. Immunoglobulins.
6. It came in from afferent lymphatic vessels that perforate the lymph node's capsule.
7. Out from the node via the efferent lymphatic.

18. Low- and high-magnification views. A. Identify the organ. B. Identify the region. C. What is normally contained within the region in (B)? D. What is the space?

1. Lymph node.
2. Hilus.
3. Blood vessels and efferent lymphatic vessel.
4. Subcapsular sinus.

19. Low- and high-power views. A. Identify the organ. B. Identify the dark region. C. Identify the indicated cells in the high-magnification view.

1. Thymus (it's highly lobulated, but the lobulation doesn't continue into the medullary regions; if we were famous histologists we could name this "pseudolobulation").
2. Thymic cortex.
3. Thymic epithelial reticular cells.

20. Low- and high-magnification views of an organ. A. Identify the vessel, which was found deep in this organ. B. What region of the organ normally contains such vessels?

1. High-endothelial venule.
2. Paracortical region only.

21. Four views of the same organ. A. Identify the area. B. Identify the area. C. Identify the vessel. D. Identify the area. E. Identify the vessel. F. Identify the cells. G. What is the function of the cells in (F)? H. What is the brown pigment in the little globules in these cells?

1. Red pulp of the spleen.
2. White pulp of the spleen.
3. Venous sinusoid.
4. Peri-arteriolar lymphoid sheath (PALS).
5. Arteriole.
6. Macrophages.
7. Fragments of worn-out erythrocytes that have been phagocytized.

22. Spleen. A. What is the name of this type of blood vessel? B. What is this erythrocyte doing? C. Why can't this process take place at conventional venules? D. What is the erythrocyte's average lifespan? E. What happens at the end of that time?

1. Venous sinus.
2. Moving from venous sinus to pulp cord, or vice versa. Looks a lot like diapedesis, but Dr. Vertel says that term can only be used with white cells.
3. The endothelial lining of the sinus has large, open spaces and a discontinuous basal lamina.
4. About 120 days.
5. See question #21.

23. Low- and high-magnification views of a lymph node. A. What is the relationship between the cells and the fibers in this organ? B. Compare it with the relationship between the cells and the fibers of the thymus. C. What is the biochemical composition of the fibers?

1. The fibers are extracellular.
2. In the thymus, the "fibers" are cytoplasmic extensions of the supporting cells, i.e. the "fibers" are cellular rather than extracellular.
3. Collagen type III (see question 16).

24. Rat lung, low- and high-magnification views. Identify the region.

Lymphoid nodule (no capsule).

25. Stomach, low- and high-magnification views. Identify the region.

Secondary lymphoid nodule.

26. Stomach/duodenal junction, low- and high-magnification views. Identify the region.

Diffuse lymphoid aggregate (infiltration).

27. Low- and high-magnification views. A. Identify the organ. B. Identify the vessel. C. What is the immediate destination of the material in this vessel?

1. Lymph node.
2. Afferent lymphatic vessel (note the valves).
3. The lymph in this vessel is headed into the subcapsular sinus of the lymph node, then to sinuses in the medulla, then finally into the efferent lymphatic that exits via the hilus .

28. Two views of similar vessels in a lymph node. A. These are examples of what type of vessel? B. The indicated leukocyte appears to be embedded within the wall of the vessel. What's going on here?

1. High endothelial venules.
2. Diapedesis (it's entering the node from the bloodstream).

29. Lymph node. Identify the cells.

Plasma cells.

30. Lymph node. A. Identify the cell. B. What is the relationship between such macrophages and the surrounding lymphocytes and plasma cells?

1. Macrophage (lots of granules, but they're not as dark or as big as the ones in a mast cell).
2. The lymphoid cells react to the presence of antigens by mounting an immunological response in which macrophages phagocytose bacteria and other microorganisms that enter the node by way of the lymph.

31. Small intestine (low, medium, high mag). A. Identify the structures. B. What is the function of the layer that covers these structures?

1. Lymphoid nodules in a Peyer's Patche. The term "Peyer's Patch" is reserved for these collections of large lymphoid nodules, usually found in the ileum.
2. Clusters of M-cells form the epithelium here. The basement membrane associated with these M-cells is perforated, so that materials within the gut lumen can filter through and be examined by the underlying cells of the immune system (macrophages, T- and B-lymphocytes), and an immune response can be mounted if necessary.

Below are links to several helpful drawings from Junquiera's textbook that summarize:

Humoral (antibody) immunity vs. cell-mediated immunity (Figure 14-2).

The life and times of a lymphocyte (Figure 14-5).

What happens after a macrophage presents a suspicious antigen to a cell of the immune system (Figure 14-6).

How the binding of immune complexes leads to destruction of invading organisms (Figure 14-11).

(Further details may be found in the text).

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