Learning Objectives
o Compare the histological organizations, location, and functions of diffuse lymphoid tissue, lymphoid follicles (nodules), and lymphoid organs
o Explain the differences between primary and secondary lymphoid organs
o Describe the cellular organization of the lymph node and correlate it with the flow of lymph and blood through it
o Describe the histological structure of the thymus and the components of the blood-thymus barrier
o Describe the microscopic organization and vasculature of the spleen and correlate it with function
SLU Slide 38: Palatine Tonsil (Note – this slide is pretty dark)
Iowa Virtual Slidebox: Palatine tonsil (180)
See if you can spot the epithelium in this preparation. What kind is it? You may be able to see some of this epithelium invaginating into the tissue. These are tonsillar crypts.
Simple squamous epithelium.
Explore the lymphatic nodules present in this tissue, as well as the surrounding diffuse lymphatic tissue. See if you can spot some plasma cells in the sea of lymphocytes!
Iowa Virtual Slidebox: Pharyngeal Tonsil (181)
The pharyngeal tonsil is lined by a different type of epithelium because of its location. What is the epithelium here?
Ciliated pseudostratified epithelium – aka respiratory epithelium.
You can see a lingual tonsil in Iowa Virtual Slidebox slide: Lingual tonsil:tongue (182)
SLU Slide 37: Lymph Node (Note – this slide is pretty dark)
Iowa Virtual Slidebox: Lymph node (177); Lymph Node (1792); Lymph Node (1868)
Examine the lymphoid nodules and germinal centers in the cortex of the lymph node. What is the difference between primary and secondary lymph nodules? What accounts for the light staining of the center of a germinal center? Note: Slide 1792 has some primary lymph nodes, whereas the other slides are secondary.
A primary lymphoid nodule consists of a solid, spherical mass of lymphocytes (B lymphocytes). A secondary lymphoid nodule is one that has been stimulated by antigens. It increases in size and develops a pale central region filled with large, immature lymphoblasts. These lymphoblasts will mature into smaller B lymphocytes. The light staining is due to the presence of lymphoblasts, which are larger and have more cytoplasm than mature lymphocytes.
Iowa Virtual Slidebox: Lymph node (1868)
Examine germinal centers at high magnification and locate the mitotic figures which are common in these centers of proliferation. What type of cells will arise from these mitotic divisions? Why were germinal centers not present in the thymus?
B cells will arise from these divisions. Germinal centers are not present in the thymus because the thymus is the site of T Lymphocyte maturation, and cells are not dividing in the thymus.
Any lymph node slide:
Try to differentiate the outer and inner (paracortical) cortical zones of the lymph node. The paracortical region, or inner cortex, is difficult to define clearly on morphological grounds. What is the importance of these two cortical zones?
The outer cortex is filled with lymphoid nodules (mostly B lymphocytes). The inner cortex is populated by diffuse lymphoid tissue and is filled with mostly T lymphocytes. The inner or paracortex is also the region in which lymphocytes enter the lymph node via high endothelial venules.
In the medulla, locate the medullary cords and sinuses. Observe the sinuses at high magnification and observe that they are not clear channels, but instead contain many cells and fibers. How does this arrangement suit the function of the lymph node?
This arrangement of cells (mostly macrophages) and fibers (reticular) helps to filter lymph. Cells and other debris get “caught” in the meshwork of reticular cells and are then phagocytized by the macrophages.
In the medullary cords, observe the presence of lymphocytes, macrophages, and numerous plasma cells. What type of lymphocytes predominate here?
Mostly B Cells.
Iowa Virtual Slidebox: Lymph node – silver stain (175)
Compare this slide to a lymph node stained with H&E. Silver stains allow you to visualize the reticular fibers present in the structure of the lymph node. Compare the reticular structure in the cortex vs. in the medulla. Write down your observations.
In this preparation, cells stain gray, and are concentrated in the cortex. The reticular fiber meshwork is important in the medulla, where it forms a system of sinuses through which lymphocytes percolate.
How do lymphocytes enter a lymph node? Trace the pathway(s) on a low magnification slide and sketch or draw it in your notebook.
Most lymphocytes enter a lymph node through the blood by flowing in through the vessels entering the hilum > capillary beds > high endothelial venules > exposure to lymph node medulla
Some can enter through afferent lymph vessels entering lymph node > through subcapsular, trabecular, and medullary sinuses.
Iowa Virtual Slidebox: Spleen (176); Spleen (178)
Identify the connective tissue capsule of the spleen and the trabeculae that invade into the substance of the spleen. Why is injury to the outer connective tissue capsule so dangerous?
Because the spleen filters blood and thus has a rich ciruculatory system, damage to this thin capsule can result in severe blood loss.
At low magnification, identify different regions of the spleen that differ in staining and cell shape. Identify and point out some of the nodules that are present in the lymphatic tissue (Also note that some of these nodules have an artery in their center). What are these regions called? What type of cells are located here?
Lymphatic nodules and Periarterial Lymphatic Sheaths are present in the white pulp. White pulp contains lymphocytes. Specifically, prior to stimulation the PALS contain T lymphocytes. After stimulation, germinal centers with B cells can be formed that displace the “central” arteriole.
What are PALS?
Periarterial lymphatic sheaths. These congregations of lymphoid cells surround arteries within the spleen.
Now, turn your attention to the tissue that surrounds these nodules and note that it contains numerous erythrocytes and stains a little pinker. What is this region called? What is located here?
This is the red pulp, filled with blood and the diffuse lymphatic tissue that filters that blood.
The circulatory pattern within the spleen is critical to its function as a blood filter. Small arterioles, after leaving the PALS, empty into the splenic cords (open circulation). Blood must filter through the cords to reach the sinusoids, which return the blood to trabecular veins, and ultimately to the splenic vein. Splenic sinusoids have a peculiar and unique structure. Are these continuous or discontinuous capillaries?
Discontinuous capillaries.
Iowa Virtual Slidebox: Spleen (298) (Silver stain)
Examine the reticular framework of the spleen, comparing the distribution in the red and white pulps.
Iowa Virtual Slidebox: Infant Thymus (271); Thymus (400); Thymus (287); Thymus (179)
Examine the thymus at low magnification and note its lobes. The structure of the thymus differs from that of a lymph node and spleen because it doesn’t filter lymph of blood, but instead produces T lymphocytes. Reflect on the differences in organization between a filtering organ and a cell-producing organ.
The connective tissue framework should be (and is) different in an organ that filters cells – Those organs a well-developed reticular meshwork, whereas reticular fibers are not prominent in the thymus.
Examine the cortex of the thymus and observe a dense layer of closely-packed, small, deeply-staining cells called thymocytes. These are immature T lymphocytes. Draw or screenshot one/some to keep in your notes.
Continue exploring until you find some larger, paler cells whose nuclei have a loose chromatin network and one or more prominent nucleoli (Hint: Iowa Slide 179 is good for examining these). What are these cells? What role do they play in the thymus? Do you know about their embryological development? Draw or screenshot some to keep in your notes.
These cells are epithelial reticular cells, and they form the supporting framework of the thymus instead of reticular fibers. They are derived from endoderm rather than mesoderm like the rest of the thymus.
Which cells mature in the thymus? What is the blood-thymus barrier? What purpose does it serve, what are its components, and where is it located?
T cells mature in the thymus. The blood-thymus barrier consists of 1) an endothelium with occluding junctions (on the blood vessel side)2) Macrophages in the perivascular connective tissue and 3) Epithelioreticular cells, which surround the thymic cortex. The purpose of the barrier is to isolate maturing T-cells away from foreign antigens, so that they can develop a sense of “self” vs. “non-self.” The barrier exists in the thymic cortex, which is where T-cell maturation occurs.
In the medulla of the thymus, you’ll see the same types of cells but in different proportions. Contrast the ratio of thymocytes to epithelial-reticular cells in the cortex and medulla.
Thymocytes (immature T cells): High in cortex, low in medulla.
Epithelial-reticular cells: Low in cortex, high in medulla.
Find a Thymic Corpuscle. How do you identify this structure? Draw or take a screenshot to add to your notes.
Bright pink, swirly aggregations of epithelial-reticular cells; can become keratinized.
We do not have a thymus belonging to an older individual in our slide collection. Compare your slide to either an image from our lecture, or an image in an atlas. What differences do you observe, and what do they represent?
Older individuals will have a thymus comprised mostly of adipose tissue, with some cellular regions. This represents the involution of the thymus.
As you complete this activity, review the characteristics that distinguish:
o Tonsils
o Lymph node
o Spleen
o Thymus
Make a table or concept map that contrasts different structural and functional properties and compare with a friend.