LYMPHATIC SYSTEM
Objectives
You should be able to identify the following features
of lymphoid tissue:
| Lymphoid tissue components | Lymph node | ||||
| Lymphocytes | C.T. capsule | ||||
| Lymphoblasts | C.T. trabeculae | ||||
| Macrophage | Hilus | ||||
| Reticular cells | Afferent lymphatic vessels | ||||
| Reticular fibers (when stained) | Efferent lymphatic vessels | ||||
| Lymphatic nodules = follicles | Subcapsular sinuses | ||||
| Germinal center | Cortex | ||||
| Corona = mantle | Nodules | ||||
| Diffuse lymphatic tissue | Cortical (trabecular) Sinuses | ||||
| Tonsil (palatine) | Paracortex | ||||
| Epithelium | Tall postcapillary venules | ||||
| Epithelial cells | Medulla | ||||
| Crypt | Medullary sinuses | ||||
| Partial capsule | Medullary cords | ||||
| Appendix | |||||
| Epithelium | |||||
| Mucosa | |||||
| Glands | |||||
Blue Histology
| The Blue Histology program has two parts for the lymphoid system. | |
| Both are relevant to today's laboratory, but skip the parts on the spleen and the thymus until the next laboratory period. |
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part 1 | |
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part 2 |
Slides
| D-33 | Lymph node (reticulum stain) | |
| D-34 | Lymph node, child (H&E) | |
| D-35 | Palatine tonsil (H&E) | |
| D-111 | Appendix (H&E) |
Optional
slide
| D-76 | Bronchus with lymph node (H&E) | |
| D-96 | Posterior tongue (H&E) | |
| D-176 | Ear canal | |
| D-108 and D-122 show other lymph node | |||
Slide descriptions
D-34 Lymph node, child (H&E)
Hold this slide up to the light and distinguish the cortex from the medulla. Observe the lymph nodules at the outer edge of the cortex. Keep in mind that your section may have been cut off center (sagittally) so that the cortex extends into the center of the section. The part of the cortex near the junction with the medulla and without nodules is that paracortex ( = lower cortex = thymus-dependent region of the node). It is where most of the T cells in the node are located.
Under low power, identify the capsule, subcapsular sinuses (mostly collapsed), lymphatic nodules, medullary sinuses, medullary cords and blood vessels of the medulla (illustration). The hilus has efferent lymphatics, with lymphocytes but no erythrocytes. Small afferent lymph vessels are harder to find but you may see one external to or within the capsule. Look especially for ones packed with lymphocytes. With the aid of your text, assure yourself of the pathway that the lymph takes in percolating through a lymph node from them to the afferent to the efferent lymph vessels. It travels mainly through sinuses (name these: _______________, ________________, _________________, check). In the medulla, for example, sinuses are interspersed between areas that are packed full of cells, the medullary cords (illustration).
In examining the other lymphatic tissues and organs (spleen, tonsils, etc.), it is useful to compare their organization with that of a lymph node. You will adequately know the histology of each lymphatic tissue if you learn the following three aspects of it: 1) the pathway of lymph or blood through the tissue; 2) any special cells of the tissue; and 3) sources of antigen to which the tissue may be exposed. In the case of a lymph node, antigens come in with the lymph drained from the overlying (perhaps infected) connective tissue.
Look at the nodules in the cortex. Identify a germinal center and the corona = mantle around it. Most of the cells in the corona are small lymphocytes. In contrast, many of the cells in the germinal centers of the nodules have large pale nuclei. This is why germinal centers stain palely.
Some of you may wish to distinguish the various cell types found in germinal centers, although this is not required. For you adventurous souls, scan the cortex for a germinal center in which the cells are not jammed too heavily on top of one another. If you have trouble finding the cell types cited below, refer to your atlas and textbook or call over an instructor. The large, pale nuclei which seem to have no cytoplasm at all around them are follicular dendritic cells (high magnification, examples). These cells actually have a lot of cytoplasm but it is stretched out into long arms that extend out of the plane of section. Their function is to hold antigen molecules on their surface and "present" them to lymphocytes. (They are one type of antigen presenting cells. Various other cells present antigen in other parts of the body.) More rarely you will see nuclei as large as those of dendritic cells, but darker and with a discrete rim of basophilic cytoplasm. The cytoplasm will have a sharp outer perimeter. These cells are lymphoblasts (examples) Their edges are discrete because they do not have cytoplasmic arms sticking out. Prolymphocytes are smaller, but with more, cytoplasm than small lymphocytes and larger, paler nuclei. Macrophages have large nuclei surrounded by substantial amounts of pink cytoplasm with a cluttered appearance.
Small lymphocytes dominate the corona but are scattered in the germinal centers as well. You also may come across a plasma cell or two (described below). You can also find the occasional lymphoblast or prolymphocyte differentiating into a small lymphocyte. The supporting reticular cells have elongate nuclei.
Turning to the cells in the medulla, the best place to look at the cells is where their density is low (illustration). The commonest cells are small lymphocytes, reticular cells and macrophages (illustration). . There are also variable numbers of plasma cells, monocytes, lymphoblasts and developmental intermediates. Do not spend too much time looking for them. We will see them better on later slides
One other important structure of the lymph node is the tall postcapillary venule (TPCV) also called a high endothelial venule (second example). These vessels are lined by specialized cuboidal epithelium (sometimes even described as columnar) instead of the usual squamous variety. They are located in the paracortex. Small arterioles enter a lymph node at the hilus, course outwards usually along medullary trabeculae, and become capillaries that loop to the surface and back. The capillaries collect into venules in the paracortex and the first segments of the venules have tall epithelium. After finding several examples, see if your partners remember their function. Then review the pathway of lymphocyte circulation that the TPCV's are involved in.
D-33 Lymph node (reticulum stain)
This slide has been stained for reticular fibers. It shows the organization of the lymph node to advantage, even though the cells barely show up. You can locate the lymph nodules, and even their germinal centers, the capsule with its underlying sinuses, trabeculae, medullary cords and sinuses and the vessels in the hilus (illustration). All of these structures are mapped out in reticular fibers. In fact, it is the stroma - the reticular cells stretched out along these fibers - that causes the various parts of the organ to form where they do. Carefully examine the reticular fibers at high power. See that they form a supporting reticulum to house wandering cells. Also find a place which shows that the reticular fibers are continuous with coarse Type I collagen fibers of the trabeculae and capsule (illustration).
D-35 Palatine tonsil (H&E)
The palatine tonsil is a specialization
of the mucosa along the pharynx. It has three components (illustration):
| 1) | a stratified squamous epithelium in various stages of infiltration by lymphocytes | |
| 2) | a lamina propria choked with small lymphocytes and peppered with lymphatic nodules | |
| 3) | dense connective tissue forming more or less of a capsule around the tonsils |
Tonsils are structures with huge numbers of lymphocytes. These cells are drawn in from the blood through tall postcapillary venules, similar to those you saw in the lymph node. The lymphocytes wander around here waiting to be stimulated by antigens that diffuse in from the decaying debris in the crypts. Many invade the epithelium; so much so that in some places you have trouble recognizing the epithelium at all (illustration). At high power such epithelium looks like dense aggregations of lymphocytes with anomalous large pale nuclei scattered among them (illustration). Observe that tonsils rest on a sheet of dense connective tissue (illustration). A good surgeon can dissect out an inflamed tonsil along this capsule with a minimum of fuss.
This tonsil has many plasma cells. Be sure that you can tell them from the small lymphocytes (illustration). A good place to look for them is right under the epithelium. The spaces between collagen fibers of the capsule happen to be other favorable places because the cells can be seen here without bunches of other cells on top of them (illustration). Make a point of looking at a number of examples until you can recognize them by their cartwheel nucleus, their basophilic cytoplasm and their Golgi body next to the nucleus. Look for the Golgi as a clear or pale area which comes and goes as you focus up and down through it, (especially at high magnification with a narrow depth of focus). You may have to look at different cells to find perfect example of these various characteristic features of the plasma cells. I suggest using both your 40X and then oil immersion objectives.
D-111 Appendix (H&E)
The appendix represents a blind outpocketing off of the large intestine. Again, your textbook has a diagram of this organ to orient you to its overall structure. The mucosal and submucosal layers have been entirely converted into a mass of lymphoid tissue (illustration). Note the analogy in structure between appendix and tonsil. Both contain enormous depots of lymphoid tissue separated from a dirty, stagnant space by an epithelium infiltrated with lymphocytes.
Optional: The epithelial cells of the glands directly adjacent to the nodules become specialized as "M" cells (illustration). They specifically transport antigens across the epithelium and also draw lymphocytes into their bosom. You may be able to find some regions of these specialized epithelial cells, characterized by 1. Close proximity to a nodule, 2. Lack of goblet cells nearby, and 3 lymphocytes infiltrated between the epithelial cells. If you can not find them do not worry too much, just remember later on in your immunology course that you did look for M cells as specializations of the epithelium in the appendix (and that they also occur in Peyer's patches).
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Optional slides
D-96 Posterior tongue
Tonsillar tissue occurs at the junction region between the anterior and posterior portions of the tongue. If you are lucky, your slide D-96 has been cut from the block that includes portions of lingual tonsils. If your tissue does not have any tonsillar tissue beat on the course chairman and, hopefully, he will have another slide to exchange for yours. Lingual tonsils have the same general structure as the palatine variety. One specialization, not shown well on your sections, is that glands empty into the crypts of lingual tonsils. This keeps them from getting infected as frequently as the palatine tonsils. It is rare to have to surgically remove infected tonsils from the tongue.
D-76 Bronchus with lymph node (H&E)
Hold this slide up and locate the lymph node as the dark blue ball of tissue about 3 mm across. It lies along a bronchus which we shall return to in a future laboratory exercise. The lymph node in this section is of interest because it drains pulmonary tissue. Many macrophages have phagocytized inhaled particles of dust. You can pick these cells out by the yellowish-black gunk in their cytoplasm (illustration). Some of these macrophages ingested particles that got washed out of the lung and into the lymph. Others may actually have phagocytized particles in the lung and then migrated in the lymph to this local lymph node to rest in peace. Note the distribution of the macrophages. They are located mainly in the sinuses, especially in the medulla and are rare in the nodules (illustration). This makes sense - at least it does to me.
Hold this slide up to the light. You see two small dark purple spots at the edge of the tissue. Try to identify them under your microscope.