LYMPHOID TISSUES OF THE IMMUNE S

Lymphoid tissue is a specialized form of reticular connective tissue. The basic structure of the stroma is reticular collagen fibers sheathed entirely by fibroreticular cells (FR cells, also called "reticular cells") and a few other elements such as CT trabeculae and blood vessels. The thymus is a notable exception to this stromal structure. Lymphoid parenchyma tissue consists of large numbers of mostly free cells packed in among the elements of the stroma. The parenchymal cells are primarily lymphocytes and their precursors.

Lymphoid tissue is defined as being loose, dense, or nodular. Loose (diffuse) lymphoid tissue is found throughout many other tissues and consists of aggregations of lymphoid cells in a reticular stroma, the stroma predominating. Dense tissue is similar except that the lymphoid cells predominate. Nodular tissue is composed of very dense aggregations of lymphoid cells in the form of follicles. Some follicles have germinal centers, sites of active production of B-lymphocytes. Most lymphoid tissue is rather disorganized and occurs within other tissue types. Some is found in particular organs and glands such as lymph nodes, spleen, thymus, and bone marrow.

LYMPH NODES function as filters and sites of immunologic surveillance of lymph. All lymph passes through multiple lymph nodes and the contents are repeatedly exposed to immune system cells. The node structure has evolved to maximize the efficiency of this interaction. The bean shaped nodes are composed of cortex and medulla within a DICT capsule. Afferent (incoming) lymph vessels branch repeatedly upstream of a node. The branches pierce the connective tissue capsule and the lumen contents enter the marginal (subcapsular) sinus. Lymph percolates through the marginal, cortical, and medullary sinuses, exiting the node in the efferent vessel at the indented hilus. The hilus is also the site at which blood vessels enter and leave the node. The capsular CT extends finger-like trabeculae of CT deep into the cortex.

The reticular-fiber-rich stroma is attached to the capsule and the trabeculae. The parenchyma is organized as very dense follicles (nodules) and less-dense internodular tissue in the cortex. The inner, deep cortex is known as the paracortex. The medullary parenchyma is organized around the stroma as clumps of moderately loose lymphoid tissue interspersed among a large number of medullary sinuses.

Lymphocytes (mostly T-cells) enter and leave the lymph node by traversing the walls of PCVs near the corticomedullary border. These PCVs have a tall, nearly columnar, endothelium. The endothelial cells' apical domains have receptors that bind to IMPs of the circulating T-lymphocytes, increasing specificity of cell transfer from blood to lymphoid tissue.

The TONSILS are aggregations of nodular and internodular tissue in the walls of the pharynx. Unlike lymph nodes, they are not located along the course of lymphatic vessels. Tonsils form an incomplete ring around the pharynx and are of three types: palatine tonsils are in the lateral walls of the oral pharynx (near the palate), lingual tonsils are at the base of the tongue, and pharyngeal tonsils (adenoids) are at the roof of the nasopharynx. Tonsils consist of dense and diffuse lymphoid tissue penetrated from the pharynx by epithelium-lined crypts. The crypts are labyrinthine tubules that are entered by things such as bacteria and other small particles in the mouth. Movement of particles down the crypts allows contact with the tonsil immune system cells. Palatine and lingual tonsil crypts are lined by stratified squamous (oral) epithelium while the pharyngeal tonsils are lined by ciliated pseudostratified (respiratory) epithelium. There is a CT capsule surrounding the tonsil. There are efferent lymph vessels but no afferent ones.

The SPLEEN is the largest lymphoid organ and consists of a CT capsule from which CT trabeculae arise to penetrate the organ. The spleen is organized into two distinct regions: red pulp and white pulp. White pulp, also called periarterial lymphoid sheath (PALS), consists of dense follicular lymphoid tissue surrounding a central artery (or arteriole). The white pulp has structure very similar to lymph node cortex. Red pulp is a complex array of reticular CT stroma, numerous blood sinuses, free blood cells, and loose lymphoid tissue; it is arranged as cords and clumps of lymphoid cells (Billroth's cords) among venous sinuses. Red pulp structure is very similar to bone marrow.

Blood from white pulp central arterioles drains into Billroth's cords and into venous sinuses. The splenic venous sinuses have discontinuous endothelial walls and incomplete basement membranes so the blood plasma and cells can leave and re-enter the sinuses repeatedly. While in Billroth's cords the blood is filtered through the stroma of FR cell-sheathed fibers and it comes in contact with numerous phagocytes and other antigen presenting cells (APCs).

The THYMUS is located under the sternum and reaches its greatest size during puberty and involutes thereafter, being replaced by mostly adipose LCT. It consists of a CT capsule that extends sheets of "interlobular" CT into the organ that create apparent lobes. Arising from the CT sheets, flat CT septae extend to the border between the cortex and medulla. Unlike the other lymphoid organs, the thymic stroma consists of a cytoreticulum created by epithelial cells (epithelioreticular cells; ER cells). The only CT present is that of the interlobular sheets, the septae, and the tunica adventitia of blood vessels. During development, lymphoblasts collect among epithelial cells and force them apart, forming a stroma of attenuated epithelial cells with long processes connected to each other by desmosomes, and a parenchyma of lymphoid cells and macrophages. Thymus tissue is not organized into follicles or lobes but is rather in a dense (cortex) or diffuse (medulla) arrangement. It is the source of new T-cell lymphocytes. T-cells are formed from stem cells in the outer cortex and mature while migrating toward the medulla. Mature T-cells enter the blood by crossing normal-morphology PCV walls near the corticomedullary border.

Lymph Node (42). First find the major features like the CT capsule and trabeculae, the cortex and medulla. Between the capsule and cortical parenchyma find the subcapsular(marginal) sinuses and identify the sinus epithelium – what kind is it? Does it cover both the cortical parenchyma and the capsular CT? In the sinus lumen identify fibroreticular cells (FR cells) by their irregularly ovoid to elongated, pale nuclei and numerous processes. Also find macrophages (mostly fixed) by their large size and eosinophilic, occasionally granular cytoplasm and lymphocytes.

In the cortex find follicles with and without germinal centers (what does it mean if a lymph node has few to no germinal centers? Or lots?). In the dark portion of the follicles find abundant lymphocytes (what type?), macrophages, and FR cells. In the pale portion (why is it pale?) find lymphocytes, macrophages, and FR cells. Why are there so many macrophages? Examine the internodular cortical parenchma: are the same cell types present in the same proportions as they are in the follicles with germinal centers? In the deep cortex (paracortex) find post-capillary venules with their distinctive tunica intima. Try to find lymphocytes crossing the PCV wall. What type of lymphocyte would these most likely be? Note that the blood vessels in the cortex and medulla, and others in the paracortex, are of normal morphology.

In the medulla identify cords and sinuses. Do the sinuses carry blood or lymph? In the cords find lymphocytes, FR cells, macrophages, and plasma cells, large, round cells with round nuclei and pale basophilic cytoplasm. How can you tell the difference between macrophages and plasma cells?

Reticular tissue (22). These trichrome-stained lymph nodes should display everything you found in #42 above but with the CT stained to stand out.

Reticular tissue, silver stain (DEMO). Make sure you have a section of lymph node and not spleen. The black reticular fibers clearly demarcate things like sinuses, follicles, medullary cords, etc. Compare with the trichrome slide.

Tonsil, Palatine (Lingual) (88). Look for stratified squamous oral epithelium lining the crypts. It is very difficult to find on these slides. Usually the entrance to the crypts from the oral cavity is not present; try to find small to medium size segments of crypts in the interior of the tonsil. The epithelium may be very thin. Note the CT capsule and penetrating trabeculae. Find follicles (with germinal centers?) and internodular, loose lymphoid tissue. Look for lymphocytes, lymphoblasts, FR cells, macrophages, and plasma cells in the appropriate locations. If your tonsil slide has no crypts, get a DEMO slide. They at least have oral epithelium.

Tonsil, Pharyngeal (Adenoid) (89). Very similar to palatine tonsil except that respiratory epithelium is found on its surface and lines the crypts. The defining epithelium type is often patchy and very hard to find. Deep in the tonsil the crypt-lining epithelium becomes stratified squamous. If you can’t identify respiratory epithelium in the crypt lining, look at the DEMO slide of pharyngeal tonsil.

Thymus (83). This is a pre-involution thymus from an infant. First identify major structural features such as the capsule, “interlobular” CT, septal CT, lobes of cortex and medulla, and Hassal’s corpuscles in the medulla. In the cortical parenchyma identify lymphocytes, lymphoblasts (especially in the deep cortex, towards the capsule), macrophages, and epithelioreticular cells (ER). Find the same cells in the medulla; are they all present and in the same relative proportions as in the cortex? Why? How can you distinguish a macrophage from an ER cell?

The flow of blood through the thymus is quite organized. Find each of the blood vessels in order – arteries in the capsular CT, in inerlobular CT, then in septal CT, arterioles and capillaries at the ends of septae, and, at the border of cortex and medulla, capillaries in cortex and medulla, PCVs near the border, veins in septal CT, interlobular CT, and finally veins in capsular CT.

Thymus, adult (DEMO). An involuted thymus. Note that most of the parenchyma has been replaced by adipose CT. How can you survive with such a small thymus?

Spleen, H&E (73). Find capsular CT and trabeculae extending into the interior of the organ. Note the trabecular blood vessels. Identify white and red pulp. In the white pulp find the central artery, the marginal zone, and germinal centers; are B or T-cells found near the marginal zone? Try to find small arteriolar branches extending into the red pulp from the white pulp. These would be penicillar arterioles.

In the red pulp find Billroth’s cords, venous sinuses, and pulp veins. Note the abundance of blood cells in Billroth’s cords making it difficult to distinguish from the sinus lumens. In the cords find macrophages, maybe plasma cells, and FR cells in addition to blood cells.

Trace the route of blood flow: capsular arteries to trabecular arteries to PALS central arterioles to penicillar arterioles to venous sinuses to pulp veins to trabecular veins to capsular veins.

Spleen, trichrome (74). Very good distinction between connective tissue and the rest of the spleen. Hard to see red and white pulp.

Spleen, silver (DEMO). Find the “ladder” pattern of venous sinus discontinuous basement membrane reticular fibers.

Lymph vessel valve, wholemount (DEMO). The valve consists of a fold od endothelium and SECT. The downstream expansion is the ‘sinus’.

Peyer’s Patches (32). If you can’t find the PP aggregations of lymphoid cells in #32, try the DEMO slide of ileum and/or jejunem. Look for large lymphoid nodules with germinal centers in the wall of the intestine. Villi are usually absent over Peyer’s Patches. You may have to examine several slides to find a good patch.

Diagram of a pig lymph node. Pigs have the most lovely classic lymph nodes. Human nodes look disordered by comparison.

Lymph node, trichrome to accentuate CT. Medulla is far left, cortex is the arc of dark reddish tissue in the middle, and the capsular CT is right.

Lymph node, H&E, medium mag. At left is the outer edge of a node. See capsular CT forming a trabecula of CT that penetrates into the cortex from left to right. There are germinal centers above and below the trabecula. Right picture -- the medulla of a LN showing medullary sinuses and parenchyma

The next 3 pictures are of a trichrome-stained lymph node at low, medium, and high mag. The CT is blue, parenchyma reddish.

Trichrome LN, med mag. Capsular CT forms a trabecula surrounded by cortical sinus.

Trichrome LN, high mag. Marginal (subcapsular) sinus leading into cortical sinus around a trabecula. The thin filaments of blue in the sinuses are FR-sheathed reticular fibers. Which portion is stained blue, the FR cells or the reticular collagen III fibers?

LN medulla, trichrome, high mag. Distinguish among parenchyma, sinus, and stroma.

An old diagram of lymphoid stroma. The black lines represent reticular fibers, the cells are fibroreticular.

Scanning EM of lymphoid stroma. The reticular fibers are covered by thin FR cell processes.

LN sinus, H&E. FR cells tend to have pale nuclei and little visible cytoplasm. Most of the rest of the cells are dark-nuclei lymphocytes.

LN sinus, H&E. FR cells, lymphocytes (probably T-cells), and a venule at the left.

Cords and clumps of parenchyma in LN medulla. H&E. The clearer areas are sinuses.

TEM of a post-capillary venule in a node. Note how thick the endothelial cells are. Most cells outside the venule are lymphocytes with a few FR cells.

The hilus of a node, H&E. The incoming artery is left, an efferent (outgoing) lymph duct is right-center and contains lots of lymphocytes.

Spleen, H&E, low mag. Three large elements of white pulp. Capsular and trabecular CT. Int he center is a longitudinal section of a large splenic pulp vein, a large vessel that drains blood out of the spleen

Spleen, H&E. Red pulp (L) and white pulp (R). The white pulp is roughly ovoid; the central arteriole is offset to the left.

Spleen, H&E. Red pulp surrounds a X-sec of white pulp containing a germinal center. Find the central artery.

Longitudinal section of white pulp. WP is the lower half of the picture. Find the central artery.

Spleen, Fast Green and Eosin, high mag. Bilroth's Cords and venous sinuses of red pulp,

Thymus, H&E, low mag. Capsular and interlobular CT. Dark cortex, lighter medulla.

Thymic cortex and medulla, H&E, high mag. You can see small blood vessels inside thin septal CT.

Corticomedullary border of thymus, H&E, high mag. The paler stain of medulla is due to a greater number of ER cells and macrophages. The darker stain many of the cortex is due to the high concentration of maturing T-cells.

Thymus, H&E, low mag. Left -- septal CT, cortex, and medulla; there's a Hassal's corpuscle in the medulla. Right -- mostly medulla with a few Hassa's corpuscles..

Palatine tonsil with germinal centers and crypt stratified squamous epithelium (center). H&E, low mag.

Tonsil with crypt and germinal centers. Does it look to you like pharyngeal or palatal epithelium?