|  | Home
| School of Anatomy and Human Biology - The University of Western Australia
| Blue Histology - Lymphoid Tissues II

Topics | Lab Guides and Images
Spleen | Spleen - H&E, reticulin
Mucosa-Associated Lymphoid Tissue Tonsils Gut-Associated Lymphoid Tissue | Palatine Tonsil - H&E
Peyer's Patch - H&E

#### Additional Resources

These links will open a new browser window.

Large Images
  Search the Large Images page with the keywords: lymphoid tissue, spleen, tonsil, GALT, gut-associated lymphoid tissue, Peyer's patch, red pulp, white pulp, central artery, splenic sinuses, splenic cords, tonsilar crypt or reticulated stratified squamous epithelium.
VScope
  Magnification & Stage Simulation: spleen, human, H&E
  Focus & Stage Simulation: spleen, reticular connective tissue, reticulin
Self Assessment
  Choose subject area "lymphoid organs and tissues" on the Quiz page. This subject area covers the Lymphoid Tissue I & II pages of this site.

### Spleen

The spleen is, like the lymph nodes, a discriminatory filter. Unlike the lymph nodes, the spleen is inserted into the blood stream. The spleen clears the blood of aged blood cells and foreign particles and is the site of immune reactions to blood-borne antigens. The spleen is not essential to life in adult individuals. Other organs can take over its functions if the spleen is removed.

The spleen is surrounded by a capsule of dense connective tissue from which branched trabecula extend into the parenchyma of the spleen (sounds familiar). The parenchyma of the spleen is termed the pulp of the spleen. Most of the pulp of a fresh, unfixed spleen is a soft, dark red mass, the red pulp. It consists of large, irregular, thin-walled blood vessels, the splenic sinusoids, interposed between sheets and strands of reticular connective tissue, the splenic cords (of Billroth). Within the red pulp small, oval or rounded greyish white areas, the white pulp, is formed by lymphoid tissue.

Branches of the splenic artery (Where does it enter the spleen?) divides into trabecular arteries (Where are they found?), which enter the white pulp, where they are called central arteries. Branches of the central artery almost all divide into smaller vessels in the marginal zone, i.e. the border between the red and white pulp. Fine branches of the central artery - penicillar arteries (cuboidal epithelium) - branch again to form arterial capillaries, which, as they exit the white pulp, are surrounded by a sheath of phagocytotic cells and reticular fibres. They are now called sheathed arteries. From here, the blood enters the red pulp. Sheathed arteries may empty the blood which they carry directly into the splenic sinusoids (closed circulation, about 90% in cats) or into the reticular connective tissue of the splenic cords (open circulation). Macrophages are, in addition to reticular cell, the main resident cell population of the splenic cords.

Blood cells which are emptied into the splenic cords re-enter the blood vessels through the endothelium of the sinusoids. The endothelial cells are elongated (in cross section they may appear cuboidal) and oriented along the long axis of the sinusoids. The endothelium of the sinusoids has no junctional complexes and its basement membrane is incomplete (forming narrow circular bands around the endothelial cells with large intervening fenestrations). Macrophages ingest aged erythrocytes, platelets and other particulate matter as they pass through the splenic cords.

The composition of the plasma membrane of erythrocytes changes as the cell ages. It is thought that these changes eventually expose erythrocyte senescence antigens, which bind blood-borne antibodies and thus tag the erythrocyte for removal by macrophages. Erythrocyte removal is also one function of the resident macrophages of the liver, although splenic macrophages take care of most of the job.

The sinusoids continue into the veins of the pulp, which empty into thin-walled trabecular veins, which eventually coalesce to form the splenic vein.

The white pulp surrounds the central arteries as a periarterial lymphoid sheath (PALS). Lymphocytes of the PALS are likely to be T-lymphocytes. In addition, we see macrophages and plasma cells in the PALS. Lymph nodules, formed by B-lymphocytes, are present along the course of the central arteries. The central arteries are typically located in the periphery of the nodule.

Spleen, human - H&E
Find a place close to the capsule where you can identify trabeculae, white pulp (possibly a nodule with a peripherally placed central artery), and red pulp. Good penicillar and sheathed arteries are very hard to identify - do not despair if you (or the demonstrator) cannot find them. As usual, it is easiest to identify macrophages by accumulations of particulate matter in their cytoplasm, which often will represent disintegrating erythrocytes.
Sketch your observations (the 10x objective should be fine).

Spleen, cat - reticulin
This slide illustrates the distribution of reticular fibres in the spleen. They often appear coarser in the red pulp, where they have a distinct, stranded organisation. The reticular fibres of the white pulp appear somewhat finer and, at times, they are arranged as concentric rings. The peripheral localisation of the central arteries in nodules is quite distinct. Occasionally you may see small rings of reticular fibres in (or close to) the periphery of the white pulp. These rings are likely to represent the reticular fibres surrounding sheathed arteries.
Sketch your observations (the 10x objective is sufficient).

###

### Mucosa-Associated Lymphoid Tissue

The mucosal lining of the alimentary canal and airways is in many ways specialised to facilitate the exchange of substances between the external environment and the body. Unfortunately, these specialisation do not just apply e.g. to components of the digested food but also pathogens. This is combined with excellent living conditions for bacteria in parts of the alimentary canal - in particular the ileum and the colon. Lymphoid tissue located beneath the mucosal epithelia, mucosa-associated lymphoid tissue (MALT), protects the body against pathogens that may enter the body via the mucosa. The importance of this task is reflected in the mass of the MALT, which corresponds to the combined mass of the other lymphoid organs and tissues.

The task that the immune cells of the MALT have to accomplish is different from that of other parts of the immune system. We do need a defense against pathogens, but it would not be a good idea to mount an immune response against components of the food. Immune cell activation therefore differs between the MALT and other lymphoid tissues.
This difference is mediated by different receptors expressed by immune cells of the MALT and by different substances which they release upon contact with an antigen. Because of their specific functions, immune cells of the MALT do not mingle with other immune cells. Epithelial cells of the vessels supplying the MALT express specific receptor which are recognized by MALT immune cells and allow their homing to the MALT during recirculation. Lastly, MALT plasma cells produce a secretable form of antibodies, immunoglobulin type A dimers, which can be taken up by epithelial cells and then released onto the epithelial surface.
Specialisation of MALT immune cells occur at the molecular level. In routine histological preparations, immune cells of the MALT look pretty much like immune cells of other lymphoid tissues.

Often MALT consists of small accumulations of lymphoid cells or one to a few lymph follicles beneath the epithelium and possibly extending into the submucosa. The tonsils and Peyer's patches are large accumulations of lymphoid tissue with associated specialisations of the epithelium.

#### Tonsils

The tonsils are accumulations of lymphoid tissue surrounding the openings of the digestive and respiratory tracts. The tonsils and smaller accumulations of lymphoid tissue, which may be found between them, are also called Waldeyer's ring.

Depending on their localisation we distinguish between

- palatine tonsils (THE tonsils), which are located in the lateral wall of the oropharynx and covered by a stratified squamous epithelium,
- lingual tonsils which are situated in the lamina propria at the root of the tongue and also covered by a stratified squamous epithelium, and
- pharyngeal tonsils (also called nasopharyngeal tonsils or adenoids) which are located in the upper posterior part of the throat (nasopharynx) and covered by a pseudostratified ciliated epithelium with goblet cells.

The tonsils do not have afferent lymph vessels. Efferent lymph vessels are present. Exposure to antigens relies on the contact of antigens with cells of the immune system across the epithelium which covers the tonsils. The epithelium of the palatine and lingual tonsils forms deep crypts into the lymphoid tissue, and the resulting increase of the surface area is one way to facilitate the contact of antigens with the immune cells. In addition, the epithelium may specialise in places to form an open meshwork of cells with an incomplete basal lamina (a reticulated epithelium) which allows the infiltration of the epithelium by lymphocytes and macrophages.
Tonsillar lymphoid nodules consist mainly of B-lymphocytes. Other areas are occupied by T-lymphocytes, activated B-lymphocytes and other cells of the immune system.

The tonsils share some histological features with lymph nodes:

1. cells in the tonsils are supported by a fine network of reticular fibres and
2. high-endothelial (postcapillary ~) venules function in the "homing" of circulating lymphocytes - this is actually a shared feature of all lymphoid tissues and organs.

The palatine tonsils are surrounded by a thick hemicapsule of connective tissue, which delimits them from the pharyngeal muscle and facilitates their removal in tonsillitis.

Tonsil, human - H&E
Identify the surface epithelium covering the lymphoid tissue of the tonsils, lymph nodules and tonsillar crypts. Have a look at the epithelium both inside and outside the crypts. Outside the crypts, the epithelium will typically look like ordinary stratified squamous epithelium. Inside the crypts, where cells of the immune system often invade the epithelium, it will be difficult to find the boundary between epithelium and lymphoid tissue. Tissue preservation is not that great, but with a little bit of patience you should be able to find high-endothelial venules in the lymphoid tissue.
Sketch the organisation of the tissue at low magnification.

#### Gut-Associated Lymphoid Tissue - GALT

Small accumulations of lymphocytes or solitary lymph follicles are found scattered in beneath the epithelium throughout the gastrointestinal tract. However, the most prominent accumulations occur in the ileum and appendix in the form of Peyer's patches. In the ileum, they form dome-shaped protrusions into the lumen. Beneath the epithelial lining of the domes, Peyer's patches extend from the lamina propria to the submucosa. Within Peyer's patches, lymph follicles with germinal centers are typically located deep in the submucosa.

The epithelium in contact with the lymphoid tissue is specialised to facilitate the contact of antigens with cells of the immune system. The epithelium appears columnar and contains cells with deeply invaginated basal surfaces - microfold cells or M-cells. Immune system cells can enter these invaginations (intraepithelial pockets) where they are exposed to materials which have been endocytosed by the epithelial cells and then released into the invaginations. Goblet cells are rare or absent in the epithelium which covers the domes.

Suitable Slides
  slides of appendix, ileum or Peyer's patches - H&E
  Slides of the appendix and ileum may be useful is there is no specially prepared slide of Peyer's patches. Extensive areas of lymphoid tissue are alway present in the appendix, but domes and specialised sections of epithelium may not. If you have a slide of the ileum hold it against the light and see if parts of the wall look darker than the rest. These parts are likely to contain lymphoid tissue and may show domes and/or specialised sections of epithelium.

Peyer's Patch, Ileum - H&E
If you hold the sections against a light surface the areas which are occupied by the lymphoid tissue should be readily visible - in H&E stained sections they appear darker than the remaining tissue. Look at these areas under the microscope. Identify the domes and their epithelial covering. The epithelium will be lower than the epithelium covering the villi and goblet cells are rare or absent. See if you can find intraepithelial pockets containing immune cells. Even if no clear pockets are present, some lymphocytes should appear scattered "over" the epithelium. In reality they will be located in the intraepithelial pockets. High-endothelial venules should be present in the lymphoid tissue beneath the domes but may be difficult to find. Next go further towards the submucosa and see if you can find lymph follicles.
Draw the epithelium covering the domes at high magnification. Include a scetch of the normal intestinal epithelium for comparison. Scetch the appearance of Peyer's patches at low magnification.

page content and construction: Lutz Slomianka
last updated: 30/01/04