The University of Western Australia

Department of Anatomy and Human Biology


HA235 - Histology - Lymphoid Tissues II


Topics Lab Guides and Images
Spleen Spleen - H&E, reticulin
Tonsils Palatine Tonsil - H&E
GALT - Appendix Appendix - H&E

Recommended readings from Histology, A Text and Atlas, M.H. Ross et al, 3rd edition


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 the adult as other organs take over its functions if it is removed.

The spleen is surrounded by a capsule of dense connective tissue from which branched trabeculae 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 either 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.


Lab: Slide 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 find - try it, but 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).


Slide SPLEEN T.S. 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).



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

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 consequent increase of the surface area is one way to facilitate the contact of antigens with the cells of the immune system. 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. postcapillary (high-endothelial-) 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.


Lab: Slide 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 postcapillary (high-endothelial-) venules in the lymphoid tissue. A little "aside" about terminology (strictly optional) ...
Sketch the organisation of the tissue at low magnification.



Appendix

The basic structure of the vermiform appendix has already been described under HA 235 - Histology - Gastrointestinal tract.

GALT (you should be able to decipher this acronym) is found throughout the gastrointestinal tract. However, the most prominent accumulations occur in the ileum in the form of Peyer's patches and in the appendix.

In the appendix, lymphoid tissue, which consists of B- and T-lymphocytes, may extend from the lamina propria into the submucosa and fill a substantial part of it. Some crypts penetrate deep into the lymphoid tissue. 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 low 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.

An extreme proliferation of lymphocytes (lymphoid hyperplasia) as a consequence of bacterial or viral stimulation may lead to the obstruction of the lumen of the appendix and thereby cause appendicitis, but this is only one of many possible causes.

The term "Peyer's patches" is usually used in conjunction with the GALT of the ileum but may be applied for GALT throughout the small and large intestines.


Lab: Slide APPENDIX CAT H&E. Crypts are more frequent than we would expect them to be in the human appendix. Lymphoid tissue is present beneath the epithelium around the entire circumference of the appendix and does (try to identify the muscularis mucosae) extend into the submucosa. Occasionally you will see crypts extending deep into the lymphoid tissue. Compare the epithelium embedded in lymphoid tissue with that closer to the lumen of the appendix. Goblet cells are rare or absent and the epithelium appears low columnar. Small, darkly stained nuclei seem to be embedded in the epithelium - they represent T-lymphocytes and other immune cells in intraepithelial pockets.
Draw a spot where you can see "normal" and specialised epithelium and lymphoid tissue.