Blood Vessels - Spleen

Blood Vessels

Blood vessels enter the lymph nodes through the hilus and travel initially in the connective tissue trabeculae that extend from the hilus into the parenchyma of the lymph node. They continue in the medullary cords towards the cortex and give off capillaries to the surrounding tissue as they do so. High-endothelial venules (or postcapillary venules) in the deep cortex have a characteristic low cuboidal epithelium - quite unlike the squamous epithelium that we usually would expect to see. Lymphocytes, which reach the lymph node via the blood stream, may migrate through this epithelium as part of their recirculation. Larger venules accompany the arteriolar branches as they leave the lymph nodes.
Lymph node, rabbit - H&E
The subcapsular and cortical sinus system was hardly (if at all) visible in the slides I looked at. Identify the connective tissue capsule and trabeculae, cortex and medulla of the lymph node, lymph nodules with germinal centres, medullary cords and postcapillary venules.

Lymph node - H&E, carbon injected This slide illustrates the distribution of macrophages in lymph nodes - take a quick look at them. Note that most of them are located in the paracortex and medulla.

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 divides into trabecular arteries, 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.
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.