The spleen is an unpaired visceral organ located on the left side of the abdominal cavity. Composed mainly of lymphoid tissue, it is part of the immune system as it is the site where lymphocytes (T and B) develop from stem cells from the bone marrow. It is highly vascular as it is supplied by the splenic artery, which arises from the celiac trunk and drained by the splenic vein, which empties into the hepatic portal vein.
The spleen lies in the left hypochondriac region, between the gastric fundus and the diaphragm. It looks like a coffee bean, with one surface being convex (diaphragmatic) and the other concave (visceral). It is 12 cm long, 7 cm wide, and 4 cm thick, weighing between 100 and 150 grams in a healthy adult. However, the size of this organ varies from individual to individual and with his/her physiological and inflammatory conditions. The splenic artery enters the spleen at the hilum (on the concave side) and immediately into trabecular arteries.
Histology
The spleen is enclosed in a dense, irregular, connective tissue capsule, which contains smooth muscle cells. This capsule is externally covered by mesothelium (serous membrane). Connective tissue trabeculae project inwardly from the inner wall of the capsule to form a complex network of connective tissue elements in the parenchyma of the organ.
The trabeculae divide the spleen into separate lobules, which are filled with red and white lymphoid pulp. The red pulp is rich in erythrocytes (red blood cells) and tortuous vascular channels. White pulp, on the other hand, is composed of collections of lymphocytes similar to the primary and secondary nodules of lymph nodes. In the white pulp, there are periarterial lymphatic sheaths, which are dense aggregate of small lymphocytes in cylindrical clusters around the central arteries.
Reticular cells and reticular fibers, which are similar to those in lymph node, can be found throughout most of the spleen parenchyma. These cells and fibers form a scaffolding which support both red and white pulp. They are located around central arteries and arranged in concentric layers which support the periarterial lymphatic sheath.
Splenic sinuses and chords are also found in the spleen. Sinuses are peculiar vascular channels, 40 μm in diameter, being lined by tapered elongated cells parallel to the long axis of the sinus.
Function
The spleen is a complex filter and also the site of development of lymphocytes. In this organ, aged and damaged erythrocytes are examined, destroyed, and removed from circulation. Meanwhile, the white pulp of the spleen has an immune surveillance function, much like the function of lymph nodes. Thus, after entering the white pulp, B lymphocytes concentrate in secondary nodule, while T lymphocytes concentrate in the peryarterial lymphatic sheath.
Below, picture of visceral organs, showing the spleen on the left side of abdominal cavity.
