Chapter 59: The Spleen

Key Points

Embryology, Anatomy, and Function of the Spleen

The spleen arises as a mesenchymal protrusion into the left dorsal mesogastrium that is first seen at the 6 to 8 mm stage of embryogenesis. Incomplete fusion in up to 20% of individuals may result in accessory spleen formation. More unusual events result in asplenia or polysplenia. In human abdominal development, most asymmetric organs began in the midline or were at the outset bilateral, with subsequent suppression of 1 side. The spleen is the exception. It starts on the left and stays there in most cases. As it develops, the spleen lies in the left upper quadrant of the abdomen and has 6 named ligamentous attachments: gastrosplenic, splenorenal, splenophrenic, splenocolic, splenopancreatic, and the presplenic fold. The peritoneum envelops the splenic vessels and the distal pancreas along the concavity of the splenic surface. The splenic artery is a celiac axis branch and travels along the cephalodorsal pancreatic surface. There are usually 2, and sometimes 3 lobular arteries arising from the main splenic artery. These vessels divide further in the parenchyma and are typically end-arteries, although in 10% of specimens there are some cross-connections. The splenic vein accompanies the artery within a number of variant patterns before it joins the superior mesenteric vein to form the portal vein. The “odd number” mnemonic of Harris (1,3,5,7,9,11), as reported in the textbook by Last on anatomy, is a useful tool to recall normal splenic dimension and location: the spleen measures 1 × 3 × 5 inches, weighs 7 ounces and abuts ribs 9 through 11.

The ultrastructure of the spleen is key to its function. The 2 main divisions are termed red and white pulp (Table 59-1). Splenic red pulp filters blood and culls out defective, senescent, or diseased erythrocytes. The red pulp also filters bacteria, parasites, and particulate matter. These functions are supported by the presence of opsonins (fibronectin, properidin, tuftsin) that are produced in the spleen. These agents help activate the complement cascade and induce granulocyte and macrophage phagocytosis. The white pulp contains lymphoid follicles (B cells) and periarteriolar lymphoid sheaths (PALS), which are populated largely by T-lymphocytes. In PALS, antigen presentation by dendritic cells and macrophages to CD4+ T-helper cells activates cell-mediated immune response. Antigen in germinal centers is processed by antigen-presenting cells to B-lymphocytes, resulting in antibody production. The spleen is the largest source of IgM.