Chapter 060. Enlargement of Lymph Nodes and Spleen (Part 4)

SplenomegalyStructure and Function of the Spleen The spleen is a reticuloendothelial organ that has its embryologic origin in the dorsal mesogastrium at about 5 weeks gestation. It arises in a series of hillocks, migrates to its normal adult location in the left upper quadrant (LUQ), and is attached to the stomach via the gastrolienal ligament and to the kidney via the lienorenal ligament. When the hillocks fail to unify into a single tissue mass, accessory spleens may develop in around 20% of persons. The function of the spleen has been elusive. Galen believed it was the source of "black...
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Chapter 060. Enlargement of Lymph Nodes and Spleen (Part 4) Chapter 060. Enlargement of Lymph Nodes and Spleen (Part 4) Splenomegaly Structure and Function of the Spleen The spleen is a reticuloendothelial organ that has its embryologic origin inthe dorsal mesogastrium at about 5 weeks gestation. It arises in a series ofhillocks, migrates to its normal adult location in the left upper quadrant (LUQ),and is attached to the stomach via the gastrolienal ligament and to the kidney viathe lienorenal ligament. When the hillocks fail to unify into a single tissue mass,accessory spleens may develop in around 20% of persons. The function of thespleen has been elusive. Galen believed it was the source of black bile ormelancholia, and the word hypochondria (literally, beneath the ribs) and the idiomto vent ones spleen attest to the beliefs that the spleen had an importantinfluence on the psyche and emotions. In humans its normal physiologic rolesseem to be the following: 1. Maintenance of quality control over erythrocytes in the red pulp by removal of senescent and defective red blood cells. The spleen accomplishes this function through a unique organization of its parenchyma and vasculature (Fig. 60-1). 2. Synthesis of antibodies in the white pulp. 3. The removal of antibody-coated bacteria and antibody-coated blood cells from the circulation. Figure 60-1 Schematic spleen structure. The spleen comprises many units of red andwhite pulp centered around small branches of the splenic artery, called centralarteries. White pulp is lymphoid in nature and contains B cell follicles, a marginalzone around the follicles, and T cell–rich areas sheathing arterioles. The red pulpareas include pulp sinuses and pulp cords. The cords are dead ends. In order toregain access to the circulation, red blood cells must traverse tiny openings in thesinusoidal lining. Stiff, damaged, or old red cells cannot enter the sinuses. (Topportion of figure from CA Janeway et al: Immunobiology, 5th ed., New York,Garland, 2001; bottom portion of figure from RS Hillman, KA Ault: Hematologyin Clinical Practice, 4th ed. New York, McGraw-Hill, 2005.) An increase in these normal functions may result in splenomegaly. The spleen is composed of red pulp and white pulp, which are Malpighisterms for the red blood–filled sinuses and reticuloendothelial cell–lined cords andthe white lymphoid follicles arrayed within the red pulp matrix. The spleen is inthe portal circulation. The reason for this is unknown but may relate to the fact thatlower blood pressure allows less rapid flow and minimizes damage to normalerythrocytes. Blood flows into the spleen at a rate of about 150 mL/min throughthe splenic artery, which ultimately ramifies into central arterioles. Some bloodgoes from the arterioles to capillaries and then to splenic veins and out of thespleen, but the majority of blood from central arterioles flows into themacrophage-lined sinuses and cords. The blood entering the sinuses reenters thecirculation through the splenic venules, but the blood entering the cords issubjected to an inspection of sorts. In order to return to the circulation, the bloodcells in the cords must squeeze through slits in the cord lining to enter the sinusesthat lead to the venules. Old and damaged erythrocytes are less deformable and areretained in the cords, where they are destroyed and their components recycled.Red cell inclusion bodies such as parasites (Chap. 203 and e18), nuclear residua(Howell-Jolly bodies, Fig. 58-6), or denatured hemoglobin (Heinz bodies) arepinched off in the process of passing through the slits, a process called pitting. Theculling of dead and damaged cells and the pitting of cells with inclusions appear tooccur without significant delay since the blood transit time through the spleen isonly slightly slower than in other organs. The spleen is also capable of assisting the host in adapting to its hostileenvironment. It has at least three adaptive functions: (1) clearance of bacteria andparticulates from the blood, (2) the generation of immune responses to certainpathogens, and (3) the generation of cellular components of the blood undercircumstances in which the marrow is unable to meet the needs (i.e.,extramedullary hematopoiesis). The latter adaptation is a recapitulation of theblood-forming function the spleen plays during gestation. In some animals, thespleen also serves a role in the vascular adaptation to stress because it stores redblood cells (often hemoconcentrated to higher hematocrits than normal) undernormal circumstances and contracts under the influence of β-adrenergicstimulation to provide the animal with an autotransfusion and improved oxygen-carrying capacity. However, the normal human spleen does not sequester or storered blood cells and does not contract in response to sympathetic stimuli. Thenormal human spleen contains approximately one-third of the total body plateletsand a significant number of marginated neutrophils. These sequestered cells areavailable when needed to respond to bleeding or infection.