The spleen is a specialized abdominal organ serving multiple functions in erythrocyte clearance, innate and adaptive immunity, and the regulation of blood volume. In general the spleen contains two structurally and functionally distinct components: white and red pulp. The white pulp of the spleen consists of secondary lymphoid tissue that provides an environment in which the cells of the immune system can interact with one another to mount adaptive immune responses to bloodborne antigens. The splenic red pulp contains macrophages that are responsible for clearing the blood of unwanted foreign substances and senescent erythrocytes, even in the absence of specific immunity. Thus, it acts as a filter for the blood.
The spleen is located within the peritoneum in the left upper quadrant of the abdomen between the fundus of the stomach and the diaphragm. It receives its blood supply from the systemic circulation via the splenic artery, which branches off the celiac trunk, and the left gastroepiploic artery.29 The blood returning from the spleen drains into the portal circulation via the splenic vein. Therefore, the spleen can become congested with blood and increase in size when there is portal vein hypertension (Chap. 56).
Approximately 10 percent of individuals have one or more accessory spleens. Accessory spleens are usually 1 cm in diameter and resemble lymph nodes. However, they usually are covered with peritoneum, as is the spleen itself. Accessory spleens typically lie along the course of the splenic artery or its gastroepiploic branch, but they may be elsewhere.30 The commonest location is near the hilus of the spleen, but approximately 1 in 6 accessory spleens can be found embedded in the tail of the pancreas, where they may be occasionally mistaken for a pancreatic mass lesion.31
The average weight of the spleen in the adult human is 135 g (range: 100 to 250 g). However, when emptied of blood it weighs only approximately 80 g. On autopsy of 539 subjects with normal spleens, there was a positive correlation between the spleen weight and both the degree of acute splenic congestion and the subject’s height and weight, but not with the subject’s sex or age.32
The splenic volume can be estimated by computed tomography (CT) of the abdomen.33 In one study, the splenic volume was calculated from the linear and the maximal cross-sectional area measurements of the spleen, using the following formula: splenic volume = 30 cm3 + 0.58 (the product of the width, length, and thickness of the spleen measured in centimeters).34 Using this formula, the mean value of the calculated splenic volume for 47 normal subjects was 214.6 cm3, with a range of 107.2 to 314.5 cm3. The calculated splenic volume did not appear to vary significantly with the subject’s age, gender, height, weight, body mass index, or the diameter of the first lumbar vertebra, the latter being considered representative of body habitus on CT.
The splenic volume also can be estimated by sonography, which provides good correlation with volumes measured by helical abdominal CT or actual volume displaced by the excised organ. In one study of 50 patients, the linear measurement by sonography that correlated most closely with CT volume was the spleen width measured on a longitudinal section with the patient in the right lateral decubitus position (correlation coefficient [r] = 0.89, p <0.001). There was also good correlation between splenic length measured in the right lateral decubitus position and CT volume (r = 0.86, p <0.001). In another postmortem analysis of 32 normal adult spleens, the ultrasonogram measurements of maximal height, width, and breadth of the spleen were compared with the actual volume displaced by the excised organ.35 The mean actual splenic volume was approximately 148 cm3 (±81 cm3 SD), whereas mean splenic volume estimated from ultrasonography was 284 cm3 (±168 cm3 SD). Despite the differences between the actual and estimated volumes, these investigators did find a roughly linear correlation between actual splenic volume and the estimated splenic volume measured by ultrasound. However, there may be operator-to-operator variation in measurement of the estimated splenic volume, making the use of sonography in longitudinal studies technically demanding.
The spleen has an open circulation, which lacks endothelial continuity from artery to vein. When isolated spleens are perfused in washout studies, erythrocytes that appear in the splenic vein appear to be flushed out from three compartments. The red cells that are flushed out first come from a compartment that presumably is formed by the splenic vessels. The erythrocytes that are flushed out next come from a second compartment, where they presumably are loosely held within the filtration beds. The erythrocytes that are flushed out last presumably were adherent to cells of the filtration beds. Although 90 percent of the blood flow passes through the splenic vessels, only approximately 10 percent of the total splenic red cells are found within this first compartment. The second compartment is perfused by 9 percent of the total inflow yet contains 70 percent of the splenic red cells. The last compartment is perfused by only 1 percent of the inflow but contains 20 percent of the splenic red cells.
These compartments reflect the anatomy of the spleen and its stroma. The stroma is composed of branched, fibroblast-like cells called reticular cells. These cells produce slender collagen fibers, the reticular fibers, which are rich in type III collagen. The reticular cells and fibers form a meshwork, or reticulum, which filters the blood. Three major types of filtration beds can be distinguished by their structure and content: the white pulp, the marginal zone, and the red pulp.
The white pulp contains the lymphocytes and other mononuclear cells that surround the arterioles branching off the splenic artery. After the splenic artery pierces the splenic capsule at the hilum, it divides into progressively smaller branches. Each branch is called a central artery because it runs through the central longitudinal axis of a distinctive filtration bed that surrounds each central artery (Fig. 6–4). This is composed of a cuff of lymphocytes called the periarteriolar lymphoid sheath (PALS). The PALS is composed mostly of T lymphocytes, about two-thirds of which are CD4+ T cells. The PALS around white pulp arterioles of the human spleen is not continuous.36 Indeed, segments of the central arterioles might not be surrounded by T cells in areas where they run through lymphoid follicles containing pale areas of activated B lymphocytes interspersed with large, pale macrophages and dendritic cells.1 The migration of T cells to the PALS is governed by stromal cell production of chemokines, primarily CCL19 and CCL21, which interact with the chemokine receptor CCR7 that is expressed by naïve T cells.37 Stromal production of these chemokines can be stimulated by certain cytokines, such as lymphotoxin.38
Structure of the spleen. A branch of the splenic artery enters the pulp and becomes a central artery. Surrounding the central artery is a periarterial lymphoid sheath (PALS). At the circumference of the PALS is the marginal zone, which generally separates the white pulp of the PALS from the red pulp. Follicles of B cells with occasional germinal centers (malpighian corpuscles) are located at the outer margins of the PALS for the depicted central artery and the PALS of central arteries that are in a different plane from that of the figure.
On gross inspection of the surface of a freshly cut spleen, these follicles appear as white dots referred to as malpighian corpuscles (Fig. 6–5). These corpuscles contain a germinal center and have the same anatomic features and functions as secondary follicles in the lymph node. Branches coming off the central artery deliver disproportionate amounts of plasma and lymphocytes to the rim of the PALS (Fig. 6–6). These branches tend to run at acute angles, leading to a selective loss of plasma from the blood, a phenomenon referred to as “skimming.” After becoming relatively depleted of plasma, the arterioles then carry plasma-reduced blood into the filtration beds of the red pulp and marginal zone. As a result, the red pulp and marginal zone beds contain relatively high concentrations of red cells.
Normal human spleen. The splenic tissue is composed of red and white pulp. The red pulp (R), shown here as masses of red cells, is imparted a red color in living tissue as a result of the natural color of hemoglobin in red cells and in stained sections as a result of the intensified red (eosinophilic) stain taken up by hemoglobin. The red pulp contains venous sinuses separated by cords of red cells (cords of Billroth), which cannot be seen in a light micrograph. The white pulp is composed of spherical aggregates of lymphocytes (lymphatic nodule [LN]) with a lighter staining germinal center and an outer, relatively thin, darker stained marginal zone, which separates white pulp from red pulp. Thick-walled central arteries are usually evident penetrating the white pulp. The central artery is cut obliquely in the white pulp at the upper left. Two arteries are seen penetrating the nodule in the center-left of the field and a single artery penetrating the white pulp in the lower-center of the field. The central artery is often seen in the lymphatic nodule in an eccentric position. Other nodules do not show a vessel in this plane of section. (Reproduced with permission from Lichtman’s Atlas of Hematology, www.accessmedicine.com.)
Normal human spleen (higher magnification of white pulp). The white pulp is composed of spherical aggregates of lymphocytes (lymphatic nodules [LN]) with a lighter staining germinal center and an outer, relatively thin, darker stained marginal zone, which separates white pulp from red pulp. The lymphatic nodules largely consist of B lymphocytes. Thick-walled central arteries are usually evident penetrating the white pulp, often in an eccentric position as noted by the asterisks. The T-cell–rich periarteriolar lymphoid sheath surrounds the central artery, which is cut longitudinally in the lymphatic nodule at the left. A single central artery penetrating the lymphatic nodule in the upper-right part of the field is in a characteristically eccentric position. R, red pulp. (Reproduced with permission from Lichtman’s Atlas of Hematology, www.accessmedicine.com.)
The marginal zone surrounds the PALS and follicles. It is composed of a reticulum, which forms a finely meshed filtration bed, serving as a vestibule for much of the blood that flows through the spleen. The marginal zone surrounds the white pulp and merges with the red pulp. It contains more lymphocytes than the red pulp. These are primarily B cells and CD4+ T cells that appear especially well equipped for rapid antibody immune responses to bloodborne antigens.39,40,41 However, like the red pulp, the marginal zone may become congested and remove damaged and senescent red cells and parasites.
The splenic red pulp is composed of a reticular meshwork, called the splenic cords of Billroth, and splenic sinuses.42 This region predominantly contains erythrocytes but also has large numbers of macrophages and dendritic cells as well as fewer numbers of granulocytes, cytolytic CD8+ T cells, and natural killer (NK) cells.
As the central arteries branch and decrease in size, the PALS also branches and decreases in diameter to but a few cells surrounding the arteriole. The small arteriole finally emerges from its sheath and then terminates in either the marginal zone or the red pulp. Here these vessels are suspended and anchored by adventitial reticular cells in the periarterial beds. They often terminate abruptly as arteriolar capillaries or as vessels with a trumpet-like flare with widened slits called interendothelial slits. The blood flows through these slits into filtration beds composed of large-meshed loculi that open to one another.
The blood in the red pulp and marginal zone drains into venous sinuses that form anastomosing, blind-ending vessels. These venous sinuses actually are specialized postcapillary venules. The endothelial cells are shaped as tapered rods that are stiffened by basal, longitudinal, intermediate cytoskeletal filaments and contractile filaments of actin and myosin. These intracellular contractile filaments can shorten the vein, causing the endothelium to buckle and form interendothelial gaps, favoring transmural passage.
The endothelial cells are attached to a basement membrane. Although this appears to be fashioned of fibers, the basement membrane actually is an extracellular membranous wall with large, regular defects that expose considerable basal endothelial surface. This includes the interendothelial slits through which blood may flow from the filtration bed and into the vein. Ordinarily the interendothelial slits are narrow or even closed unless forced apart by cells in transmural transit or by endothelial contraction.
Splenic arterioles terminate at varied distances from the walls of venous vessels. Blood flowing from arterioles that terminate at the venous vessel wall may flow directly into the splenic vein. However, blood flowing from arterioles that terminate at a distance from a vein must traffic through the spleen. In so doing, the blood either may pass quickly through a nonsinusoidal venous aperture or slowly through sinusoidal interendothelial slits and the fibroblast stroma.
The fibroblast stroma contains reticular cells and myofibroblast cells, which are also called barrier cells. The latter may fuse with each other to form a syncytial membrane that connects the arterial terminals with venous interendothelial slits or apertures. Like other myofibroblasts, these cells contain actin and myosin and may contract, thereby approximating splenic arterial and venous vessels with one another. Thus, the fibroblast stroma may affect the relative proportion of blood that flows through the stroma or the sinusal interendothelial slits. Such redistribution might occur during periods of acute physiologic stress, allow for increased expulsion of red cells from the spleen, and account for some of the increase in hematocrit observed during strenuous exercise.43
Mixed within the stroma of the red pulp and marginal zone are monocytes and macrophages. As the blood passes through the stroma, monocytes adhere to the stroma, where the microenvironment is conducive to their maturation into macrophages and large, dendritic, lysosome-rich phagocytes. These cells assist the reticular cells in mechanical filtration. More importantly, these cells have phagocytic activity that allows them to ingest imperfect erythrocytes, store platelets, and remove infectious agents, such as plasmodia, from the circulation.44 In addition, the monocytes and macrophages have nonphagocytic functions, such as the presentation of antigens to T cells or the elaboration of immunomodulatory cytokines.
Collectively, the anatomy of the spleen allows the marginal zone and red pulp to cull defective erythrocytes. As the blood passes slowly through the sinusoidal interendothelial slits and the fibroblast stroma, the erythrocytes must undergo alterations in shape to squeeze through the mechanical barrier generated by this filtration compartment. Normal red cells that are supple may pass through readily because the interendothelial slits can open to approximately 0.5 μm. However, erythrocytes containing large, rigid inclusions, such as plasmodium-containing erythrocytes, are delayed or sequestered.45 Antibody-coated red cells, as present in autoimmune hemolytic anemia, are also recognized and removed by macrophages in the splenic red pulp. Polymorphisms of FcγRII (CD32) or FcγRIII (CD16) that affect immunoglobulin (Ig) G binding in vitro can alter the efficiency of clearance of antibody-coated red cells in vivo.46
When these filtration beds sequester imperfect red cells, the blood pools inside the spleen, causing stasis and congestion. This stimulates sphincter-like contraction of the distal vein, resulting in proximal plasma transudation that produces a viscous luminal mass of high-hematocrit blood. During episodes of enhanced red cell sequestration, as occurs during malarial crises or hemolytic episodes in a small proportion of patients with sickle cell disease, the splenic volume and weight may increase 10- to 20-fold (Chap. 56).47 Although the white pulp may enlarge, particularly in germinal centers, the marginal zones and red pulp become greatly widened with pooled erythrocytes and macrophages in this setting.
Regulation of Blood Volume
The spleen also can play a role in modulating blood volume. Release of high-hematocrit blood through splenic contraction occurs in response to activation of the baroreflex, which also may be activated during conditions of decreased blood pressure and cardiac output.48,49 On the other hand, physiologic agents such as atrial natriuretic peptide, nitric oxide, and adrenomedullin can induce fluid extravasation from the splenic circulation into lymphatic reservoirs.50 Excessive splenic extravasation can contribute to the inability to maintain adequate intravascular volume during septic shock. There also is evidence that the splenic afferent and renal sympathetic nerves play a role in maintaining renal microvascular tone.50 This splenorenal reflex can influence blood pressure and, during septic shock, help promote renal sodium and water reabsorption and release of the vasoconstrictor angiotensin II. On the other hand, in portal hypertension, the splenorenal reflex can promote renal sodium and water retention and possibly play a role in the hemodynamic complications of portal hypertension through neurohormonal modulation of the mesenteric vascular bed.
The spleen and its responses to antigens are similar to those of lymph nodes, the major difference being that the spleen is the major site of immune responses to bloodborne antigens, while lymph nodes are involved in responses to antigens in the lymph.42 Antigens and lymphocytes enter the spleen through the vascular sinuses, because the spleen lacks high endothelial venules. Upon entry, the lymphocytes home to the white pulp. T cells, which express the chemokine receptor CCR7, migrate to the PALS in response to CCL19 and CCL21, and B cells, which express CXCR5, migrate to the lymphoid follicles in response to CXCL13.37,51 Dendritic cells also express CCR7 and hence migrate to the same area as do naïve T cells. T and B cells migrate within these compartments for about 5 and 7 hours, respectively. In the absence of an immune response, these cells migrate through a reticulum arranged around the circumference of the central artery.
Upon immune activation in response to antigen, the lymphocytes may remain in the spleen to sustain a primary or secondary immune response. Activation of B cells is initiated in the marginal zones that are adjacent to CD4+ T cells in the PALS. Activated B cells then migrate into germinal centers or into the red pulp.52 Lymphoid nodules appear and expand by recruiting lymphocytes from the blood and the peripheral zone of the follicles, termed the mantle zone. These cells then proliferate and differentiate in the center of a lymphoid nodule, forming a germinal center.53 In their path from the marginal zone to the follicles, B cells pass into the PALS, where they remain in contact with T lymphocytes for a few hours, allowing ample time for T- and B-cell interaction in response to antigens. If they are not recruited in an immune response to antigen, both T and B lymphocytes exit the spleen via deep efferent lymphatics, not the splenic veins.
These efferent lymphatics are not distinguished as separate structures within the PALS, being quite thin-walled and often packed with efferent lymphocytes. However, they are important in moving nonreactive lymphocytes out of the spleen and in producing high-hematocrit pulp blood. After leaving the spleen, the efferent lymphocytes become the afferent lymphatics of the perisplenic mesenteric lymph nodes or empty into the thoracic duct. This duct empties into the left subclavian vein, thus returning the lymphocytes to the venous circulation.