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88 Part II: The Organization of the Lymphohematopoietic Tissues Chapter 6: The Organization and Structure of Lymphoid Tissues 89

an environment in which the cells of the immune system can interact SPLENIC ARCHITECTURE
with one another to mount adaptive immune responses to bloodborne The spleen has an open circulation, which lacks endothelial continu-
antigens. The splenic red pulp contains macrophages that are responsi- ity from artery to vein. When isolated spleens are perfused in washout
ble for clearing the blood of unwanted foreign substances and senescent studies, erythrocytes that appear in the splenic vein appear to be flushed
erythrocytes, even in the absence of specific immunity. Thus, it acts as out from three compartments. The red cells that are flushed out first
a filter for the blood.
come from a compartment that presumably is formed by the splenic
vessels. The erythrocytes that are flushed out next come from a second
SPLENIC ANATOMY compartment, where they presumably are loosely held within the filtra-
The spleen is located within the peritoneum in the left upper quadrant tion beds. The erythrocytes that are flushed out last presumably were
of the abdomen between the fundus of the stomach and the diaphragm. adherent to cells of the filtration beds. Although 90 percent of the blood
It receives its blood supply from the systemic circulation via the splenic flow passes through the splenic vessels, only approximately 10 percent
artery, which branches off the celiac trunk, and the left gastroepiploic of the total splenic red cells are found within this first compartment.
artery. The blood returning from the spleen drains into the portal The second compartment is perfused by 9 percent of the total inflow
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circulation via the splenic vein. Therefore, the spleen can become con- yet contains 70 percent of the splenic red cells. The last compartment is
gested with blood and increase in size when there is portal vein hyper- perfused by only 1 percent of the inflow but contains 20 percent of the
tension (Chap. 56). splenic red cells.
Approximately 10 percent of individuals have one or more acces- These compartments reflect the anatomy of the spleen and its
sory spleens. Accessory spleens are usually 1 cm in diameter and resem- stroma. The stroma is composed of branched, fibroblast-like cells called
ble lymph nodes. However, they usually are covered with peritoneum, reticular cells. These cells produce slender collagen fibers, the reticular
as is the spleen itself. Accessory spleens typically lie along the course fibers, which are rich in type III collagen. The reticular cells and fibers
of the splenic artery or its gastroepiploic branch, but they may be else- form a meshwork, or reticulum, which filters the blood. Three major
where. The commonest location is near the hilus of the spleen, but types of filtration beds can be distinguished by their structure and con-
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approximately 1 in 6 accessory spleens can be found embedded in the tent: the white pulp, the marginal zone, and the red pulp.
tail of the pancreas, where they may be occasionally mistaken for a pan-
creatic mass lesion. 31 White Pulp
The average weight of the spleen in the adult human is 135 g The white pulp contains the lymphocytes and other mononuclear cells
(range: 100 to 250 g). However, when emptied of blood it weighs only that surround the arterioles branching off the splenic artery. After the
approximately 80 g. On autopsy of 539 subjects with normal spleens, splenic artery pierces the splenic capsule at the hilum, it divides into
there was a positive correlation between the spleen weight and both the progressively smaller branches. Each branch is called a central artery
degree of acute splenic congestion and the subject’s height and weight, because it runs through the central longitudinal axis of a distinctive fil-
but not with the subject’s sex or age. 32 tration bed that surrounds each central artery (Fig. 6–4). This is com-
The splenic volume can be estimated by computed tomography posed of a cuff of lymphocytes called the periarteriolar lymphoid sheath
(CT) of the abdomen. In one study, the splenic volume was calculated (PALS). The PALS is composed mostly of T lymphocytes, about two-
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from the linear and the maximal cross-sectional area measurements of thirds of which are CD4+ T cells. The PALS around white pulp arterioles
the spleen, using the following formula: splenic volume = 30 cm + 0.58 of the human spleen is not continuous. Indeed, segments of the central
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(the product of the width, length, and thickness of the spleen measured arterioles might not be surrounded by T cells in areas where they run
in centimeters). Using this formula, the mean value of the calculated through lymphoid follicles containing pale areas of activated B lympho-
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splenic volume for 47 normal subjects was 214.6 cm , with a range of cytes interspersed with large, pale macrophages and dendritic cells. The
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107.2 to 314.5 cm . The calculated splenic volume did not appear to vary migration of T cells to the PALS is governed by stromal cell production
3
significantly with the subject’s age, gender, height, weight, body mass of chemokines, primarily CCL19 and CCL21, which interact with the
index, or the diameter of the first lumbar vertebra, the latter being con- chemokine receptor CCR7 that is expressed by naïve T cells. Stromal
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sidered representative of body habitus on CT. production of these chemokines can be stimulated by certain cytokines,
The splenic volume also can be estimated by sonography, which such as lymphotoxin. 38
provides good correlation with volumes measured by helical abdomi- On gross inspection of the surface of a freshly cut spleen, these
nal CT or actual volume displaced by the excised organ. In one study follicles appear as white dots referred to as malpighian corpus-
of 50 patients, the linear measurement by sonography that correlated cles (Fig. 6–5). These corpuscles contain a germinal center and have
most closely with CT volume was the spleen width measured on a the same anatomic features and functions as secondary follicles in the
longitudinal section with the patient in the right lateral decubitus lymph node. Branches coming off the central artery deliver dispropor-
position (correlation coefficient [r] = 0.89, p <0.001). There was also tionate amounts of plasma and lymphocytes to the rim of the PALS
good correlation between splenic length measured in the right lateral (Fig. 6–6). These branches tend to run at acute angles, leading to a selec-
decubitus position and CT volume (r = 0.86, p <0.001). In another post- tive loss of plasma from the blood, a phenomenon referred to as “skim-
mortem analysis of 32 normal adult spleens, the ultrasonogram mea- ming.” After becoming relatively depleted of plasma, the arterioles then
surements of maximal height, width, and breadth of the spleen were carry plasma-reduced blood into the filtration beds of the red pulp and
compared with the actual volume displaced by the excised organ. The marginal zone. As a result, the red pulp and marginal zone beds contain
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mean actual splenic volume was approximately 148 cm (±81 cm SD), relatively high concentrations of red cells.
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whereas mean splenic volume estimated from ultrasonography was
284 cm (±168 cm SD). Despite the differences between the actual and The Marginal Zone
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estimated volumes, these investigators did find a roughly linear correla- The marginal zone surrounds the PALS and follicles. It is composed of a
tion between actual splenic volume and the estimated splenic volume reticulum, which forms a finely meshed filtration bed, serving as a ves-
measured by ultrasound. However, there may be operator-to-operator tibule for much of the blood that flows through the spleen. The marginal
variation in measurement of the estimated splenic volume, making the zone surrounds the white pulp and merges with the red pulp. It contains
use of sonography in longitudinal studies technically demanding. more lymphocytes than the red pulp. These are primarily B cells and

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