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The spleen is a dark purplish, highly vascular, coffee bean-shaped organ of mesodermal origin situated in the left upper quadrant of the abdomen at the level of the eighth to eleventh ribs between the fundus of the stomach, the diaphragm, the splenic flexure of the colon, and the left kidney (Figure 27–1). The adult spleen weighs 100-150 g, measures about 12 × 7 × 4 cm, and usually cannot be palpated. It is attached to adjacent viscera, the abdominal wall, and the diaphragm by peritoneal folds or “ligaments.” The gastrosplenic ligament carries the short gastric vessels. The other ligaments are avascular except in patients with portal hypertension or myelofibrosis.
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The splenic capsule consists of peritoneum overlying a 1- to 2-mm fibroelastic layer that contains a few smooth muscle cells. The fibroelastic layer sends into the pulp numerous fibrous bands (trabeculae) that form the framework of the spleen. Corrosion cast studies demonstrate that the spleen consists of specific segments based on arterial supply numbering between two and six separated by an avascular plane.
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The splenic artery enters the hilum of the spleen, branches into the trabecular arteries, and then branches into the central arteries that course through the surrounding white pulp and send radial branches to the peripheral marginal zone and the more distant red pulp. The white pulp consists of lymphatic tissue including T cells adjacent to the central artery (periarteriolar lymphoid sheets [PALS]), with a surrounding area containing lymphoid follicles rich in B cells interspersed with dendritic and reticular cells important in antigen presentation. The vascular spaces of the marginal zone between the red and white pulp channel blood into the splenic Billroth cords and out to the associated sinuses. The red pulp vascular structures have a noncontiguous basement membrane that filters cells such as senescent erythrocytes into the macrophage-lined sinuses.
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Accessory spleens (spleniculi) are seen in 10%-15% of the normal population and are located primarily in the gastrosplenic, gastrocolic, and lienorenal ligaments, but they can also be found throughout the peritoneal cavity in the omentum, bowel mesentery, and pelvis. Accessory spleens probably result from a failure of infusion of splenic embryologic tissues. Ordinarily of no significance, they may play a role in recurrence of certain hematologic disorders for which splenectomy is performed. Removal of accessory spleens may lead to remission of disease in these patients. Accessory spleens are more difficult to identify with laparoscopic procedures, but the use of a hand port has allowed identification and resection of accessory spleens with a minimally invasive approach. Patients who fail to respond to initial splenectomy should undergo scanning with technetium 99m-labeled red cells or indium 111-labeled platelets to identify potential sites of missed accessory spleens and can be identified intraoperatively with a hand-held gamma counter.
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