Anatomy of Spleen

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### Gross Anatomy - **Location:** Located in the left upper quadrant (LUQ) of the abdomen, inferior to the diaphragm, posterior to the stomach, and anterior to the left kidney. - **Shape:** Ovoid and somewhat flattened, resembling a large bean. - **Size:** Approximately 12 cm long, 7 cm wide, and 3-4 cm thick. Weighs about 150-200 grams in adults. - **Surfaces:** - **Diaphragmatic surface:** Smooth and convex, conforming to the concavity of the diaphragm. - **Visceral surface:** Irregular, with impressions for the stomach (gastric impression), left kidney (renal impression), and splenic flexure of the colon (colic impression). - **Borders:** - **Anterior border:** Often notched, especially superiorly. - **Posterior border:** Smooth and rounded. - **Hilum:** A fissure on the visceral surface, near the gastric impression, where the splenic artery enters and the splenic vein exits. ### Ligaments and Fixation - **Gastrosplenic ligament:** Connects the greater curvature of the stomach to the hilum of the spleen. Contains the short gastric and left gastroepiploic vessels. - **Lienorenal (splenorenal) ligament:** Connects the hilum of the spleen to the anterior surface of the left kidney. Contains the splenic artery and vein, and the tail of the pancreas. - **Phrenicocolic ligament:** A fold of peritoneum that extends from the diaphragm to the splenic flexure of the colon, providing support for the inferior pole of the spleen. ### Blood Supply and Drainage - **Arterial Supply:** - **Splenic artery:** Largest branch of the celiac trunk. It is tortuous and runs along the superior border of the pancreas, dividing into 5-6 terminal branches at the splenic hilum. - **Venous Drainage:** - **Splenic vein:** Formed by several veins leaving the hilum. It runs posterior to the pancreas and unites with the superior mesenteric vein (SMV) to form the hepatic portal vein. - **Lymphatic Drainage:** Lymphatic vessels leave the hilum and drain into pancreaticosplenic lymph nodes, then to celiac lymph nodes. - **Innervation:** Primarily sympathetic fibers from the celiac plexus, which control vasoconstriction and contraction of the splenic capsule. ### Histology - **Capsule:** Dense fibroelastic connective tissue, often containing smooth muscle cells, especially in some animals. It is covered by visceral peritoneum. - **Trabeculae:** Connective tissue septa that extend from the capsule into the parenchyma, carrying blood vessels. - **Parenchyma (Splenic Pulp):** Divided into two main components: - #### White Pulp - **Description:** Composed of lymphoid tissue, appearing as white spots on a fresh cut surface. - **Structure:** Arranged around central arteries (branches of the splenic artery) forming periarteriolar lymphoid sheaths (PALS) rich in T-lymphocytes. - **Germinal Centers:** Within the PALS, B-lymphocytes are organized into lymphoid follicles, often with germinal centers, where B-cell proliferation and differentiation occur. - **Function:** Immune surveillance and initiation of immune responses against blood-borne antigens. - #### Red Pulp - **Description:** Appears reddish-brown and constitutes the majority of the splenic volume. - **Structure:** Consists of splenic cords (cords of Billroth) and splenic sinusoids (venous sinuses). - **Splenic Cords:** Network of reticular cells and fibers, macrophages, lymphocytes, plasma cells, and blood cells. - **Splenic Sinusoids:** Specialized vascular channels lined by endothelial cells with wide intercellular gaps, facilitating the movement of blood cells. - **Function:** - **Filtration of blood:** Macrophages remove old, damaged, or abnormal red blood cells and platelets. - **Storage:** Stores red blood cells, platelets, and monocytes. - **Hematopoiesis:** Can resume red blood cell production in cases of severe anemia (extramedullary hematopoiesis). ### Functions - **Hematopoietic:** In fetal life, it produces blood cells. In adults, it removes senescent red blood cells and platelets. - **Immunological:** Filters antigens from the blood, initiates immune responses, and produces antibodies (white pulp). - **Reservoir:** Stores blood, platelets, and monocytes. - **Phagocytosis:** Macrophages in the red pulp engulf cellular debris, microorganisms, and old blood cells.