Spleen

The spleen is an intraperitoneal organ located in the left upper quadrant of the abdomen that originates from the mesoderm. As it lies deep within the rib cage, the spleen is generally not palpable except in particularly slender individuals or if it is pathologically enlarged (splenomegaly). It is supplied by the splenic artery (a branch of the celiac trunk) and drained by the splenic vein, which joins the superior mesenteric vein to form the hepatic portal vein. The lymphatics drain into the celiac lymph nodes. The spleen receives its sympathetic and parasympathetic innervation via the celiac plexus and the vagus nerve respectively. The spleen is composed of the white pulp and the red pulp, which are separated by the marginal zone. The red pulp consists of blood-filled sinusoids and cords that filter abnormal red blood cells and platelets. The white pulp consists of the periarterial lymphatic sheath and lymphoid follicles, which are involved in the production of antibodies. Marginal-zone macrophages are involved in the clearance of encapsulated pathogens. In embryos, the spleen is the site of hematopoiesis. The spleen is a well-vascularized organ that is susceptible to injury and rupture from blunt abdominal injury. Asplenic patients should receive vaccinations against encapsulated organisms such as Streptococcus pneumoniae, Neisseria meningitidis, and H. influenzae.

Overview

Characteristics

• Coffee bean-shaped, intraperitoneal organ
• The spleen has two poles (superior and inferior) and two surfaces (diaphragmatic and visceral).
• Usually not palpable as it lies deep in the rib cage, but may be felt in individuals of slender build or those with splenomegaly.

Location

• Left upper quadrant (LUQ) of the abdomen
• Protected by the left 9^th to 11^th ribs
• Neighboring structures of the spleen
  • Diaphragm (posterior and superior)
  • Stomach (anterior and medial)
  • Left kidney (inferior and medial)
  • Left colic flexure (inferior)
  • Tail of the pancreas (at the hilum of the spleen)

The “odd” spleen: 1-3-5-7-9-11 --> The spleen is 1 inch (thickness) by 3 inches (breadth) by 5 inches (length), weighs 7 ounces, and lies between the 9^th and 11^th ribs.

Function

• Immune response
• Filters old or misshapen RBCs and platelets out of circulation
• In embryos: hematopoiesis

Vasculature and innervation

• Splenic hilum
  • Located on the visceral surface of the spleen
  • Entry and exit point for splenic vasculature
  • Only part of spleen that is not covered in peritoneum

	Vessel	Course	Notable features
Arteries	Splenic artery	Celiac trunk → superior border of pancreas → hilum of spleen → branches into central and penicillar arterioles	Vessels that arise from the splenic artery Short gastric arteries Left gastroepiploic artery Located inside the splenorenal ligament Supplies the pancreas while flowing superior to it (retroperitoneal course until the hilum of the spleen)
Veins	Splenic vein	Hilum of spleen → meets superior mesenteric vein to form the portal vein	Veins that empty into the splenic vein Short gastric vein Left gastroepiploic vein Pancreatic veins from body and tail of pancreas Inferior mesenteric vein
Lymphatics	Celiac nodes	Follow course of splenic artery	Efferent lymphatics only
Innervation	Celiac plexus Vagus nerve	Follow course of splenic artery	Sympathetic (celiac plexus) and parasympathetic (vagus nerve) innervation

The spleen has only a thin capsule and is a well-vascularized organ, making it susceptible to injury and rupture, especially through trauma to the LUQ or left lower ribs. Splenic rupture should therefore always be considered in blunt abdominal injuries!

Ligaments

The splenic ligaments connect the spleen to neighboring structures, contain important vessels and are part of the greater omentum.

	Gastrosplenic ligament	Splenorenal ligament
Connected structure	Greater curvature of the stomach	Left kidney
Ligament contents	Short gastric vessels, gastroepiploic vessels	Splenic artery, splenic vein, tail of pancreas (the only intraperitoneal part of that organ)
Notes	Separates greater sac and lesser sac

The spleen is made of red pulp and white pulp, separated by the marginal zone.

Red pulp (75%)

Red pulp is composed of splenic cords and sinuses (sinusoids)

• Splenic cords: reticular meshwork filled with blood (open circulation system); filters blood from damaged erythrocytes
• Spleen sinusoids: fenestrated vessels that prevent old or malformed RBCs/platelets from re-entering venous circulation
  • Open circulation: blood empties from sheathed capillaries into the splenic cords and then enters the sinusoids through slits in the vessel wall.
  • Closed circulation: Blood empties from sheathed capillaries of the red pulp directly into the sinusoids.
• Macrophages: found in the cords and around the sinusoids
  • Phagocytosis of damaged RBCs/platelets that do not reenter circulation
  • Capture of viruses and opsonized pathogens that enter the red pulp
• Blood flow: splenic artery → arterioles → red pulp (cords → sinusoids) → venules → splenic vein → portal circulation

White pulp (25%)

• Periarteriolar lymphatic sheath (PALS)
  • Surrounds the arterioles
  • Dense lymphoid tissue containing T lymphocytes
• Follicles
  • Main component of white pulp
  • Close to PALS
  • Contain B lymphocytes
• Marginal zone
  • Located between the red pulp and white pulp
  • Contains antigen-presenting cells (APCs): macrophages and specialized B cells (marginal zone B cells)

Red pulp

• Filters abnormal RBCs/platelets: macrophages capture and break down defective RBCs/platelets via phagocytosis.
• Fetal erythropoiesis
• Sequestration of platelets: Up to ⅓ of the body's platelets are stored in the spleen.

White pulp

The white pulp is the lymphatic tissue of the spleen. After maturation in the primary lymphatic organs, lymphocytes migrate into the spleen (as a secondary lymphoid organ), where they lie dormant until activation. After differentiation to effector cells, activated B and T lymphocytes reenter the bloodstream via the red pulp to mount an immune response in inflamed tissue.

• Mounts immune response to blood-borne antigens and infections
  • Macrophages and other APCs in the PALS and marginal zone trap the antigen from circulation and present it to the lymphocytes of the spleen.
    • Splenic macrophages eliminate encapsulated bacteria.
    • Patients undergoing splenectomy should receive vaccination against pneumococcus, meningococcus, and H. influenza b
  • B cells in follicles differentiate into plasma cells → production of antibodies (e.g., IgM) and memory cells

The spleen is the major site of IgM production in the body. It also contains a large store of macrophages capable of capturing opsonized bacteria. Opsonization is the body's primary defense against encapsulated bacteria. Therefore, splenectomy → ↓ IgM production (→ ↓ complement activation → ↓ opsonization) → ↓ bacterial clearance of opsonized bacteria → ↑ susceptibility to infection by encapsulated bacteria. Asplenic patients must be vaccinated against infection with encapsulated organisms!

• Organ of mesodermal origin
• Arises within the dorsal mesentery, posterior to the stomach
  • Mesentery between spleen and gut tube → gastrosplenic ligament
  • Mesentery between spleen and dorsal body wall → splenorenal ligament
• Has foregut blood supply (i.e., celiac trunk), although it is not a foregut derivative
• Involved in fetal erythropoiesis: produces RBCs from 10–28 weeks

Prenatally, the spleen is involved in the formation of blood! Postnatally, extramedullary hematopoiesis in the spleen would not be physiological but rather an indicator of a hemato-oncological disease (e.g., bone marrow involvement in leukemia or lymphoma)!

• See splenomegaly.
• Splenic rupture
• Asplenia

The spleen can become enlarged (splenomegaly) due to various disorders: increased immunological burden (e.g., infectious mononucleosis), increased RBC removal and destruction (e.g., hemolytic anemia), blood congestion (e.g., portal hypertension), and malignant infiltration (e.g., leukemia, lymphoma).