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, medial, and posterior)
  • 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 (thick) 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

The spleen has only a thin capsule and is well-vascularized, so it is 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.

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

Red pulp

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

• Splenic cords: a reticular meshwork filled with blood (open circulation system) that filters the blood from damaged erythrocytes
• Spleen sinusoids: long vessels with a fenestrated ring-like (“barrel hoop”) basement membrane that prevent old or malformed RBCs/platelets from reentering 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

• Periarteriolar lymphatic sheath
  • Surrounds the arterioles
  • Dense lymphoid tissue containing T lymphocytes
• Splenic follicles
  • Main component of white pulp
  • Close to periarteriolar lymphatic sheath
  • Contain B lymphocytes
• Splenic 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, it mounts immune response to blood-borne antigens and infections. 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.

• Macrophages and other APCs in the periarteriolar lymphatic sheath and marginal zone trap the antigen from circulation and present it to the lymphocytes of the spleen.
• Splenic macrophages capture the opsonized encapsulated bacteria.
• B cells in follicles differentiate into plasma cells, which produce antibodies (e.g., IgM) and form memory cells

Splenectomy → ↓ IgM production → ↓ complement activation → ↓ C3b opsonization → ↓ 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 hematooncological disease (e.g., bone marrow involvement in leukemia or lymphoma).

• Splenic infarction
• Splenomegaly
• Splenic rupture, splenic laceration
• Splenic abscess
• Asplenia, complications of splenectomy
• Hypersplenism

A variety of disorders can cause splenomegaly, including 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).