Spleen

Microscopic anatomy

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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)

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Spleen Manifestations in Disease

The spleen’s primary roles are filtering blood and mounting immune responses. Its dysfunction or enlargement is a key sign of various systemic diseases. Spleen manifestations can be broadly categorized into Splenomegaly (enlargement) and Hyposplenism/Asplenia (decreased or absent function).

Splenomegaly (Enlarged Spleen)

Enlargement occurs via three main mechanisms:

1. Congestion: Due to backup of blood.
2. Infiltration: By abnormal cells (e.g., cancer, lipid-laden macrophages).
3. Work Hypertrophy/Hyperplasia: Increased filtering or immune activity.

Clinical Presentation:

• Often asymptomatic; may present with early satiety, abdominal fullness, or LUQ pain.
• Palpable spleen below the left costal margin (a normal spleen is usually not palpable).

Key Causes & Associations:

• Congestive Splenomegaly:

  • Portal Hypertension: Most common cause. Cirrhosis, portal/splenic vein thrombosis.
  • Congestive Heart Failure (CHF): Right-sided heart failure leads to systemic venous congestion.

• Hematologic Disorders:

  • Hemolytic Anemias: Hereditary spherocytosis, Sickle Cell Disease (in children, before autosplenectomy), Thalassemia major, Autoimmune Hemolytic Anemia. The spleen enlarges as it removes defective RBCs.
  • Myeloproliferative Neoplasms: CML, polycythemia vera, essential thrombocythemia, myelofibrosis. Often cause massive splenomegaly (>1000g or >8 cm below costal margin).
  • Leukemias/Lymphomas: Hairy cell leukemia (massive), CLL, Hodgkin & Non-Hodgkin lymphomas.

• Infections:

  • Infectious Mononucleosis (EBV): Splenomegaly is common; risk of spontaneous rupture with minor trauma.
  • Malaria & Leishmaniasis (Kala-azar): Classic causes of massive splenomegaly.
  • Bacterial: Endocarditis, tuberculosis.

• Infiltrative/Storage Diseases:

  • Gaucher Disease: Classic lysosomal storage disease causing massive splenomegaly due to accumulation of glucocerebrosides in macrophages (“Gaucher cells”).
  • Niemann-Pick Disease: Accumulation of sphingomyelin.
  • Sarcoidosis & Amyloidosis: Infiltration by granulomas or amyloid protein, respectively.

Hyposplenism & Asplenia

This refers to decreased or absent splenic function, leading to impaired blood filtering and immune response.

Pathophysiology/Etiology:

• Splenectomy: Surgical removal (e.g., for trauma, ITP, hereditary spherocytosis).
• Autosplenectomy: Repetitive infarction and fibrosis, classically seen in Sickle Cell Disease in adults.
• Congenital Asplenia: Rare developmental failure.
• Functional Hyposplenism: Spleen is present but dysfunctional (e.g., celiac disease, SLE, inflammatory bowel disease).

Diagnosis (Hallmark Peripheral Smear Findings):

• Howell-Jolly bodies: Basophilic nuclear remnants in RBCs (normally removed by spleen). Their presence is a key sign of hyposplenism.
• Target cells (Codocytes): Due to altered RBC membrane.
• Thrombocytosis: Loss of splenic sequestration of platelets.
• Acanthocytes (Spur cells): Spiky RBCs.

Complications & Management:

• Overwhelming Post-Splenectomy Infection (OPSI): High risk of fulminant sepsis from encapsulated organisms due to loss of opsonizing IgM production and phagocytosis.
  • Key Pathogens: Streptococcus pneumoniae (most common), Haemophilus influenzae type b, Neisseria meningitidis.
  • Mnemonic: SHiN
• Prevention:
  • Vaccinations: Administer vaccines against pneumococcus, meningococcus, and H. influenzae type b, ideally before elective splenectomy or as soon as possible after.
  • Prophylactic Antibiotics: Often prescribed, especially for children and the immunocompromised (e.g., penicillin).
  • Patient Education: Advise patients to seek immediate medical care for fevers.

Disease	Spleen Finding	Mechanism
Sickle Cell Disease	Splenomegaly (in children) → Autosplenectomy (in adults)	Vaso-occlusion from sickled RBCs causes initial splenic sequestration crisis, followed by recurrent infarctions leading to a shrunken, fibrotic spleen.
Hereditary Spherocytosis	Splenomegaly	”Work hypertrophy” from clearing spherocytic RBCs. Splenectomy is curative.
Infectious Mononucleosis (EBV)	Splenomegaly	Lymphoid hyperplasia in response to viral infection. High risk of rupture.
Portal Hypertension	Congestive Splenomegaly	Increased pressure in the splenic vein obstructs outflow.
Myelofibrosis	Massive Splenomegaly	Extramedullary hematopoiesis (blood cell production shifts to spleen as bone marrow fails).
Hairy Cell Leukemia	Massive Splenomegaly	Infiltration of the red pulp by neoplastic B-cells.
ITP	Normal Spleen Size or Mild Splenomegaly	Spleen is the site of platelet destruction, but does not typically cause significant enlargement.