Microscopic anatomy

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)

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.