Hematopoiesis: Definition, Types & Processes

What is Hematopoiesis?

• Definition: Hematopoiesis is the process of blood cell production.
• Etymology: Derived from Greek - "Hama" (blood) + "Poisis" (to make something).
• Other Names: Hemopoiesis, hematogenesis, hemogenesis.
• Lifespan: Begins before birth and continues throughout life to maintain a steady blood supply.

Types of Blood Cells Produced

1. Red Blood Cells (Erythrocytes):

   • Function: Carry oxygen from lungs to organs and return carbon dioxide to lungs.
   • Production: Called erythropoiesis.

2. White Blood Cells (Leukocytes):

   • Function: Fight infections, protect against germs, and destroy abnormal cells.
   • Types: Neutrophils, Basophils, Eosinophils, Monocytes, Lymphocytes (B-cells, T-cells, NK cells).
   • Production: Called leukopoiesis.
   • Note: Neutrophils, basophils, eosinophils are grouped as granulocytes.

3. Platelets (Thrombocytes):

   • Function: Form clots to prevent blood loss from injuries.
   • Production: Called thrombopoiesis.

Hematopoiesis Locations

• Bone Marrow: Primary site for hematopoiesis (medullary hematopoiesis).
• Other sites: Liver and spleen (extramedullary hematopoiesis), especially before birth or when bone marrow is insufficient.
• Before Birth:
  • Week 3: Initial RBCs made in yolk sac.
  • Months 2-3: Liver and spleen produce RBCs, WBCs, and platelets.
  • Month 5: Majority of blood cells from bone marrow.
• After Birth:
  • Bone marrow primarily generates blood cells; thymus develops T-cell lymphocytes.
  • Exception: Certain diseases may shift production to prenatal sites (liver, spleen).

Hematopoiesis Process

• Begins with a hematopoietic stem cell (HSC) which develops into specific precursor cells:
  • Erythropoiesis: HSC -> Erythroblast -> Reticulocyte -> Mature RBC.
  • Leukopoiesis (Granulocytes): HSC -> Myeloblast -> Myelocyte -> Basophil/Eosinophil/Neutrophil.
  • Leukopoiesis (Mononuclear Cells):
    • Monocytes: HSC -> Monoblast -> Mature Monocyte.
    • Lymphocytes: HSC -> Lymphoblast -> T-cells/B-cells/NK cells.
  • Thrombopoiesis: HSC -> Megakaryoblast -> Megakaryocyte -> Platelets.

Duration of Hematopoiesis

• Depends on body's needs; increases during illness.
• Lifespan of cells:
  • RBCs: ~120 days.
  • WBCs: Few hours to days.
  • Platelets: 5-9 days.

Issues in Hematopoiesis

• Red Blood Cells:
  • Anemia: Too few RBCs; leads to fatigue.
  • Erythrocytosis: Too many RBCs; can thicken blood, risk heart attack/stroke.
• White Blood Cells:
  • Leukopenia: Too few WBCs; increased infection risk.
  • Leukocytosis: Too many WBCs; can indicate infection/blood disorder.
• Platelets:
  • Thrombocytopenia: Too few platelets; risk of prolonged bleeding.
  • Thrombocytosis: Too many platelets; risk of clots.

Conditions Affecting Hematopoiesis

• Age-Related: Bone marrow fat deposition reduces hematopoiesis space.
• Diseases: Blood disorders and cancers like leukemia disrupt production.
• Medications: Certain drugs, including chemotherapy, can affect blood cell counts.

Hematopoiesis is crucial for maintaining health by ensuring a steady supply of blood cells, and it's influenced by various internal and external factors.