RBC Production

Overview of Erythropoiesis

Definition: The process of red blood cell (RBC) production in the bone marrow
Location: Primarily occurs in the bone marrow in adults. Can occur in the liver and spleen during fetal development and in certain disease states (extramedullary hematopoiesis)
Regulation: Tightly regulated to maintain a constant red cell mass and adequate oxygen delivery to tissues

Hematopoietic Stem Cell (HSC):
- Self-renewing cells that can differentiate into all blood cell lineages
Common Myeloid Progenitor (CMP):
- Gives rise to all myeloid lineages (erythrocytes, granulocytes, monocytes, megakaryocytes)
Burst-Forming Unit-Erythroid (BFU-E):
- Early erythroid progenitor
- Less responsive to erythropoietin (EPO) than CFU-E
Colony-Forming Unit-Erythroid (CFU-E):
- More mature erythroid progenitor
- Highly responsive to EPO
Erythroid Precursors (Normoblasts/Erythroblasts):
- Pronormoblast/Proerythroblast:
  - Large cell with a high nucleus-to-cytoplasm (N:C) ratio
  - Basophilic cytoplasm
- Basophilic Normoblast:
  - Smaller than pronormoblast
  - Intensely basophilic cytoplasm
- Polychromatic Normoblast:
  - Smaller than basophilic normoblast
  - Gray-blue cytoplasm due to the presence of both ribosomes and hemoglobin
- Orthochromic Normoblast:
  - Smaller than polychromatic normoblast
  - Pink cytoplasm due to increased hemoglobin
  - Nucleus is condensed and about to be extruded
Reticulocyte:
- Anucleated red blood cell with residual RNA
- Slightly larger than mature RBCs
- Spends 1-2 days in the bone marrow and 1 day in the peripheral blood before maturing into an erythrocyte
Erythrocyte (Mature Red Blood Cell):
- Anucleated biconcave disc filled with hemoglobin
- Lifespan of approximately 120 days

Erythropoietin (EPO):
- Primary growth factor for erythropoiesis
- Produced mainly by the kidneys in response to hypoxia
- Stimulates the proliferation and differentiation of erythroid progenitors (BFU-E and CFU-E)
- Prevents apoptosis (programmed cell death) of erythroid precursors
Iron:
- Essential for heme synthesis
- Sources: Dietary intake, recycling from senescent RBCs, and storage in the liver and spleen
- Transported in the plasma by transferrin
- Stored in cells as ferritin and hemosiderin
Vitamins:
- Vitamin B12 (Cobalamin) and Folate (Folic Acid):
  - Essential for DNA synthesis
  - Deficiency leads to impaired cell division and maturation, resulting in megaloblastic anemia
Cytokines and Growth Factors:
- Interleukin-3 (IL-3):
- Multi-lineage growth factor that affects early hematopoietic progenitors
- Stem Cell Factor (SCF):
  - Supports the survival and proliferation of HSCs and early progenitors

Components:
- Heme: Iron-containing porphyrin ring
- Globin: Protein chains (alpha, beta, gamma, delta)
Process:
- Porphyrin Synthesis:
  - Occurs in mitochondria and cytoplasm
  - Starts with the condensation of succinyl CoA and glycine
  - Involves a series of enzymatic reactions
  - Key enzyme: ALA synthase (rate-limiting step)
- Iron Incorporation:
  - Iron (Fe2+) is incorporated into protoporphyrin to form heme
  - Key enzyme: Ferrochelatase
- Globin Synthesis:
  - Occurs on ribosomes in the cytoplasm
  - Different globin chains are produced depending on the stage of development (embryonic, fetal, adult)
- Hemoglobin Assembly:
  - Four globin chains (two alpha-like and two beta-like) combine to form a globin tetramer
  - Each globin chain binds to one heme molecule
Types of Hemoglobin:
- Hemoglobin A (HbA):
  - α2β2 (two alpha and two beta chains)
  - Predominant hemoglobin in adults
- Hemoglobin A2 (HbA2):
  - α2δ2 (two alpha and two delta chains)
  - Minor component in adults
- Hemoglobin F (HbF):
  - α2γ2 (two alpha and two gamma chains)
  - Predominant hemoglobin in fetuses and newborns
  - Higher affinity for oxygen than HbA

Hypoxia:
- Low oxygen levels stimulate EPO production
- Causes include high altitude, lung disease, heart failure, and anemia
Anemia:
- Decreased red cell mass stimulates erythropoiesis
Hemorrhage:
- Blood loss leads to decreased oxygen delivery and increased erythropoiesis
Kidney Disease:
- Impaired EPO production, leading to decreased erythropoiesis
Inflammation:
- Chronic inflammation can suppress erythropoiesis through increased hepcidin production and reduced EPO responsiveness
Nutritional Deficiencies:
- Iron, vitamin B12, or folate deficiency can impair erythropoiesis

Complete Blood Count (CBC):
- Hemoglobin (HGB): Measures the amount of hemoglobin in the blood
- Hematocrit (HCT): Measures the percentage of blood volume occupied by red blood cells
- Red Blood Cell Count (RBC): Measures the number of red blood cells in a volume of blood
- MCV, MCH, MCHC
Reticulocyte Count:
- Measures the percentage or absolute number of reticulocytes in the blood
- Indicates the rate of red blood cell production by the bone marrow
Peripheral Blood Smear:
- Evaluates the morphology (size, shape, color) of red blood cells
- Can identify abnormalities such as:
  - Anisocytosis (variation in size)
  - Poikilocytosis (variation in shape)
  - Hypochromia (decreased color)
  - Polychromasia (increased blueness due to residual RNA in reticulocytes)
Iron Studies:
- Serum Iron: Measures the amount of iron in the blood
- Total Iron-Binding Capacity (TIBC): Measures the blood’s capacity to bind iron
- Transferrin Saturation: The percentage of transferrin saturated with iron
- Ferritin: Measures the body’s iron stores
Erythropoietin (EPO) Level:
- Measures the amount of EPO in the blood
- Useful for differentiating causes of anemia and polycythemia
Bone Marrow Examination:
- Provides information about the cellularity and maturation of erythroid precursors in the bone marrow
- Can identify abnormalities such as:
  - Dysplasia (abnormal cell development)
  - Infections
  - Malignancies