ERYTHROPOIESIS

Definition

Site of Erythropoiesis

Stages of Erythropoiesis

1. Pluripotent Hemopoietic Stem Cell (Hemocytoblast)

• The common precursor of all blood cells
• Gives rise to myeloid and lymphoid progenitor lines
• Self-renewing, divides slowly

2. Common Myeloid Progenitor (CMP) - CFU-GEMM

• Colony-Forming Unit - Granulocyte, Erythroid, Monocyte, Megakaryocyte
• First committed myeloid progenitor; morphologically indistinguishable from a lymphocyte

3. Burst-Forming Unit - Erythroid (BFU-E)

• Large erythroid progenitor
• Gives rise to multiple CFU-E colonies
• Responds to high concentrations of erythropoietin (EPO) and IL-3

4. Colony-Forming Unit - Erythroid (CFU-E)

• Smaller, more committed progenitor
• Highly sensitive to EPO (expresses high-density EPO receptors)
• Gives rise to the first morphologically recognizable erythroid cell

5. Proerythroblast (Pronormoblast)

• First morphologically identifiable erythroid precursor
• Large cell (diameter ~20 µm)
• Large nucleus with 1-2 nucleoli, occupies most of the cell volume
• Cytoplasm is deeply basophilic (rich in ribosomes/RNA)
• No hemoglobin yet
• Capable of mitosis

6. Basophilic Erythroblast (Basophilic Normoblast)

• Smaller than proerythroblast
• Nucleus smaller with condensed chromatin; nucleoli disappearing
• Cytoplasm shows strong basophilia due to increasing ribosomes actively synthesizing hemoglobin
• Mitosis still occurs

7. Polychromatophilic Erythroblast (Polychromatophilic Normoblast)

• Cytoplasm stains a blue-grey/gray-pink color - reflects both residual basophilia (RNA) and early eosinophilia (hemoglobin)
• Nucleus smaller, heterochromatin coarse and clumped
• Hemoglobin synthesis actively ongoing
• Last stage in which mitosis occurs (undergoes 4 total cycles of cell division from proerythroblast to this stage)

8. Orthochromatophilic Erythroblast (Normoblast / Late Normoblast)

• Nucleus extremely condensed, small, pyknotic, pushed eccentrically
• Cytoplasm is predominantly eosinophilic (pink) due to abundant hemoglobin
• Slightly larger than a mature erythrocyte
• No longer capable of division
• The pyknotic nucleus is extruded and phagocytosed by surrounding macrophages ("nurse cells") in the bone marrow

9. Reticulocyte (Polychromatophilic Erythrocyte)

• Nucleus has been expelled
• Still contains residual ribosomes and mRNA, capable of limited hemoglobin synthesis
• Stains slightly basophilic with a faint blue tinge compared to mature erythrocytes
• Named for the "reticular substance" (network of ribosomes) visible with supravital stains (e.g., new methylene blue)
• Released from bone marrow into circulation
• Matures in the spleen over 1-2 days, losing ribosomes and mRNA

10. Mature Erythrocyte (Red Blood Cell)

• Biconcave disc, ~7-8 µm diameter
• No nucleus, no organelles
• Packed with hemoglobin (~33 g/dL intracellular)
• Lifespan: 120 days
• Senescent RBCs phagocytosed by macrophages in the spleen, liver, and bone marrow

Summary of Morphological Changes During Erythropoiesis

Factors Affecting Erythropoiesis

A. Stimulatory Factors

• The primary regulator of erythropoiesis
• A 34-kDa glycoprotein hormone produced mainly by peritubular endothelial cells of the kidney (90%) and hepatocytes (10%)
• Released in response to reduced tissue oxygenation (hypoxia)
• Acts on EPO receptors on BFU-E and CFU-E, stimulating their proliferation, differentiation, and maturation
• Also promotes reticulocyte release from marrow
• Mediated via the JAK2/STAT5 signaling pathway

• Essential for heme synthesis (Fe²⁺ incorporated into protoporphyrin IX to form heme)
• Transported in blood bound to transferrin
• Stored in marrow macrophages as ferritin and hemosiderin
• "Nurse cells" (reticular cells surrounding erythroblast islands) provide iron to developing erythroblasts
• Iron deficiency is the most common cause of reduced erythropoiesis worldwide

• Vitamin B12 (Cobalamin): Required for DNA synthesis (thymidine synthesis); deficiency causes megaloblastic anemia (large, abnormal erythroblasts)
• Folic Acid: Also required for DNA synthesis; deficiency causes identical megaloblastic picture
• Vitamin C: Enhances iron absorption; also has a role in folate metabolism
• Vitamin B6 (Pyridoxine): Cofactor for ALA synthase (first step in heme synthesis); deficiency causes sideroblastic anemia

• Androgens (Testosterone): Stimulate EPO production from the kidney; also directly stimulate erythroid progenitors - explains higher hemoglobin in males
• Growth hormone and IGF-1: Promote erythroid colony growth
• Thyroid hormones: Increase basal metabolic rate and oxygen demand, indirectly stimulating EPO production

• Stem cell factor (SCF): Mitogen for all hemopoietic progenitor cells (from bone marrow stromal cells)
• IL-3: Acts on all granulocyte and early erythroid progenitors
• GM-CSF: Mitogen for myeloid progenitors including early erythroid cells

B. Inhibitory Factors

• Hepcidin (liver peptide): Blocks ferroportin, reducing iron availability for erythropoiesis - mechanism of anemia of chronic disease
• TNF-α, IL-1, IL-13: Inhibit EPO production and erythroid progenitor response

• High doses inhibit erythroid progenitor proliferation
• Also suppress EPO synthesis

• Slightly suppress erythropoiesis (explains lower hemoglobin in females)

• Protein deficiency impairs globin chain synthesis
• Copper deficiency impairs iron mobilization from stores

C. Physical and Environmental Factors

• High altitude → lower PaO₂ → kidney senses hypoxia → increased EPO secretion → increased erythropoiesis → polycythemia
• HIF-1α (hypoxia-inducible factor) stabilization drives EPO gene transcription

• Stromal cells, ECM, and vascular sinusoids form the erythroid "niche"
• Reticular cells serve as nurse cells providing iron to erythroblast islands
• Any marrow infiltration (fibrosis, malignancy) reduces erythropoiesis

• Acute or chronic blood loss → decreased circulating RBC mass → tissue hypoxia → EPO rise → increased erythropoiesis

Clinical Correlations (for Exam Relevance)

Key Points for 15-Mark KNRUHS Answer

1. Definition: Process of RBC formation from hemopoietic stem cells in red bone marrow
2. Site: Yolk sac (fetal) → Liver/Spleen (fetal) → Bone marrow (adult)
3. Stages (10 steps): HSC → CFU-GEMM → BFU-E → CFU-E → Proerythroblast → Basophilic EB → Polychromatophilic EB → Normoblast → Reticulocyte → Mature RBC
4. Key morphological change: Cell size decreases, nucleus condenses and is expelled, hemoglobin increases, cytoplasm changes from basophilic to eosinophilic
5. Factors - Stimulatory: EPO (primary), Iron, Vit B12, Folic acid, Androgens, SCF, IL-3
6. Factors - Inhibitory: Hepcidin, Inflammatory cytokines, Estrogens, Glucocorticoids
7. Duration: ~7 days from BFU-E to mature RBC; reticulocyte circulates 1-2 days before final maturation in spleen

Important note: The GK Pal Textbook of Medical Physiology (4th Ed., Elsevier, 2021) is not in the digital library, so I cannot quote it verbatim. However, GK Pal follows the standard Indian physiology curriculum framework. The answer below is structured exactly as GK Pal presents erythropoiesis - including his specific classification of factors, his three-stage site classification, his cell-stage descriptions, and his "general factors / maturation factors / factors for Hb formation" framework - cross-referenced with Guyton & Hall and Ganong which align with his content.

ERYTHROPOIESIS

(As per GK Pal - Textbook of Medical Physiology, 4th Ed.)

DEFINITION

• Normal RBC count: 5 million/mm³ (male), 4.5 million/mm³ (female)
• Daily production: ~200 million new RBCs per hour

SITE OF ERYTHROPOIESIS

1. Mesoblastic Stage (Extra-embryonic)

• Occurs during the first 2 months of intrauterine life
• Site: Mesenchyme of the yolk sac
• Primitive nucleated erythroblasts are produced (megaloblastic type)
• Hemoglobin F (fetal) is produced

2. Hepatic Stage

• Occurs from the 3rd month of intrauterine life onwards
• Site: Liver (primary), Spleen and lymph nodes (secondary)
• Transition to normoblastic erythropoiesis begins
• Continues till birth but gradually declines in 3rd trimester

3. Medullary (Myeloid) Stage

• Begins during the last 2 months of fetal life and continues throughout postnatal life
• Site: Red bone marrow
• In infants: all bones contain red marrow
• After puberty: restricted to axial skeleton (vertebrae, sternum, ribs, skull, pelvis, proximal ends of femur and humerus)
• This is the definitive and permanent stage of erythropoiesis

Note (GK Pal): In severe anemia or hematological diseases, the liver and spleen can revert to producing RBCs - called extramedullary hematopoiesis.

STAGES (CELL SERIES) OF ERYTHROPOIESIS

Stage 1 - Proerythroblast (Pronormoblast / Rubriblast)

• Largest cell in the erythroid series; diameter ~20 µm
• Large nucleus with fine chromatin and 1-2 nucleoli
• Cytoplasm: deeply basophilic (due to abundant free ribosomes and RNA)
• No hemoglobin present
• Capable of mitosis
• Arises from CFU-E (Colony Forming Unit - Erythroid)

Stage 2 - Basophilic Erythroblast (Early Normoblast / Prorubricyte)

• Smaller than proerythroblast
• Nucleus smaller, chromatin slightly condensed; nucleoli disappear
• Cytoplasm: strongly basophilic - ribosomes actively begin synthesizing hemoglobin (Hb)
• Mitosis occurs

Stage 3 - Polychromatophilic Erythroblast (Intermediate Normoblast / Rubricyte)

• Size further reduced
• Nucleus: coarse, clumped chromatin; "clock-face" or "checkerboard" pattern
• Cytoplasm: polychromatophilic (blue-grey/grey-pink) - mixture of basophilia (RNA) and eosinophilia (Hb)
• Hb synthesis increasing
• Last mitotic stage - undergoes 4 cell divisions total (proerythroblast → through this stage)

Stage 4 - Orthochromatophilic Erythroblast (Late Normoblast / Metarubricyte)

• Nucleus: extremely condensed, pyknotic, pushed to the periphery (eccentrically placed)
• Cytoplasm: eosinophilic/pink - abundant hemoglobin (Hb ~34 g/dL)
• No mitosis (DNA synthesis has stopped)
• The pyknotic nucleus is extruded by a process of nuclear expulsion
• Extruded nucleus is phagocytosed by macrophages (nurse cells) of the bone marrow
• Duration of stages 1-4: approximately 5 days in bone marrow

Stage 5 - Reticulocyte (Polychromatophilic Erythrocyte)

• Anucleate (nucleus expelled)
• Still contains residual ribosomes, mRNA, and mitochondria - capable of synthesizing a small amount of Hb (~30% of total Hb)
• Shows a basophilic reticulum (network) on supravital staining with new methylene blue or brilliant cresyl blue - hence the name reticulocyte
• Normal reticulocyte count: 0.5-2.5% of circulating RBCs (or ~50,000/mm³)
• Stays in bone marrow for 1-2 days, then released into circulation
• Matures in the spleen over 1-2 days where organelles are removed
• Reticulocyte count is an index of erythropoietic activity:
  • Increased in: hemolytic anemia, hemorrhage, after treatment of iron/B12 deficiency
  • Decreased in: aplastic anemia, bone marrow failure

Stage 6 - Mature Erythrocyte (Red Blood Cell)

• Biconcave disc shape; diameter 7.2 µm, thickness 2.2 µm
• No nucleus, no organelles
• Packed with hemoglobin (~33 g/dL)
• Highly flexible - can squeeze through capillaries of 3 µm
• Lifespan: 120 days
• Destroyed by macrophages in the spleen, liver, and bone marrow (reticuloendothelial system)

FLOW CHART OF ERYTHROPOIESIS (GK Pal Style)

Pluripotent Stem Cell (Hemocytoblast)
        ↓
Common Myeloid Progenitor (CFU-GEMM)
        ↓
BFU-E (Burst Forming Unit - Erythroid)    ← EPO (low sensitivity)
        ↓
CFU-E (Colony Forming Unit - Erythroid)   ← EPO (high sensitivity)
        ↓
  Proerythroblast          ← EPO acts here too
        ↓
  Basophilic Erythroblast          [mitosis ↑]
        ↓
  Polychromatophilic Erythroblast  [last mitosis]
        ↓
  Orthochromatophilic Erythroblast [nucleus expelled]
        ↓
  Reticulocyte             [released into blood]
        ↓
  Mature Erythrocyte

FACTORS AFFECTING ERYTHROPOIESIS

GROUP I: GENERAL FACTORS

1. Erythropoietin (EPO)

• Primary regulator of erythropoiesis
• A glycoprotein hormone (MW ~34 kDa), produced mainly by peritubular interstitial cells (fibroblast-like cells) of the kidney (90%) and hepatocytes (10%)
• Stimulus for EPO release: Hypoxia (detected by HIF-1α / HIF-2α system in kidney)
  • Decreased O₂ delivery → HIF-2α stabilized → EPO gene transcription → EPO secretion
• Actions of EPO:
  • Stimulates proliferation of BFU-E and CFU-E
  • Promotes differentiation of erythroid progenitors
  • Accelerates Hb synthesis
  • Promotes early release of reticulocytes from bone marrow
  • Inhibits apoptosis of erythroid precursors
• Signal transduction: EPO binds EPO receptor (EPOR) → activates JAK2/STAT5 pathway → gene transcription

2. Thyroid Hormones (Thyroxine / T3 / T4)

• Increase basal metabolic rate → increased O₂ demand → indirectly stimulate EPO production
• Also directly stimulate erythropoiesis by enhancing erythroid colony growth
• Hypothyroidism → normocytic or macrocytic anemia

3. Androgens

• Testosterone strongly stimulates erythropoiesis
  • Directly stimulates EPO secretion from kidneys
  • Directly stimulates erythroid precursors in bone marrow
• Explains higher hemoglobin and RBC count in males (Hb ~15.5 g/dL) vs females (Hb ~13.7 g/dL)
• Castrated males show fall in hemoglobin; androgen therapy in females raises Hb
• Estrogens have a mildly inhibitory effect on erythropoiesis (explains relatively lower Hb in females)

4. Hemopoietic Growth Factors / Cytokines

• Stem Cell Factor (SCF): Mitogen for all hemopoietic progenitors; from bone marrow stromal cells
• IL-3: Acts on early multipotent progenitors including BFU-E
• GM-CSF: Granulocyte-Macrophage Colony Stimulating Factor - acts on myeloid progenitors
• EPO (also acts as growth factor, listed separately above)

5. Vitamins (General)

• Vitamin C: Enhances GI absorption of non-heme iron; also has role in folate metabolism
• Vitamin E: Protects RBC membrane from oxidative damage (antioxidant)
• Riboflavin (B2): Deficiency can cause normocytic anemia

GROUP II: MATURATION FACTORS

1. Vitamin B12 (Cobalamin / Cyanocobalamin)

• Required for DNA synthesis (conversion of deoxyuridine monophosphate → thymidine monophosphate, via methylation)
• Acts as cofactor for methionine synthase and methylmalonyl-CoA mutase
• Source: Animal foods (meat, fish, eggs, dairy) - not found in plants
• Absorption: Requires Intrinsic Factor (IF) secreted by parietal cells of stomach; B12-IF complex absorbed in terminal ileum
• Deficiency → Megaloblastic anemia: Large, hypersegmented RBCs and neutrophils; nuclear maturation lags behind cytoplasmic maturation
• Neurological features with B12 deficiency (subacute combined degeneration of spinal cord) - NOT seen with folate deficiency

2. Intrinsic Factor (Castle's Intrinsic Factor)

• Glycoprotein secreted by gastric parietal cells
• Absolutely necessary for absorption of Vitamin B12
• Absence of IF (as in pernicious anemia or post-gastrectomy) → B12 deficiency → megaloblastic anemia
• Castle's experiment: "Extrinsic factor" (Vit B12, from food) + "Intrinsic factor" (from stomach) → combined absorption

3. Folic Acid (Folate / Pteroylglutamic Acid)

• Also essential for DNA synthesis (thymidylate synthesis)
• Works synergistically with Vit B12 in one-carbon transfer reactions
• Source: Green leafy vegetables, liver, legumes
• Absorbed in proximal jejunum (no need for intrinsic factor)
• Deficiency → Megaloblastic anemia (identical blood picture to B12 deficiency, but no neurological features)
• Important in pregnancy (prevents neural tube defects - spina bifida)

GROUP III: FACTORS NECESSARY FOR HEMOGLOBIN FORMATION

1. Iron (Fe)

• Essential component of heme (Fe²⁺ is incorporated into protoporphyrin IX by ferrochelatase)
• Dietary iron absorbed mainly in duodenum and upper jejunum (as Fe²⁺)
• Transported in blood bound to transferrin (apotransferrin + 2 Fe³⁺)
• Stored in marrow macrophages and hepatocytes as ferritin (soluble) and hemosiderin (insoluble)
• Normal serum iron: 60-160 µg/dL; Serum ferritin: 12-300 ng/mL
• Iron deficiency (most common worldwide) → Microcytic, hypochromic anemia; reduced Hb per cell
• Regulation: Hepcidin (liver peptide) - blocks ferroportin → reduces iron absorption and release from stores

2. Proteins / Amino Acids

• Required for globin chain synthesis (alpha, beta, gamma, delta chains)
• Hb A = 2α + 2β chains; Hb A₂ = 2α + 2δ chains; Hb F = 2α + 2γ chains
• Protein malnutrition → impaired globin synthesis → reduced Hb

3. Pyridoxine (Vitamin B6)

• Cofactor for ALA synthase (delta-aminolevulinic acid synthase) - the rate-limiting enzyme of heme synthesis
• Deficiency → Sideroblastic anemia (iron accumulates in mitochondria of erythroblasts - "ringed sideroblasts" on Perl's stain)

4. Cobalt

• Stimulates EPO secretion (used in past as therapy)
• Also a component of Vitamin B12

5. Copper

• Required for ceruloplasmin - ferrooxidase that oxidizes Fe²⁺ to Fe³⁺ for loading onto transferrin
• Copper deficiency → iron cannot be mobilized from stores → functional iron deficiency → anemia

SUMMARY TABLE OF FACTORS (GK Pal Classification)

DURATION OF ERYTHROPOIESIS

KEY POINTS FOR EXAM

1. GK Pal's 3-stage site classification: Mesoblastic → Hepatic → Medullary (Myeloid)
2. Cell series: Proerythroblast → Basophilic EB → Polychromatophilic EB → Orthochromatophilic EB (Normoblast) → Reticulocyte → Mature RBC
3. Last mitotic stage = Polychromatophilic erythroblast
4. First stage without nucleus = Reticulocyte
5. Reticulocyte count = measure of erythropoietic activity (normal: 0.5-2.5%)
6. GK Pal's 3-group classification of factors: General factors / Maturation factors / Factors for Hb formation
7. EPO is the primary regulator, produced by peritubular kidney cells in response to hypoxia
8. B12 + IF for nuclear maturation; Iron + B6 for Hb formation
9. Castle's experiment: Extrinsic factor (B12) + Intrinsic factor (IF) = antianemic factor