Erythropoiesis: Definition, Stages, Regulation, and Iron Deficiency Anemia

1. Definition of Erythropoiesis

• Histology: A Text and Atlas with Correlated Cell and Molecular Biology
• Basic Medical Biochemistry: A Clinical Approach, 6e

2. Stages of Erythropoiesis

A. Commitment Phase (Progenitor Cells)

The transcription factor GATA-1 is required for terminal differentiation of MEP cells into the definitive erythroid lineage.

Stages of erythrocytic differentiation from CMP cells - Histology: A Text and Atlas

B. Maturation Phase (Morphologically Recognizable Precursors)

As RNA falls, hemoglobin rises - progressive shift from basophilia to eosinophilia in cytoplasm

Stage 1: Proerythroblast (Pronormoblast)

• Size: 12-20 µm in diameter (large cell)
• Nucleus: Large, round, with 1-2 visible nucleoli; fine euchromatin
• Cytoplasm: Mildly basophilic (free ribosomes beginning to accumulate)
• Activity: Components for hemoglobin (Hgb) production begin to accumulate
• Duration: ~24 hours
• Mitosis: Capable of division
• Key point: First microscopically recognizable erythroid precursor

Stage 2: Basophilic Erythroblast (Early Normoblast)

• Size: Smaller (10-16 µm); arises by mitotic division from proerythroblast
• Nucleus: Smaller, progressively more heterochromatic; no visible nucleoli
• Cytoplasm: Strong basophilia due to abundant polyribosomes actively synthesizing Hgb
• Duration: ~24 hours
• Mitosis: Capable of division
• Key point: Peak RNA content; maximum basophilia of cytoplasm

Stage 3: Polychromatophilic Erythroblast (Intermediate Normoblast)

• Size: Smaller than basophilic erythroblast
• Nucleus: Smaller; coarse heterochromatin forms a "checkerboard" pattern
• Cytoplasm: Mixed staining - gray/lilac color (both basophilia from RNA and eosinophilia from accumulating Hgb)
• Duration: ~30 hours
• Mitosis: This is the LAST stage capable of mitosis
• Key point: Hemoglobin production begins in earnest; cytoplasm reflects RNA-Hgb transition

Stage 4: Orthochromatophilic Erythroblast (Late Normoblast / Normoblast)

• Size: Only slightly larger than a mature erythrocyte
• Nucleus: Small, compact, densely stained (pyknotic); no chromatin pattern visible
• Cytoplasm: Eosinophilic (large amount of Hgb); slight residual basophilia
• Duration: ~48 hours
• Mitosis: No longer capable of division
• Key event: Nucleus is ejected (extruded) from the cell; nuclear fragments left behind are called Howell-Jolly bodies

Stage 5: Polychromatophilic Erythrocyte (Reticulocyte)

• Origin: Orthochromatophilic erythroblast after nuclear extrusion
• Characteristics: Acidophilic cytoplasm with trace of earlier gray; retains polyribosomes and mRNA capable of synthesizing Hgb
• Staining: With supravital stains (e.g., brilliant cresyl blue), polyribosomes clump and form a reticular network - hence "reticulocyte"
• Normal count: 1-2% of total erythrocyte count in peripheral blood
• Transit: 24 hours in bone marrow, then 24 hours in peripheral circulation, then matured in the spleen where ribosomes/mRNA are lost
• Clinical significance: Reticulocytosis indicates accelerated erythropoiesis (e.g., after hemorrhage)

Stage 6: Mature Erythrocyte (Red Blood Cell)

• Size: 7-8 µm in diameter; biconcave disc
• Appearance: Acidophilic; no nucleus, no organelles
• Life span: ~120 days in circulation
• Fate: Senescent erythrocytes are phagocytosed by macrophages in spleen, liver, and bone marrow

General maturation trend: Each successive division produces a smaller cell; the nucleus shrinks and becomes pyknotic; nucleoli disappear; the cytoplasm shifts from deep blue (basophilic) → gray/lilac → pink (eosinophilic); RNA falls while Hgb rises.

3. Regulation of Erythropoiesis

A. Erythropoietin (EPO) - Primary Regulator

EPO released by kidney in response to hypoxia stimulates erythroid maturation - Basic Medical Biochemistry, 6e

• Source: Produced and secreted primarily by peritubular interstitial cells of the kidney (90%); small amount from liver (10%)
• Stimulus: Tissue hypoxia (decreased oxygen delivery to the kidney)
• Mechanism: Hypoxia stabilizes HIF-1α (Hypoxia-Inducible Factor), which stimulates EPO gene transcription
• Molecular weight: 34 kDa glycoprotein hormone
• Site of action: EPO acts on specific receptors (ErP and CFU-E) via the JAK2/STAT5 signaling pathway
• Effect: Stimulates proliferation, differentiation, and survival of erythroid progenitors; accelerates maturation from BFU-E through reticulocyte release
• Negative feedback: As RBC mass increases and oxygen delivery improves, EPO secretion falls

Mutations in the EPO receptor that prevent binding of the phosphatase SHP-1 result in sustained JAK2/STAT5 activation and polycythemia, as the receptor cannot be inactivated.

B. Growth Factors and Cytokines

C. Iron and Hepcidin

• Iron is essential for hemoglobin synthesis; its availability limits erythropoiesis
• Hepcidin (produced by liver) is the master regulator of iron homeostasis:
  • Inhibits ferroportin on enterocytes and macrophages, reducing iron release into plasma
  • Increased by: IL-6 (inflammation), iron loading
  • Decreased by: EPO, iron deficiency, erythroferrone (released by erythroid precursors)
  • When erythropoiesis is stimulated, erythroid precursors secrete erythroferrone, which suppresses hepcidin, allowing more iron absorption

D. Other Regulators

• Androgens: Stimulate EPO production (explains higher RBC count in males)
• Thyroid hormone: Increases EPO sensitivity; hypothyroidism causes mild anemia
• Corticosteroids: Generally stimulate erythropoiesis
• Vitamin B12 and Folate: Required for DNA synthesis during rapid cell division; deficiency causes megaloblastic anemia

E. Feedback Summary

Tissue Hypoxia
     ↓
Kidney senses low O₂
     ↓
EPO secretion ↑
     ↓
BFU-E/CFU-E stimulated → Erythroid maturation → More RBCs
     ↓
O₂ delivery improves
     ↓
EPO secretion ↓ (negative feedback)

4. Note on Iron Deficiency Anemia (IDA)

Definition

Iron Metabolism (Normal)

Iron cycle: gut absorption → plasma transferrin → marrow erythroid precursors → RBCs → recycled by macrophages - Robbins, Cotran & Kumar Pathologic Basis of Disease

• Total body iron: ~3,450 mg in males; ~2,450 mg in females
• Functional pool: Hemoglobin (2,100 mg), myoglobin (300 mg), enzymes (catalase, cytochromes)
• Storage pool: Ferritin (in hepatocytes/macrophages) and hemosiderin
• Transport: Transferrin carries iron from gut → bone marrow → erythroid precursors (via transferrin receptor-mediated endocytosis)
• Daily intake needed: ~1-2 mg absorbed; dietary intake is 10-20 mg/day; only 1-2% of non-heme iron and ~20% of heme iron is absorbed

Etiology (Causes)

1. Dietary lack: Rare in high-resource countries; common in infants (breast milk provides only 0.3 mg/L iron), vegans, elderly with restricted diets
2. Impaired absorption: Malabsorption (celiac/sprue, steatorrhea), gastrectomy (reduced acidity in duodenum), H. pylori infection, dietary inhibitors (tannins, oxalates, phosphates, carbonates)
3. Increased requirement: Pregnancy, infants/children/adolescents (rapid growth), premenopausal women
4. Chronic blood loss: Most common cause in developed countries - GI bleeding (peptic ulcer, carcinoma, hookworm), menorrhagia, urinary tract bleeding

In adult males and postmenopausal females, iron deficiency almost always indicates GI blood loss, which must be investigated to exclude colorectal carcinoma.

Pathogenesis

Morphology

Peripheral blood smear in IDA: hypochromic microcytic RBCs with expanded central pallor and pencil cells - Robbins, Cotran & Kumar

• Peripheral blood: Microcytic, hypochromic RBCs; zone of central pallor enlarged (normally 1/3 cell diameter; in IDA, Hgb seen only at narrow rim)
• Poikilocytosis: "Pencil cells" (elongated red cells) are characteristic
• Bone marrow: Mild-moderate increase in erythroid progenitors; absent stainable iron in macrophages on Prussian blue stain (diagnostically significant)

Clinical Features

• Fatigue, pallor, exertional dyspnea, palpitations, headache

• Koilonychia (spoon-shaped nails)
• Alopecia (hair loss)
• Atrophic glossitis (smooth, sore tongue)
• Angular stomatitis (cracking at mouth corners)
• Pica (craving for non-food items: clay, ice [pagophagia], flour)
• Gastric mucosal atrophy and intestinal malabsorption
• Plummer-Vinson syndrome (rare triad): microcytic hypochromic anemia + atrophic glossitis + esophageal webs

Laboratory Diagnosis

Treatment

• Oral iron supplementation: Ferrous sulfate 325 mg TDS (elemental iron ~65 mg/dose); taken on an empty stomach with vitamin C to enhance absorption
• Response: Reticulocytosis appears within 5-7 days; followed by steady rise in Hgb and normalization of red cell indices
• Duration: Continue for 3-6 months after Hgb normalization to replenish iron stores
• Parenteral iron: For those with malabsorption, intolerance to oral iron, or ongoing blood loss
• Treat underlying cause: Identify and treat the source of blood loss or deficiency

Sources

• Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 736-746
• Basic Medical Biochemistry: A Clinical Approach, 6e, p. 1536
• Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 612-615