Erythropoiesis: Stages and Controlling Factors

Definition

Stages of Erythropoiesis

Phase 1: Committed Progenitor Stages (Morphologically Unrecognizable)

Phase 2: Morphologically Recognizable Stages (in Bone Marrow)

1. Proerythroblast (Pronormoblast)

• Largest of the recognizable precursors (~20 µm); scarce in marrow
• Nucleus: Large, occupies most of the cell; loose, lacy (euchromatic) chromatin; multiple prominent nucleoli
• Cytoplasm: Deeply basophilic (reflects abundant ribosomes/polyribosomes; no hemoglobin yet)
• Golgi apparatus may appear as a pale perinuclear zone
• Capable of mitosis; duration ~20 hours

2. Basophilic Erythroblast (Early Normoblast)

• Smaller than the proerythroblast (arises by mitotic division)
• Nucleus: Smaller; chromatin begins to coarsen and condense; nucleoli disappear
• Cytoplasm: Strongly basophilic — reflects the increasing number of ribosomes and polyribosomes now actively synthesizing hemoglobin
• Capable of mitosis; duration ~20 hours

3. Polychromatophilic Erythroblast (Intermediate Normoblast)

• Further reduced cell size
• Nucleus: Smaller still; coarser, clumped heterochromatin (checkerboard pattern)
• Cytoplasm: Mixed blue-gray (polychromatic) — hemoglobin accumulation introduces eosinophilia, but residual RNA retains basophilia; the two staining qualities coexist
• Active hemoglobin synthesis
• Capable of mitosis (last stage that divides); duration ~25 hours
• This is the last EPO-dependent stage (along with earlier erythroblasts)

4. Orthochromatophilic Erythroblast (Late Normoblast / Normoblast)

• Cell is now smaller, slightly larger than a mature erythrocyte
• Nucleus: Extremely condensed (pyknotic); no visible chromatin pattern; occupies a small fraction of cell volume
• Cytoplasm: Nearly eosinophilic/pink — hemoglobin now predominates; very little RNA remains
• Cannot divide; duration ~30 hours
• Late in this stage: the pyknotic nucleus is ejected (extruded) and phagocytosed by macrophages at the erythroblastic island

5. Reticulocyte (Polychromatophilic Erythrocyte)

• Enucleate cell — nucleus has been expelled
• Still contains residual polyribosomes and mRNA, capable of some hemoglobin synthesis
• With brilliant cresyl blue stain: ribosomes precipitate into a faint blue network → "reticulum" (hence the name)
• Released from bone marrow into the peripheral bloodstream
• Circulates for 1–2 days, then matures in the spleen where ribosomes and mRNA are lost
• Normally constitutes ~1% of circulating red cells

6. Mature Erythrocyte

• Biconcave disc, ~7–8 µm
• No nucleus, no organelles; entirely eosinophilic cytoplasm filled with hemoglobin
• Life span: ~120 days
• Senescent cells are phagocytosed by macrophages in the spleen, bone marrow, and liver; iron is recycled

Key sequential changes summarized: cell size ↓ → nuclear size ↓ → nuclear chromatin condensation ↑ → nucleoli disappear → cytoplasm: basophilic → polychromatic → eosinophilic → nucleus ejected → reticulocyte → erythrocyte.

Factors Controlling Erythropoiesis

1. Erythropoietin (EPO) — Primary Hormonal Regulator

• A 34 kDa glycoprotein (30.4 kDa protein core), produced mainly by peritubular interstitial cells of the kidney (and a minor amount by the liver)
• Regulated by a classic negative feedback loop: hypoxia → ↑ EPO → ↑ erythropoiesis → ↑ O₂ delivery → ↓ EPO release
• Mechanism: EPO binds the EPO receptor (EPO-R) on erythroid progenitors, activating the JAK2/STAT5 signaling pathway; this suppresses apoptosis and promotes proliferation/differentiation
• EPO receptors are most densely expressed on CFU-E cells
• Steady-state serum EPO: 10–30 U/L; in severe anemia/hypoxia: up to 10,000 U/L
• EPO is an essential survival factor from CFU-E through the basophilic erythroblast stage — without it, these cells rapidly undergo apoptosis
• Bone marrow is not a storage site for erythrocytes; erythrocytes are released as soon as formed

2. Oxygen Tension (Hypoxia-Sensing Mechanism)

• The kidney acts as an oxygen sensor via the HIF (Hypoxia-Inducible Factor) pathway
• In hypoxia: HIF-1α/HIF-2α are stabilized (normally degraded by prolyl hydroxylases under normoxia) → bind HRE (hypoxia-response elements) in the EPO gene promoter → ↑ EPO transcription
• HIF-2α is the principal isoform controlling EPO synthesis in renal peritubular cells
• Conditions stimulating EPO: high altitude, anemia, cardiopulmonary disease, hemorrhage, hemolysis

3. Iron

• Essential for hemoglobin (heme) synthesis; iron deficiency is the most common cause of anemia worldwide
• Iron supply must precisely match demand: ~1 mg/day absorbed from diet (5–10% of dietary intake); the majority comes from macrophage recycling of senescent RBCs
• HRI (Heme-Regulated Inhibitor / eIF-2α kinase): In iron-sufficient states, free heme binds HRI and inhibits eIF-2α phosphorylation → globin synthesis proceeds. In iron deficiency, HRI phosphorylates eIF-2α and suppresses mTORC1 → reduced globin synthesis → microcytic erythrocytes
• Iron deficiency also partially suppresses HIF-2α → reduces EPO production → inappropriately low reticulocyte count

4. Vitamin B₁₂ (Cobalamin) and Folic Acid

• Both are essential for DNA synthesis (purine and pyrimidine synthesis), required for the rapid cell divisions of erythropoiesis
• Deficiency → inability of erythroid progenitors to complete cell division → megaloblastic anemia (ineffective erythropoiesis with large abnormal cells)
• The stage most affected is at the transition from EPO-dependent effects to hemoglobin synthesis (around the CFU-E/proerythroblast boundary)
• Increased serum bilirubin, LDH, and accelerated iron turnover are hallmarks of the resulting ineffective erythropoiesis

5. Growth Factors and Cytokines

6. The Erythroblastic Island Microenvironment

• Erythroblasts mature in close contact with central macrophages in erythroblastic islands within the bone marrow
• Macrophages supply ferritin and iron to developing erythroblasts, assist in enucleation, and support proliferation
• Osteoblasts in the hematopoietic niche can also locally produce EPO, modulating the response to systemic anemia

7. Negative Feedback

• Adequate oxygen delivery → reduced HIF-2α → ↓ EPO → ↓ erythropoiesis
• Mature erythrocytes themselves do not feed back directly, but their oxygen-carrying capacity determines the hypoxic stimulus
• Excess EPO → polycythemia → increased blood viscosity, which can paradoxically reduce tissue O₂ delivery

Summary Table

• Junqueira's Basic Histology: Text and Atlas, 17e, pp. 644–645
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology, Plate 10.3, pp. 776–777
• Basic Medical Biochemistry: A Clinical Approach, 6e, p. 1536
• Brenner and Rector's The Kidney, 2-Volume Set, pp. 2465–2469