Stages of Erythropoiesis

Commitment Phase (Progenitor Cells — not visible by light microscopy)

1. Hematopoietic Stem Cell (HSC) → Common Myeloid Progenitor (CMP)
2. CMP → Megakaryocyte/Erythrocyte Progenitor (MEP), under influence of erythropoietin (EPO), IL-3, and IL-4
3. MEP → Erythrocyte-committed Progenitor (ErP), driven by transcription factor GATA-1
   • This ErP passes through BFU-E (Burst-Forming Unit–Erythroid) and CFU-E (Colony-Forming Unit–Erythroid) stages

Morphologically Recognizable Stages

1. Proerythroblast (~20 hours)

• Largest erythroid precursor: 12–20 μm in diameter
• Large spherical nucleus with 1–2 visible nucleoli
• Mild cytoplasmic basophilia from free ribosomes
• Hemoglobin (Hgb) synthesis begins
• Capable of mitosis

2. Basophilic Erythroblast (~20 hours)

• Smaller than proerythroblast (10–16 μm)
• Nucleus becomes smaller and progressively more heterochromatic
• Strong cytoplasmic basophilia due to abundant free polyribosomes actively synthesizing Hgb
• As Hgb accumulates, cytoplasm begins to stain with eosin
• Capable of mitosis

3. Polychromatophilic Erythroblast (~25–30 hours)

• Cytoplasm shows both basophilia (ribosomes) and acidophilia (Hgb) — the "mixed color" staining
• May appear gray/lilac overall, or show distinct pink and purple regions
• Nucleus smaller with coarse heterochromatin in a checkerboard pattern
• Last stage capable of mitosis

4. Orthochromatophilic Erythroblast / Normoblast (~48 hours)

• Small, compact, densely stained pyknotic nucleus
• Cytoplasm is strongly eosinophilic (large amount of Hgb)
• Size only slightly larger than a mature erythrocyte
• No longer capable of division
• Late in this stage, the nucleus is ejected (extruded) from the cell
• Small nuclear remnants left behind = Howell-Jolly bodies

5. Reticulocyte / Polychromatophilic Erythrocyte (~3 days)

• Nucleus has been extruded; the cell enters blood sinusoids
• Still retains polyribosomes capable of synthesizing Hgb → mild residual basophilia
• Polyribosomes form a reticular network when stained with brilliant cresyl blue — hence "reticulocyte"
• Resides ~24 hrs in bone marrow, then ~24–48 hrs in peripheral circulation
• Normally 1–2% of circulating red blood cells
• Matures in the spleen, where ribosomes and mRNA are lost

6. Mature Erythrocyte

• Biconcave disc, no nucleus, no organelles
• Fully hemoglobin-loaded
• Lifespan ~120 days
• Senescent erythrocytes are phagocytosed by macrophages in spleen, bone marrow, and liver; heme is recycled → bilirubin → bile

Key Changes Throughout Erythropoiesis

Regulation

• Erythropoietin (EPO): A 34-kDa glycoprotein produced by the kidney in response to hypoxia; stimulates proliferation and maturation of ErPs
• GATA-1: Transcription factor essential for commitment to the erythroid lineage
• Vitamins/minerals: Iron (Hgb synthesis), Vitamin B12 and folate (DNA synthesis for cell division)