Myeloblasts: Earliest Myeloid Cells in Bone Marrow

Morphology

• Large cells (15–20 µm)
• High nuclear-to-cytoplasmic ratio (N:C ~5:1 to 7:1)
• Round/oval nucleus with fine, evenly dispersed chromatin (lacy)
• 1–3 prominent nucleoli
• Basophilic cytoplasm (due to free ribosomes)
• Few or no cytoplasmic granules (rare azurophilic granules/primary granules may appear)
• No Auer rods (if present → acute promyelocytic leukemia)

Lineage & Development

Derived from hematopoietic stem cells (HSC) → multipotent progenitors → common myeloid progenitors (CMP) → granulocyte-monocyte progenitors (GMP) → myeloblasts → proliferate/differentiate into:

1. Promyelocytes (primary granules appear)
2. Myelocytes (secondary/specific granules: neutrophilic, eosinophilic, basophilic)
3. Metamyelocytes (indented nucleus)
4. Band cells (horseshoe nucleus)
5. Mature neutrophils (segmented nucleus)

Parallel pathways for monocytes, eosinophils, basophils, erythrocytes, megakaryocytes.

Clinical Significance

• Normal: <5% of bone marrow nucleated cells; absent in peripheral blood.
• Increased in Peripheral Blood: "Left shift" = stress, infection, inflammation, G-CSF effect.
• Acute Myeloid Leukemia (AML): ≥20% blasts in marrow/blood (WHO). Myeloblasts with Auer rods = APL (AML-M3).
• Myelodysplastic Syndromes (MDS): 5–19% blasts (RAEB).
• Myeloproliferative Neoplasms: Blast phase transformation.

Identification requires expert morphologic review + flow cytometry (CD34+, CD117+, HLA-DR+, CD13+, CD33+, MPO+) + cytogenetics/molecular for classification.