Broadly, any hematolymphoid precursor is a blast. Erythroblast, Myeloblast, Monoblast, Lymphoblast, Immunoblast, Plasmablast and Megakaryoblast.

They all have scanty amphophilic to basophilic cytoplasm, high N/C ratio, fine or open chromatin with or without nucleoli. Occassionally shows a Paranuclear hoff

Basically, a mitotically active neoplastic cell will have cytoplasmic basophilia and loosened but diffuse chromatin. Nucleolus is actually not necessary for it's morphological distinction.

Mature WBC cells tends to show regularly clumped chromatin. Hence terming their blasts as "hyperchromatic" is disputable.

Agranular blasts

Both pics are WBC precursors for sure. Agranular blasts of WBC are always M0, M1 and lymphoid blasts. (M1 may show granularity in 3% blasts.)

Left picture shows blast size larger than 2 - 3 RBCs. So we may rule out a lymphoblast. Agranular myeloblasts in AML are reported M0/M1 from morphology alone. Further typing requires cytogenetics and Flow cytometry.

Right picture shows a blast with nucleus sized as same as normal RBC. Scantier cytoplasm. Much more diffuse chromatin. Hence lymphoblast.

Granular blasts

Granularity rules out a lymphoblast. So the differentials are whether they are Myeloblast M2 & M3; and Monoblasts of M4 and M5.

Granular blasts may have cytoplasmic basophilic "rods" which gives further clue to the class of AML. They are called Auer Rods. They are fused lysosomes with all the leukocytic enzymes as in their primary granules.

Within M3, there exists a variant called M3V (Microgranular variant) where the blast granules are sparse and nucleus is notched. More on that in the following pictures.

The blasts here are large with scanty but appreciable granules.

The blast on top shows an Auer Rod. The blast below has abundant cytoplasm unlike usual myeloblasts & shows 2 lobes of the nucleus.

Cytoplasm is abundant with scanty granules but have auer rods. And the nucleus is bilobed. Clearly a Monoblast.

Report it as M4/M5 in a resource limited setup.

Blasts here are very similar to the monoblasts. But smaller than monoblasts. Cytoplasm is not that abundant, rarely any auer rods. Nuclei are "Buttock" or "Butterfly" shaped rather than calling it bilobed.

The only myeloblast with such a feature is AML microgranular blast. The microgranular variant shows similar genetics of the M3, hence it is a promyelocytic leukemia but with totally different morphology.

(Ignore the lymphocyte in the bottom)

Blast with abundant granules and Auer rod. (May have abundant auer rods in one cell - "Faggot cell"). Smooth round nucleus. No doubts, we call it AML M3. The hypergranular promyelocyte has abundant granules as seen, unlike the Microgranular promyelocyte.

Ignore the smaller cells which are lymphocytes. Concentrate on the largest cell, with the pale grey nucleolus. Compared to the cytoplasmic volume, and the magnification, the granules are not that abundant. A single Auer rod may be present in some but not much.

By exclusion, we report this granular blast as M2. This is the most common variant.


Based on granules appearing in Romanowski stain and the properties in cytochemical stain.


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Basophilic dark blue granules that cover the nucleus. Composed of Histamine, Chondroitin sulfate, Tryptase, Carboxypeptidase A3 etc. Suggests Basophils (from myelocyte stage)

Eosinophilic granules composed of Major Basic Protein, Eosinophilic PerOxidase, Eosinophil Derived Neurotoxin, Charcot Leyden crystals derived from spermine and mucin. Suggests Eosinophilic precursors from myelocyte downwards.

Azurophilic granules are primary granules. Not specific if present in a blast. Consists of a spectrum of enzymes. Main ones being - Elastase, Myeloperoxidase, Lysozymes, Cathepsin, BPIP, Azurocidin, etc Suggests Neutrophils, Monocytes, Monoblasts and any myeloid blasts upto promyelocytes.


  • MPO - Myeloperoxidase

  • SBB - Sudan Black - B

  • NSE - Non specific esterase

  • PAS - Per-Iodic Acid Schiff

  • Acid phosphatase

MPO and SBB are hand in hand positive for granular myeloblasts and monoblasts. Which are M2, M3, M4 and M5. And negative for any lymphoid blasts.

NSE is positive for any monocytic cell. M4 and M5. And for megakaryoblasts in M7.

PAS, by definition shows block positivity in Lymphoblasts of ALL. If at all, it shows some punctate / fine positivity for AML M6 and M7 as we expect erythroblast and megakaryoblasts in them respectively.

Acid phosphatase is negative for AML by definition.