Primary Hematopoietic and Lymphoid Tissue ›› Bone Marrow ›› Normal

Myelocyte*


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› Microscopic Features:

• 2-3x larger than a mature RBC
• Intermediate nuclear to cytoplasmic ratio (more cytoplasm than promyelocyte)
• Eccentrically placed oval Nucleus (No nucleus indentation) with more mature chromatin (clumped)
• Perinuclear clearing is common
• Nucleoli are absent (Note: absent nucleoli from this stage onward)
• More cytoplasm with only rare or absent primary (azurophilic) granules
• Secondary (specific) granules of neutrophilic (lilac), eosinophilic (red-orange), or basophilic (blue) color are present based on precursor type

› Normal % blood-PB, marrow-BM, lymphoid tissue-LN:

• PB: None
• BM: Scattered (~10%)
• LN: None

› May Resemble:

• Promyelocyte
• Metamyelocyte
• Monocyte
• Reactive lymphocyte
• Plasma cell

› Differential Diagnoses:

If in blood or increased numbers in bone marrow: Chronic Myelogenous Leukemia (CML) Leukemoid reaction Post-GCSF therapy Chronic Neutrophilic Leukemia

› Classic Immunophenotype:

• CD34-
• CD117-
• HLA-DR-
• CD13+
• CD33+
• CD15+
• CD11b+/-
• CD16-

› Cartoon Image:


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› Misc:

• Myelocyte bulge (increased myelocytes) is commonly noted in CML patients