Primary Hematopoietic and Lymphoid Tissue ›› Bone Marrow ›› Normal Monoblast* Click Here for Full Size › Microscopic Features: 3-4x larger than a mature RBC High nuclear to cytoplasmic ratio Round to slightly Indented Nucleus with immature chromatin (not clumped) Prominent nucleoli Cytoplasm is scant, gray to pale blue (more cytoplasm than a Myeloblast and with rare or absent granules vacuoles are rare but sometimes noted in cytoplasm › Normal % blood-PB, marrow-BM, lymphoid tissue-LN: PB: None BM: 1% LN: None › May Resemble: Promonocyte Myeloblast Pronormoblast Mature Monocyte Reactive lymphocyte Lymphoma cell (e.g. Leukemic phase of Burkitt) Carcinoma cell › Differential Diagnoses: If in blood or increased numbers in bone marrow: AML: Acute Monocytic Leukemia AML: Acute Myelomonocytic Leukemia Chronic Myelomonocytic Leukemia › Classic Immunophenotype: CD34+/- CD64+ CD4dim+ CD14- › Cartoon Image: Click and drag for direct comparison › Misc: Note: In Monocyte differentiated leukemias, total blast count is equal to Monoblasts + Promonocytes (since promonocytes are equivalent to blasts)