myeloblasts

Myeloblasts are immature precursor cells of the myeloid lineage in the bone marrow. They are the immediate precursors of granulocytes—neutrophils, eosinophils, and basophils—and arise from hematopoietic stem cells through a series of differentiation steps. In normal marrow, myeloblasts represent a small fraction of nucleated cells.

Morphology and development: Myeloblasts are relatively large cells with a high nucleus-to-cytoplasm ratio and round to oval nuclei that have delicate chromatin and one or more prominent nucleoli. The cytoplasm is scant and basophilic. They typically lack visible granules, which begin to appear as development proceeds to promyelocytes and beyond. Myeloblasts progress through stages including promyelocytes, myelocytes, and metamyelocytes before becoming mature granulocytes.

Immunophenotype and markers: In immunophenotyping, myeloblasts express early myeloid and stem cell markers such as CD34, CD33, CD13, and often CD117 (c-Kit). They may show myeloperoxidase (MPO) activity and can express HLA-DR in many cases, though expression patterns vary by subtype. Lymphoid lineage markers are absent or minimal, helping distinguish myeloid blasts from lymphoblasts.

Clinical significance: Myeloblasts are normally a minority in healthy bone marrow. In acute myeloid leukemia, there is clonal expansion of myeloid blasts with an elevated blast count in bone marrow or peripheral blood. The diagnostic threshold for AML is generally 20% blasts. Auer rods, when present in myeloblasts, support a diagnosis of AML. Abnormal proliferation of myeloblasts disrupts normal hematopoiesis and can lead to cytopenias.