Myeloblast: A type of immature white blood cell that forms in the bone marrow. Myeloblasts become mature white blood cells called granulocytes (neutrophils, basophils, and eosinophils.) Myeloblasts are the primitive or “precursor” cells. Precursor notes a primitive member of the cell line that proliferates more cells of its type unless prompted with the proper stimulus to produce cells which show further differentiation along the cell line.