Promyelocytes represent the second developmental stage of granulopoiesis between myeloblasts and myelocytes. Cells vary in size from 15 - 25 µm, have a distinct basophilic cytoplasm with coarse, bright red primary (azurophilic) granules, and often have a perinuclear halo. Nuclei range in shape from round to oval and are usually eccentric, with fine chromatin and several nucleoli.
The normal promyelocyte must be differentiated from the pathologic blastic promyelocyte of acute promyelocytic leukemia . In contrast to the normal promyelocytes, leukemic promyelocytes have dense and coarse granules in the classical hypergranular type of AML M3 with almost invisible perinuclear clearing, bigger nuclei of variable shapes and occasionally characteristic Auer rods.