Most of the blasts are agranular. Auer rods are infrequent. NSE and PAS are generally negative. NSE and PAS are generally negative. In the classic M3 the majority of the proliferating cells are abnormal promyelocytes with numerous primary type granules.
Auer rods are frequent and often multiple .
Recognition of M3 leukemias is critical because M3s can be initially treated with retinoic acid rather than toxic chemotherapy. In the microgranular variant, M3v, the leukemic cells have a monocytic appearance with clefted angel-wing nuclei and abundant cytoplasm having at best indistinct cytoplasmic granulation. The cytochemical, immunophenotypic and chromosomal features are indentical to the classic M3. Differentiation along both myeloid and monocytic lines . This is the M5a or poorly differentiated form of acute monoblastic leukemia .