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It is characterized by the appearance of a bent nucleus, cytoplasmic granules, and the absence of visible nucleoli. (If the nucleus is not yet bent, then it is likely a myelocyte.)
Histology image: 01805ooa – Histology Learning System at Boston University - "Bone Marrow and Hemopoiesis: bone marrow smear, neutrophilic metamyelocyte and mature PMN"