Metamyelocytes

Metamyelocytes are immature granulocyte precursors in the bone marrow and represent a developmental stage in neutrophil maturation, occurring after myelocytes and before band neutrophils. They arise during granulopoiesis as myeloid precursors differentiate along the neutrophil lineage; metamyelocytes then mature into band neutrophils and finally segmented neutrophils that enter the bloodstream and tissues to participate in innate immunity. In humans, maturation proceeds in the bone marrow; occasional metamyelocytes may be released into the peripheral blood during extreme demand, but their presence is generally abnormal.

Morphology: Metamyelocytes are typically smaller than myelocytes, with a non-segmented nucleus that has a pronounced indentation, giving a kidney-shaped appearance. The chromatin is coarsely clumped, and nucleoli are absent. The cytoplasm contains neutrophilic granules, including azurophilic granules, and, with further maturation, secondary granules become more evident.

Clinical significance: In clinical hematology, metamyelocytes are not normally present in peripheral blood. Their appearance in blood constitutes a left shift and reflects accelerated granulopoiesis in response to infection, inflammation, or marrow stress. Persistent circulating metamyelocytes can be seen in certain myeloid disorders, including leukemia, and may indicate abnormal hematopoiesis.