metamyelocyte

Metamyelocyte is an immature neutrophil precursor in the granulocyte lineage. It lies between the myelocyte stage and the band neutrophil in normal hematopoiesis, and it is usually retained in the bone marrow. During increased demand for neutrophils, metamyelocytes can be released into the peripheral blood as part of a left shift, alongside other immature forms.

Morphology and identification are the primary means of recognizing metamyelocytes on a stained smear. The nucleus is kidney- or bean-shaped with a deep indentation and lacks the lobulated segmentation seen in mature neutrophils. The chromatin is coarser than in earlier precursors, and the cytoplasm contains neutrophilic granules. Compared with myelocytes, metamyelocytes have a more condensed and indented nucleus and a less foamy cytoplasm. They are typically smaller than mature neutrophils and are considered immature cells that do not yet function as fully competent phagocytes.

Clinical significance and context are important for interpretation. The appearance of metamyelocytes in peripheral blood is uncommon in healthy adults and usually signals a hematologic response to infection, inflammation, or marrow stimulation. A left shift, including metamyelocytes and bands, can be seen in severe bacterial infections, certain inflammatory states, or myeloproliferative disorders and some leukemias. Their presence prompts correlation with clinical findings and additional laboratory evaluation to determine the underlying cause and to distinguish reactive processes from malignant conditions.