Metamyelocytes: An Intermediate Stage in Granulocyte Development
Metamyelocytes are immature granulocytes that represent an intermediate stage in neutrophil development, occurring between myelocytes and band neutrophils in the maturation process of granulocytic cells. They are normally found in the bone marrow but may appear in peripheral blood during certain pathological conditions.
Characteristics of Metamyelocytes
- Morphology: Metamyelocytes have a kidney-shaped or indented nucleus (unlike the round nucleus of myelocytes) but have not yet developed the band or segmented form of mature neutrophils
- Size: Generally 10-15 μm in diameter
- Location: Primarily found in bone marrow during normal hematopoiesis
- Development sequence: Myeloblast → Promyelocyte → Myelocyte → Metamyelocyte → Band neutrophil → Segmented neutrophil
Clinical Significance
Presence in Peripheral Blood
The appearance of metamyelocytes in peripheral blood (along with other immature granulocytes) is referred to as a "left shift" and indicates increased bone marrow activity 1. This can occur in:
- Infections and inflammatory conditions
- Leukemic processes
- Myeloproliferative disorders
- Bone marrow recovery after chemotherapy or radiation
Diagnostic Value
Chronic Myeloid Leukemia (CML): The hallmark of CML diagnosis includes leukocytosis with immature granulocytes in peripheral blood, primarily metamyelocytes, myelocytes, and promyelocytes 1. This is a key diagnostic feature that helps distinguish CML from other myeloproliferative disorders.
Inflammatory Response: Increased metamyelocyte counts in peripheral blood can serve as a biomarker for enhanced bone marrow activity in response to infection or inflammation 2.
Prognostic Indicator: In some hematologic malignancies, the presence of a left shift (including metamyelocytes) at diagnosis has been correlated with disease outcomes 3.
Identification Methods
Microscopic Examination
- Light microscopy with Wright-Giemsa stain is the traditional method for identifying metamyelocytes in bone marrow aspirates or peripheral blood smears
Flow Cytometry
- Modern flow cytometric methods can enumerate and classify immature granulocytes including metamyelocytes
- Metamyelocytes can be identified as CD16-/CD11b+ cells (IG stage 2) 4
Pathological Implications
Vitamin Deficiencies
- In vitamin B12 and folate deficiency, giant metamyelocytes may appear, representing abnormal development of granulocyte precursors 5
Myelodysplastic Syndromes
- Dysplastic metamyelocytes may be observed in myelodysplastic syndromes, showing abnormal nuclear configuration or granulation 1
Biological Role
Metamyelocytes represent a critical stage in neutrophil maturation where:
- The nucleus begins to change from round to indented shape
- Specific granules continue to form
- The cell loses its capacity for division (unlike earlier precursors such as myelocytes)
- The cell continues to develop the enzymatic and functional capabilities needed for mature neutrophil function 6
In summary, metamyelocytes are important intermediate cells in granulocyte development that normally reside in bone marrow. Their appearance in peripheral blood serves as an important diagnostic and prognostic indicator in various hematological and inflammatory conditions.