Myelomonocytic

Myelomonocytic is an adjective used in hematology to describe cells, precursors, or diseases that show differentiation along both the myeloid (granulocytic) and monocytic lineages. In normal hematopoiesis, cells of the myeloid lineage arise from the common myeloid progenitor, and the myelomonocytic pathway includes precursors of neutrophils, monocytes, and macrophages. Maturation is promoted by cytokines such as GM-CSF, G-CSF, and M-CSF.

In clinical practice, the term is most often applied to leukemias with dual myeloid and monocytic differentiation. Acute myelomonocytic leukemia (AML-M4) is the FAB designation for AML with prominent myeloid and monocytic features; in the WHO classification it is described as AML with myelomonocytic differentiation. Patients typically present with symptoms of acute leukemia, and laboratory findings may include anemia, thrombocytopenia, and circulating blasts containing both granulocytic and monocytic characteristics. Immunophenotyping often shows myeloid markers such as CD33 and CD13 together with monocytic markers like CD14; cytochemical staining may reveal non-specific esterase activity in monocytic cells and variable myeloperoxidase activity. Cytogenetic abnormalities such as inv(16)/t(16;16) are associated with certain myelomonocytic leukemias, and prognosis depends on cytogenetic and molecular features.

The term also appears in non-malignant contexts, describing bone marrow or peripheral blood populations that coexpress myeloid and monocytic markers. Chronic myelomonocytic leukemia (CMML) is a related clonal myeloid neoplasm characterized by persistent monocytosis with dysplasia, illustrating a broader use of the concept of myelomonocytic differentiation.