Monoblasts

Monoblasts are immature precursor cells in the monocytic branch of hematopoiesis. They arise from the myeloid lineage, specifically from common myeloid progenitors, and differentiate along the monocytic pathway through promonocytes to mature monocytes. In normal development, monoblasts proliferate in the bone marrow before entering the bloodstream as monocytes that can migrate into tissues and differentiate further into macrophages and dendritic cells.

Morphology and development characterize monoblasts as relatively large cells with a round to oval nucleus, fine chromatin, and one or more nucleoli. The cytoplasm is typically basophilic and may be scant to moderate in amount. Monoblasts are early-stage blasts that lack the fully mature features of monocytes but are committed to the monocytic lineage.

Immunophenotyping and markers help distinguish monoblasts from other blasts. They commonly express myeloid-associated markers such as CD33 and CD13 and may exhibit HLA-DR. They typically lack markers associated with lymphoid lineages (for example, CD3 and CD19). As monoblasts differentiate toward promonocytes and then monocytes, they may acquire additional monocytic markers such as CD14 and CD64.

Clinical significance involves their role in disease when monocytic precursors proliferate abnormally. An excess of monoblasts characterizes acute monocytic leukemia (AML M5), where blasts predominate in bone marrow and peripheral blood. Monoblast proliferation can lead to tissue infiltration and cytopenias, and diagnosis relies on morphology, flow cytometry, and cytogenetic or molecular studies.