Granuloblasts

Granuloblasts are immature cells in the granulocytic lineage of hematopoiesis, precursors that give rise to granulocytes—neutrophils, eosinophils, and basophils—within the bone marrow. They represent an early stage in the development of the granulocytic component of the blood.

The term granuloblast is historic and not consistently used in modern hematology. In many contemporary texts, the earliest identifiable granulocytic precursors are the myeloblasts, followed by promyelocytes and successive maturation stages. Some authors use granuloblasts to refer more generally to early granulocytic precursors, including both myeloblasts and promyelocytes.

Morphologically, granuloblasts typically exhibit a high nucleus-to-cytoplasm ratio, a large round or oval nucleus with fine chromatin, and one or more prominent nucleoli. The cytoplasm is relatively scant and basophilic, with little or no granulation in the earliest cells; as maturation proceeds, cytoplasmic granules begin to appear and stage-specific features become evident.

Development and identification rely on light microscopy and, in modern practice, often immunophenotyping. Granuloblasts arise from common myeloid progenitors under the influence of growth factors such as granulocyte colony-stimulating factor. Immunophenotypically, early granulocytic precursors may express markers such as CD34 and myeloperoxidase, with later stages showing markers associated with mature granulocytes (for example, CD16 and CD15 in neutrophils).

Clinical significance centers on the role of granulocytic precursors in normal hematopoiesis and in disorders such as myeloid leukemias, where an increased number of primitive granulocytic precursors may be observed in the bone marrow. Granuloblasts are part of the diagnostic vocabulary when assessing granulocytic differentiation and maturation.

See also: myeloblast, promyelocyte, granulocyte, granulopoiesis.