Promyelozyten

Promyelozyten, or promyelocytes, are an immature granulocytic precursor in hematopoiesis. They represent an early stage of the myeloid lineage in the bone marrow, formed between myeloblasts and myelocytes during granulopoiesis. In healthy individuals, promyelocytes are confined largely to the bone marrow and are rarely seen in peripheral blood.

Morphologically, promyelocytes are typically 12–20 micrometers in diameter with a large, oval nucleus that often contains one or two prominent nucleoli. The cytoplasm is basophilic and packed with primary (azurophilic) granules that stain darkly, which can obscure the nucleus. Secondary granules appear later as cells mature into myelocytes.

Promyelocytes arise from myeloblasts through cellular division and differentiation as part of granulopoiesis. They begin to synthesize lysosomal enzymes stored in the primary granules, contributing to the cells' eventual role in innate immunity. They further differentiate into myelocytes, metamyelocytes, and mature granulocytes such as neutrophils, eosinophils, and basophils.

Abnormalities in promyelocyte numbers or maturation are seen in hematologic disorders. Acute promyelocytic leukemia, a type of acute myeloid leukemia (APL/M3), is characterized by an accumulation of promyelocytes and is associated with a risk of disseminated intravascular coagulation. Diagnosis combines morphological evaluation with cytochemical staining (myeloperoxidase positivity) and immunophenotyping.