Myelopoiesis is the regulated formation of myeloid cells, including eosinophilic granulocytes, basophilic granulocytes, neutrophilic granulocytes, and monocytes. In hematology, myelopoiesis is the production of blood cells in the bone marrow.
The myeloid progenitor can differentiate in the bone marrow into granulocytes, macrophages (mature monocytes), mast cells (whose blood-borne progenitor is not well defined), and dendritic cells of the innate immune system. The granulocytes, also called polymorphonuclear leukocytes because of their oddly shaped nuclei, give rise to three short lived cell types including eosinophils, basophils, and neutrophils. A granulocyte differentiates into a distinct cell type by a process called granulopoiesis. In this process it first transforms from a common myeloblast (myeloid progenitor) to a common promyelocyte. This promyelocyte gives rise to a unique myelocyte that for the first time can be classified as a eosinophil, basophil, or neutrophil progenitor based on the histological staining affinity (eosinophilic, basophilic, or neutral granules). The unique myelocyte next differentiates into a metamyelocyte and then a band cell, with a "C" shaped nucleus, before becoming a mature eosinophil, basophil, or neutrophil. Macrophages come from monoblast progenitors that differentriate into promonocytes, which mature into monocytes. Monocytes eventually enter the tissues and become macrophages.