The earliest morphologically distinct myeloid cell is a myeloblast. Cell with myeloid commitment at stages of differentiation between the pleuripotant stem cell and myeloblasts have been identified but these lack morphological characters of a myeloid lineage. Myeloid cells share a common precursors with the cells of the monocyte-macrophage system in the form of CFU-GM. CFU-GM matures into CFU-G, a precursor for myeloid cells and CFU-M, a precursor for cells of the monocyte-macrophage series. There are five morphological distinct stages of myeloid precursors, myeloblast, promyelocyte, myelocyte, metamyelocyte and the band form. Only the first three are capable of cell division. Myeloid cells in the peripheral blood are classified into neutrophils, eosinophils and basophils depending on the staining characteristics of the cytoplasmic granules. Morphological evidence of commitment one of the myeloid lines is seen at the myelocyte stage. Electron microscopically commitment can be seen at the promyelocyte stage.
Morphology of Myeloid Precursors