The myeloblast is a unipotent stem cell, which will differentiate into one of the effectors of the granulocyte series. The stimulation by G-CSF and other cytokines triggers maturation, differentiation, proliferation and cell survival. It is found in the bone marrow.
These cells descend from the primitive reticulum cells, which are found in the stroma of the marrow. There is also an intermediate phase between the myeloblast and these primitive reticulum cell, namely the hemocytoblast. At this time several developing blood cell lines are available, like erythropoiesis and thrombopoiesis. The granulopoiesis is regulated by humoral agents, like colony-stimulating factor (CSF) and interleukin 3.
Location in the body
The myeloblasts reside extravascularly in the marrow. The hematopoiesis takes place in the extravascular cavities between the sinuses of the marrow. The wall of the sinuses is composed of two different types of cells, the endothelial cells and the adventitial reticular cells. The hemopoietic cells are aligned in cords or wedges between these sinuses, the myeloblasts and other granular progenitors are concentrated in the subcortical regions of these hemopoietic cords.
Myeloblasts are rather small cells with a diameter between 14 and 18μm. The major part is occupied by a large oval nucleus composed of very fine nonaggregated chromatin and possesses 3 or more nucleoli. The cytoplasm has basophilic character and is devoid of granules, which is a major difference with its successor, the promyelocyte. The nucleolus is the site of assembly of ribosomal proteins, which are located in various particles dispersed over the cytoplasm. Mitochondria are present but have a rather small size.
The main features that distinguish a myeloblast from a lymphoblast upon microscopic examination are the presence of more prominent nucleoli, the nuclear chromatin being less condensed, and cytoplasmic granules are present.
The granulopoiesis consists of 5 stages, of which the myeloblast is the first recognizable cell. Next in the differentiation sequence is the monoblast and the promyelocyte, which can develop into one of the three different precursor cells: neutrophilic, basophilic or eosinophilic myelocytes. This proliferation needs five divisions before the final stage is obtained. These divisions all take place in the first three stages of granulopoiesis.
The most common problem with malfunctioning myeloblasts is acute myeloblastic leukemia. The main clinical features are caused by failure of the hemopoiesis with anemia, hemorrhage and infection as result. There is a progressive accumulation of leukemic cells, because some blast progenitor cells renew themselves and will have a limited differentiated division. Sometimes acute myeloblastic leukemia can be initiated by earlier hematologic disorder, like myelodysplastic syndrome, pancytopenia, or hypoplasia of the bone marrow.
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