The myeloblast is a unipotent stem cell which differentiates into the effectors of the

granulocyte Granulocytes are cells in the innate immune system characterized by the presence of specific granules in their cytoplasm. Such granules distinguish them from the various agranulocytes. All myeloblastic granulocytes are polymorphonuclear. They ha ...

series. It is found in the

bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...

. Stimulation of myeloblasts by G-CSF and other

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in au ...

s triggers maturation, differentiation, proliferation and cell survival.

Structure

Myeloblasts reside extravascularly in 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, endothelial cells and adventitial reticular cells. The hemopoietic cells are aligned in cords or wedges between these sinuses, with myeloblasts and other granular progenitors 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 Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

composed of very fine nonaggregated

chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important ...

and possessing 3 or more

nucleoli The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of s ...

. The

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

has a basophilic character and is devoid of granules, which is a major difference from the myeloblast's successor, the promyelocyte. The

nucleolus The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of sign ...

is the site of assembly of ribosomal proteins, which are located in various particles dispersed over the cytoplasm.

Mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

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 cytoplasmic granules, the lesser degree of condensation in the nuclear chromatin, and the increased prominence of the nucleoli.Figure 12-14 in:

Development

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 cells, namely the

hemocytoblast Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells. This process is called haematopoiesis. In vertebrates, the very first definitive HSCs arise from the ventral endothelial wall of the embryonic aorta within t ...

. At this time several developing

blood cell A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes) ...

lines are available, like erythropoiesis and thrombopoiesis. Granulopoiesis is regulated by humoral agents, like

colony-stimulating factor Colony-stimulating factors (CSFs) are secreted glycoproteins that bind to receptor proteins on the surfaces of hemopoietic stem cells, thereby activating intracellular signaling pathways that can cause the cells to proliferate and differentia ...

(CSF) and interleukin 3.

Function

Granulopoiesis consists of 5 stages, in which the myeloblast is the first recognizable cell. Next in the differentiation sequence is the

monoblast Monoblasts are the committed progenitor cells that differentiated from a committed macrophage or dendritic cell precursor (MDP) in the process of hematopoiesis. They are the first developmental stage in the monocyte series leading to a macrophage ...

and the promyelocyte, which can develop into one of three different precursor cells: the neutrophilic, basophilic or eosinophilic

myelocyte A myelocyte is a young cell of the granulocytic series, occurring normally in bone marrow (can be found in circulating blood when caused by certain diseases). Structure When stained with the usual dyes, the cytoplasm is distinctly basophil ...

. This proliferation takes five divisions before the final stage is obtained. These divisions all take place in the first three stages of granulopoiesis. File:Leukopoiese.JPG File:Granulopoiesis.jpg

Clinical significance

The most common problem with malfunctioning myeloblasts is acute myeloblastic leukemia. The main clinical features of acute myeloblastic leukemia are caused by failure of hemopoiesis with

anemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, t ...

hemorrhage Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, v ...

and infection as a result. There is a progressive accumulation of leukemic cells, because some blast progenitor cells renew themselves and have a limited differentiated division. Sometimes acute myeloblastic leukemia can be initiated by earlier hematologic disorders, like myelodysplastic syndrome,

pancytopenia Pancytopenia is a medical condition in which there is significant reduction in the number of almost all blood cells ( red blood cells, white blood cells, platelets, monocytes, lymphocytes, etc.). If only two parameters from the complete blood ...

, or

hypoplasia Hypoplasia (from Ancient Greek ὑπo- ''hypo-'' 'under' + πλάσις ''plasis'' 'formation'; adjective form ''hypoplastic'') is underdevelopment or incomplete development of a tissue or organ.bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...

Structure

Development

Function

Clinical significance

See also

References

Further reading