Myeloblasts are the most immature form of granulocytes within granulopoiesis and arise from multipotent stem cells of the bone marrow. Granulocytes are involved in the defense against infection. When there is a deficiency of granulocytes, this deficiency may result from a previous deficiency of myeloblasts and results in immunodeficiency in the sense of immune deficiency.
What is a myeloblast?
Granulocytes belong to the leukocytes. They are the group of white blood cells that perform important tasks in the immune system and are thus significantly involved in the defense against infections. Leukocytes are involved, for example, in the recognition of foreign antibodies, in the formation of antigens and in phagocytosis. Granulocytes, more precisely, attack pathogens and render them harmless to the organism. The formation of the cells takes place in the bone marrow and has multipotent precursor cells as its basis. The formation processes are summarized under the term granulopoiesis, which is considered part of hematopoiesis. Multipotent stem cells of the bone marrow develop into the so-called myeloblasts, which are known as the smallest precursor of granulocytes within granulopoiesis. They thus arise from hematopoietic stem cells and represent their first differentiation on the way to becoming granulocytes. Consequently, the cells are also referred to as the most immature form of granulocytes.
Anatomy and structure
Myeloblasts are the only cells in granulopoiesis that do not exhibit granulation. The cells are equipped with a round or oval nucleus that has fuzzy nucleoli. The cytoplasm of myeloblasts appears pale bluish because of its basophilia. All myeloblasts have a size between twelve and 20 micrometers. The chromatin structure of myeloblasts is considered reticular. Surrounding the nucleus of the cells is a Golgi apparatus, which appears perinuclear as a zone of brightening. Unlike the so-called proerythroblasts, myeloblasts do not possess any plasma protrusions. Myeloblasts belong to the so-called “white series”. Among the nucleated cells, they form a proportion of less than five percent. Their precursor cells are called hemocytoblasts. In the stage after the myeloblast, the nascent granulocyte precursors are called promyelocytes. On the way of the myeloblast to the full-fledged granulocyte there are further cell stages. The metamyelocyte is followed by the rod-nucleated granulocyte and finally the segment-nucleated granulocyte.
Function and tasks
The function of myeloblasts is to differentiate into granulocytes. Thus, myeloblasts in themselves have no active roles in the human immune system and are not yet involved in the recognition and defense against exogenous pathogens. They merely form a developmental stage of the granulocytes, which are responsible for recognizing and defending against pathogens. With their participation in granulopoiesis, they are also involved in hematopoiesis on a larger scale. As such, hematopoiesis is the formation of blood in the bone marrow. Without the formation of granulocytes from myeloblasts, the patient’s defense against infection is impaired. For example, if there are too few myeloblasts from which granulocytes are formed, there are too few immune system defense cells available in the patient’s blood. This makes the patient immunocompromised and more susceptible to infections of all kinds. The excessive development of granulocytes from myeloblasts is an indication of an excessively strong immune system and can be a sign of disease. The term neutropenia is used to describe a lack of granulocytes. In granulocytosis, there is an increased number of granulocytes. Myeloblasts, as the precursor stage of granulocytes, are involved in both one and the other. Thus, although myeloblasts per se do not have an active immunologic function, they nevertheless have an essential effect on the functioning of the immune system.
Diseases
In a condition called neutrophil granulocytosis, neutrophil granulocytes exceed the 6.3 G/l threshold. This form of excessive formation of granulocytes from myeloblasts may refer to leukemias or other malignant tumor diseases, but it may also accompany infectious diseases, inflammation, or stress. Eosinophilic granulocytosis is the term used for an abnormal proliferation of eosinophilic granulocytes in the peripheral blood. In most cases, the abnormal granulopoiesis is due to an allergic reaction.In some cases, the phenomenon is further observed in parasite infestation. In basophilic granulocytosis, basophilic granulocytes proliferate above the limit. Usually this form of granulocytosis occurs together with eosinophilic granulocytosis and is favored by allergies or hypersensitivity reactions. Parasitoses and hyperlipidemia are also conceivable causes. In pathological reduction of neutrophil granulocytes, neutrophils and other granulocytes are proportionally absent within the blood. This neutropenia is the most common leukopenia. For the patient, the phenomenon is critical because his ability to defend against infections is significantly reduced by the decrease in granulocytes. In particular, affected individuals are much more susceptible to bacterial infections. Neutropenia can occur when too few granulocytes are produced from myeloblasts. This is the case in the context of inadequate bone marrow proliferation. In this phenomenon, decreased differentiation of granulocytes from myeloblasts is triggered due to a deficiency of certain substances, such as a deficiency of folic acid. In addition, displacement of hematopoiesis can lead to decreased granulocyte formation from myeloblasts. Such displacement occurs, for example, in neoplasms, but can also be a side effect of cytotoxic drugs that disrupt hematopoiesis. Theoretically, the processes and cell stages of granulopoiesis may also be affected on a genetic basis, such as in the context of certain mutations.
