Hematopoiesis is the technical language word for blood formation. It is a highly complex process that takes place largely in the bone marrow.
What is hematopoiesis?
Erythrocytes or red blood cells are the most abundant cells in human blood. Among other things, they serve to transport oxygen from the lungs to the organs, bones, and tissues. Erythrocytes make the blood appear red. Click to enlarge. Blood formation serves to supply the body with blood cells. It is important that it takes place continuously as well as according to the current need, so that there is always a sufficient number. The different blood cells have different average lifetimes. For example, erythrocytes, the red blood cells, live for about 120 days, while thrombocytes, the blood platelets, only live for about 5 to 12 days. Ultimately, billions of new blood cells are formed in the bone marrow of a healthy adult day after day. The starting point for hematopoiesis is a multipotent hematopoietic stem cell, which then undergoes cell divisions and differentiation steps so that it becomes increasingly specialized. The term “multipotent” means that all developmental paths are still open to the cell in question; its further fate has not yet been determined. The first important differentiation of the multipotent cell then takes place into either a myeloid or a lymphoid progenitor cell. Now its further development is determined for it, which means that only a few variants of development are still open to it.
Function and task
In order for the initial multipotent stem cell to give rise to finished blood cells, which then perform specific tasks for the body depending on the cell type, different paths are now taken. Four developmental options are open to the myeloid progenitor cell. It can become an erythrocyte, a platelet, a granulocyte or a monocyte. The erythrocytes are the red blood cells. They are responsible for oxygen and carbon dioxide transport. Their formation process is called erythropoiesis. The earliest cell stage of erythropoiesis is the proerythroblast. This is a relatively large cell, 20µm in diameter, with a centrally located nucleus. The proerythroblast gives rise to smaller and smaller erythroblasts through cell divisions. Their cell diameter decreases continuously, while the hemoglobin content increases. In the last developmental step, which still takes place in the bone marrow, the erythroblasts expel their nuclei. They thus become reticulocytes. These can be distinguished microscopically from the finished red blood cells by the so-called substantia granulofilamentosa. Their number in the peripheral blood is proportional to the degree of erythropoiesis occurring at that time. Predominantly in the spleen, maturation to the erythrocyte finally occurs. Platelets are also called blood platelets. Their function is to close tissue defects. Consequently, they play an important role in wound healing and blood clotting. Platelet topoiesis also proceeds through several intermediate stages. Specifically, these are called hemcytoblast, megakaryoblast, promegakaryocyte, and megakaryocyte. Finally, platelets lace off from the megakaryocytes. Granulocytes are in the service of cellular immune defense. They develop through the hemacytoblast, myeloblast, promyelocyte, myelocyte and metamyelocyte stages. This then gives rise to the rod-nucleated neutrophil granulocyte, which differentiates once again into the segment-nucleated neutrophil granulocyte. Ultimately, segmental nuclei account for 45 to 70% of all leukocytes in peripheral blood. Lymphocytes are components of the blood. They belong to the natural “killer cells” as well as to the white blood cells, the leukocytes. In the picture, lymphocytes destroy cancer cells. White: lymphocytes, green: cancer cells. Click to enlarge. Monocytes develop through the hemacytoblast, monoblast, promonocyte, and monocyte stages. Monocytes first circulate in the blood, but then migrate out into the tissues where they become macrophages. These are scavenger cells that phagocytize potentially pathogenic substances and thus render them harmless. Lymphocytes have the task of rendering infectious agents and the body’s own degenerated tissue harmless. Lymphopoiesis, like the other types of hematopoiesis, begins in the bone marrow. Some lymphocytes remain there until the end of their development.They are called B lymphocytes. For other lymphocytes, the final differentiation takes place in the thymus. They are then called T lymphocytes.
Diseases and disorders
Precisely because hematopoiesis plays such an important role in the smooth functioning of numerous bodily functions, disruptions quickly lead to sometimes life-threatening diseases. A mild example of impaired hematopoiesis is anemia. It is based on disturbed erythropoiesis, which is caused in particular by a lack of substrates such as vitamin B12, iron or folic acid. Chronic infections and rheumatic diseases can also cause the formation of erythrocytes to proceed too slowly for current needs. In addition, a number of other causes of anemia are possible. Only rarely does pathologically increased erythropoiesis occur. In most cases, the reason for such anemia is tumor disease. If the thrombocytopoiesis does not correspond to the just existing demand, this condition is called thrombocytopenia. There is a deficiency of platelets, which can be dangerous especially in case of injuries. In such cases, bleeding can hardly be stopped. An excess of platelets, on the other hand, is called thrombocytosis. This is usually caused by myeloproliferative diseases in which cell development is disturbed. Temporary thrombocytosis can also occur as a result of splenectomy or major blood loss. Leukopenia, i.e. a reduction in the white cell series, should be clarified without fail. Since leukocytes play an important role in the immune defense system, even mild infections can take a life-threatening course in this case. Here, too, a formation disorder in the bone marrow can be the cause, but sometimes an increased consumption, as it can occur in the context of an infectious disease, is the reason. Therapy depends on the cause. In cases of severe leukopenia, antibiotics as well as antifungals are administered to support the body’s weakened defenses.
