From around the end of the fourth embryonic month (the beginning of the medullary phase ), the human bone marrow is the most important blood- forming organ in humans. It fills the cavities of the bones (medullary cavity and cavities of the cancellous bone ). Almost all human blood cell types are produced in the bone marrow.
The newborn is hematopoietic bone marrow found in the marrow cavities of almost all the bones, while in adults only in the sternum , the ribs , the skull bones, the collarbone , the vertebrae , the pelvis , shoulder blade and stem Windwärts directed ends of the upper arm and thigh bones can be found. On average, an adult has about 2,600 g of bone marrow, which is 4.6% of the body weight or about twice the weight of the liver . About half of this mass is made up of red bone marrow, the rest mainly of fat marrow. About 10% of all blood in the body is found in the bone marrow.
The spinal cord (lat. Medulla spinalis ) is not a bone marrow, but rather, as a component of the central nervous system, consists of sensitive nerve tissue.
The inner cavities of the bones are covered with a fine layer of connective tissue , the endostrum ; starting from it, the cavities are traversed with reticular connective tissue . This tissue is nourished by blood vessels that enter the bone ( vasa nutriciae ) and expand there to form elongated caverns ( sinusoids ). The walls of these caverns are formed by a special tissue that looks like an epithelium , but is made up of reticular cells .
The task of this tissue is not only to supply the tissue behind it, as every capillary wall does, but also to offer the blood cells produced behind it an opportunity to pass into the blood. Granulocytes , monocytes and thrombocytes are actively involved, but the erythrocytes are not able to do this, as there are obviously no permanent gaps in the vein wall that would be suitable for this. It is therefore believed that the reticulo-endothelial cells diverge when erythrocytes are to be released into the bloodstream. In addition, the erythrocytes probably have to deform strongly, so that this process would also represent a functional test for the finished red blood cells.
There are no lymphatic vessels in the bone marrow.
Red bone marrow
The blood-forming cells are only found in the red bone marrow (Latin: Medulla ossium rubra ). It is about 400 grams, 180 grams of which are involved in erythropoiesis , the generation of erythrocytes , and leukopoiesis, the generation of leukocytes . The remaining 40 grams are accounted for by cells that produce platelets .
While in infants the red marrow is found all over the bone, in adults the red marrow is concentrated on the flat and short bones. In the shaft of the long bones ( diaphysis ), the red bone marrow is replaced by fat marrow with age. But fat cells are also found within the red bone marrow, from 35% in the vertebral bodies to 75% in the ribs.
Yellow bone marrow (fat marrow)
In the yellow bone marrow (lat. Medulla ossium flava ), particularly large amounts of fat are stored in the reticular cells , which makes the marrow appear yellow. Yellow bone marrow cannot produce blood cells, nor can it be converted into blood-forming cells, since specialized body cells cannot develop back into pluripotent cells. The only way in which yellow bone marrow can develop into red again is as a metastasis in the context of leukemia . However, if larger amounts of blood are needed, e.g. B. after major blood loss, in bones in which red bone marrow is still present, this can expand and displace the yellow bone marrow.
White bone marrow (gelatinous bone marrow)
In this variant of the yellow bone marrow, the fat deposits are replaced by water, hence the gelatinous appearance. White bone marrow can no longer be converted back to red bone marrow, so it is an irreversibly degenerated form of the marrow. This variant usually occurs in old age or in the seriously ill.
Blood formation takes place in the bone marrow. Although researchers have long debated whether the different blood cells are formed from one, two or three different stem cells, it now appears that a single stem cell is the origin of all blood cells: the hemocytoblast . When it divides , it creates two cells: a new hemocytoblast and a precursor cell , which is the beginning of the development series erythropoiesis , granulopoiesis , lymphopoiesis , thrombocytopoiesis and monocytopoiesis , at the end of which are the different types of blood cells. The factors that lead to the different forms are so far largely unknown; only that a group of active ingredients, the poetins , is involved is considered safe.
Hemocytoblasts are not only found in the bone marrow, but are also found to a small extent in the blood. In adults, these cells rarely divide.
When a proerythroblast results from the division process described above , new erythrocytes are produced ( erythropoiesis ). The blood cells mature in four to five days from the proerythroblast via the erythroblast (also called macroblast ) and the normoblast , which expels its cell nucleus , to the reticulocyte . Remnants of the RNA can still be seen in it, which run through the cell like a network ( substantia granulofilamentosa ). About 0.8% of these reticulocytes already find their way into the bloodstream, although they are only the last precursor to normal red blood cells. After a day or two, the last remnants of the RNA have also disappeared; now the erythrocyte is mature and is expelled into the bloodstream.
Each proerythroblast produces 16 to 32 erythrocytes in four to five division steps. Usually 10% to 15% of these are malformed and perish.
In granulopoiesis (syn. Granulocytopoiesis), the precursor cells develop into myeloblasts , which in turn develop into promyelocytes . Not only the granulocytes probably originate from this stage , but also the monocytes (see below: Monocytopoiesis).
Metamyelocytes are the direct precursors of the three types of granulocytes and are the next stage in development. It takes four to seven days for the promyelocyte to develop this far, and another four to six days for the final stages of the granulocytes to develop. 16 mature granulocytes arise from one promyelocyte.
The division of the promyelocytes leads to the first, still immature form of granulocytes, which now still have an unsegmented cell nucleus ( rod-core granulocyte ). Over the next few days, three to four constrictions appear, which divide the cell nucleus into segments that are, however, connected to one another; the cell nucleus is not divided. As soon as this segmentation is completed, the granulocyte is considered mature ( segmented granulocyte ) and is channeled into the bloodstream.
Like the granulocyte, the monocyte also descends from the promyelocyte ; at least the assumption is suggested by enzyme histochemical investigations. They are removed from the bone marrow without further maturation, but can transform themselves into other cell types of the reticulo-endothelial system in other parts of the body, probably also into mast cells .
Lymphoblasts develop from lymphocyte stem cells.
On the one hand, these produce pro-T lymphocytes, which pass through the bone marrow in an undifferentiated state. They settle in the thymus and there their further development into T lymphocytes takes place.
On the other hand, pro-B lymphocytes are formed that differentiate in the bone marrow and then colonize lymphatic organs (spleen, lymph nodes, tonsils, etc.) as B lymphocytes.
Thrombocytes (blood platelets) are constrictions from the plasma of megakaryocytes (bone marrow giant cells) that are used up during this process. The partitions of parts of the plasma lemma unite to form parting walls and crevices. The resulting platelet is surrounded with a membrane until it finally constricts.
Diseases of the bone marrow
Harmful active pharmaceutical ingredients
Most cytotoxic drugs such as B. chlorambucil , melphalan , busulfan , thioguanine , cyclophosphamide , ifosfamide , imatinib , trofosfamide , carmustine , lomustine , mitobronitol , vinblastine , vindesine , cisplatin , etoposide and teniposide and biguanides , chloramphenicol , clozapine , hydantoin and pyrazinamide act bone marrow damaging.
Bone marrow edema
Bone marrow edema or bone marrow edema syndrome (BMES) is a painful disease mostly of the thigh bone (femoral condyle) directly above the knee joint, less often the hip or even less often other bones, the cause of which has not yet been clarified. The term transient osteoporosis , also known as transitory osteoporosis, is often used for bone marrow edema in the Anglo-American language area. Transient osteoporosis is a temporary form of bone marrow edema that never becomes chronic. The classic leading symptom of transient osteoporosis is a spontaneous stabbing pain in the knee or hip joint area . While the x-ray often does not show any pathological findings, the diagnosis can usually be made reliably with magnetic resonance imaging (MRI). Transient osteoporosis affects middle-aged men and significantly less often women; if the etiology is unknown, it is also known as idiopathic bone marrow edema syndrome (BMES).
Bone marrow donation
Bone marrow donation is a method of obtaining blood stem cells . These are needed, for example, if someone has leukemia (blood cancer) or other malignant diseases of the blood-forming system. In this case, a transplant of the blood stem cells offers a chance of recovery. Since bone marrow donation in this sense is only one method for obtaining blood stem cells, the term blood stem cell donation is often used today, with bone marrow donation in the actual sense as the removal of blood stem cells through punctures in the term blood stem cell donation.
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- W. Tackmann: Repetition of histology . Berlin
- F. Krück et al .: Pathophysiology . Urban & Schwarzenberg, ISBN 3-541-12551-9