Megakaryocyte
Megakaryocytes are among the hematopoietic cells in the bone marrow. You are precursor cells of platelets ( platelets), which play a central role in blood clotting. Having a diameter of up to 0.1 mm are megakaryocytes of the largest cells in the human organism.
Construction
Megakaryocytes are the largest cells in the bone marrow, their diameter is 35-150 microns. The nucleus is irregularly lobed, containing coarse chromatin and no visible nucleoli. The light microscope, it may seem as if several nuclei present.
In the cytoplasm there are numerous mitochondria, a well-developed endoplasmic reticulum, an extensive Golgi apparatus and a large number of ribosomes. It can be found also granules, as they occur in platelets. They are in α - granules, granules and lysosomes electron density differences and contain the proteins and substances which are important for platelet function, including ADP, ATP, calcium, growth and coagulation factors.
Occurrence
The total number of megakaryocytes is about 6 million per kg body weight. The majority of megakaryocytes found in the red bone marrow. They make about 1 %, however, only a small proportion of the local cells.
In a small number ( five to seven per milliliter ), they are also found in the circulating blood, but then for the most part, but not completely filtered out in the pulmonary capillaries. However, these circulating cells then contain almost no cytoplasm. The number of circulating megakaryocytes is increased in neonates and infants and mothers immediately after birth, as well as infections, inflammations, malignant neoplasm, disseminated intravascular coagulation, and myeloproliferative disorders.
Megakaryopoiesis
The development of megakaryocytes from specific stem cells ( Hämozytoblasten ) is referred to as bone marrow megakaryopoiesis. It lasts about 10 days.
Under the influence of thrombopoietin (TPO ) is due to pluripotent stem cells in the bone marrow by mitotic division megakaryoblasts. These have a diameter of 20 to 30 microns, a basophilic cytoplasm and numerous nucleoli in nucleus. You no longer have the ability to divide, but can continue to multiply their DNA ( endomitosis ). After several of these endomitoses they are polyploid and eventually contain up to 64 complete sets of chromosomes ( 64N ). Megakaryoblasts contain definition, no granulation in the cytoplasm. This only occurs at the next stage of maturation, the promegakaryocytes. In the further course of development, the basophilia of the cytoplasm is declining, while the Azurgranulation increases. Relative to the volume of the cell nucleus, the volume of the cytoplasm. The DNA synthesis comes to a halt. The mature megakaryocyte is usually very zytoplasmareich with azurophilem cytoplasm.
Thrombopoiesis
Thrombopoiesis refers to the formation of thrombocytes ( platelets) by megakaryocytes. A single cell can release several hundred to a thousand platelets.
After the cytoplasm of megakaryocytes has matured to converts it into so-called Proplatelets. It formed several thick streamers pseudopodia that penetrate into the sinusoids of the bone marrow. There they form pearl necklace- like constrictions in which can be found platelet material that was transported there along microfilaments from the center of the cell. Entirely pinched off to form platelets. Unknown is far but if it is fully developed platelets, who will give into the bloodstream, or whether the formation is completed only in the vessels.
Diseases
In essential thrombocythemia an uncontrolled high number of platelets is released into the bloodstream of the megakaryocytes, so that it has a concentration of up to 500,000 per ul ( normal value: 150.000-350.000/μl ) reach. The concentration of other blood components, however, is normal. The causes of essential thrombocythemia lie in an increased sensitivity of the megakaryocytes to thrombopoietin. As a result, found in the bone marrow, an increased number of abnormally large, mature megakaryocytes. When diagnosing However, other causes of thrombocytosis, such as infection, anemia, or other diseases of the bone marrow must be excluded.
Thrombocytopenia in the number of circulating platelets is reduced. In addition to a variety of possible causes outside of the bone marrow, the disease can also be caused by an impaired formation in the bone marrow. In aplastic disorder of the bone marrow, the number of megakaryocytes is reduced or completely lacking. This is either a genetic disorder, as in the Fanconi anemia justified or can be acquired. These include bone marrow damage by poisoning or radiation, bone marrow infiltration by malignant tumors, displacement by uncontrolled proliferating cells in leukemia or obliteration of the blood-forming bone marrow myelofibrosis. Furthermore, disturbances of maturation, the number of mature due to deficiency of vitamin B12 or folic acid, reduce functional megakaryocytes reversible.