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The megakaryocyte is a large blood cell originating in the bone marrow. Inside the megakaryocyte, the main substance of the cell, known as cytoplasm, breaks up into tiny fragments to form what are called platelets, or thrombocytes. Platelets are involved in blood clotting and blood vessel repair, and are found at sites of injury. Megakaryocytes are able to shape their cytoplasm into long protrusions, which extend through the gaps between cells in a blood vessel wall. This way, they release platelets into the circulation.
Stem cells in the bone marrow develop into immature cells called megakaryoblasts, which in turn develop into megakaryocytes. The megakaryocyte is the largest cell originating in the bone marrow and is of giant size, at least ten times larger than a red blood cell. It has a nucleus which is irregular in shape and which contains much more DNA than the nuclei of normal cells.
A structure known as the Golgi complex is found inside the cytoplasm of the megakaryocyte. The Golgi complex makes proteins, some of which are later found inside platelets in the form of platelet granules. Platelet granules are involved in blood coagulation. They are also used in a process where platelets stick to the cells lining blood vessels, in order to repair gaps.
Platelets usually survive for around ten days, and each megakaryocyte will typically produce about 4,000 platelets during its lifespan. Around two-thirds of platelets are normally in the circulation, with the rest contained within the spleen. Platelets are continually being produced to replace those which have died, and in a healthy individual a steady level in the blood is maintained. Sometimes, diseases occur which affect megakaryoctes, and these have a subsequent effect on platelet production.
In thrombocytopenia, the number of platelets is lower than normal, and this can result from problems with their production, a decrease in platelet lifespan, or a relative lowering in numbers when the blood is diluted. One cause is a defect in the process that releases platelets from the megakaryocyte. The platelets remain fixed to the cytoplasm and fewer end up in the circulation. Symptoms of thrombocytopenia include excessive bleeding and red spots on the skin. Treatment will vary according to the specific cause.
Primary thrombocytosis is a disease where there is over-production of megakaryocytes, with a resulting abnormal increase in the number of platelets. The spleen may be enlarged, and there may be other signs such as hemorrhages, especially in the gut, and thrombosis, or clot formation, inside large blood vessels. Unless complications occur, the disorder can be treated successfully with drugs that lower platelet production.