Megakaryocyte

In general, megakaryocytes are 10 to 15 times larger than a typical red blood cell, averaging 50–100 μm in diameter. During its maturation, the megakaryocyte grows in size and replicates its DNA without cytokinesis in a process called endomitosis. As a result, the nucleus of the megakaryocyte can become very large and lobulated, which, under a light microscope, can give the false impression that there are several nuclei. In some cases, the nucleus may contain up to 64N DNA, or 32 copies of the normal complement of DNA in a human cell.

The cytoplasm, just as the platelets that bud off from it, contains α-granules and dense bodies.

Development

Megakaryocytes are derived from

Once the cell has completed differentiation and become a mature megakaryocyte, it begins the process of producing platelets. The maturation process occurs via endomitotic synchronous replication whereby the cytoplasmic volume enlarges as the number of chromosomes multiplies without cellular division. The cell ceases its growth at 4N, 8N or 16N, becomes granular, and begins to produce platelets.[6] Thrombopoietin plays a role in inducing the megakaryocyte to form small proto-platelet processes. Platelets are held within these internal membranes within the cytoplasm of megakaryocytes. There are two proposed mechanisms for platelet release. In one scenario, these proto-platelet processes break up explosively to become platelets.^[7] It is possible to visualize the spontaneous release of platelets using holotomographic live-cell imaging. Alternatively, the cell may form platelet ribbons into blood vessels. The ribbons are formed via pseudopodia and they are able to continuously emit platelets into circulation. In either scenario, each of these proto-platelet processes can give rise to 2000–5000 new platelets upon breakup. Overall, 2/3 of these newly produced platelets will remain in circulation while 1/3 will be sequestered by the spleen.^{[medical citation needed]}

Thrombopoietin (TPO) is a 353-amino acid protein encoded on chromosome 3p27. TPO is primarily synthesized in the liver^[8] but can be made by kidneys, testes, brain, and even bone marrow stromal cells. It has high homology with erythropoietin. It is essential for the formation of an adequate quantity of platelets.

After budding off platelets, what remains is mainly the cell nucleus. This crosses the

Megakaryocytes are directly responsible for producing platelets, which are needed for the formation of a thrombus, or blood clot. There are several diseases that are directly attributable to abnormal megakaryocyte function or abnormal platelet function.^[11]

Essential thrombocythemia

Congenital amegakaryocytic thrombocytopenia

Bone marrow/stem cell transplant is the only remedy for this genetic disease. Frequent platelet transfusions are required to keep the patient from bleeding to death until transplant has been completed, although this is not always the case.

There appears to be no generic resource for CAMT patients on the web and this is potentially due to the rarity of the disease.

History

This section needs expansion. You can help by adding to it. (March 2018)

In 1906, James Homer Wright provided evidence that megakaryocytes give rise to blood platelets.^[17]

Kelemen first coined the term "thrombopoietin" to describe the humoral substance responsible for the production of platelets.^[18]

See also

• List of distinct cell types in the adult human body

References