Thrombopoiesis

Megakaryocytes

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Thrombopoiesis is the process of formation of thrombocytes (platelets).

Stages

Thrombopoiesis Pathway

Stem Cell

As with erythrocytes, thrombocytes are derived from multipotential myeloid stem cells (CFU-GEMM). In the bone marrow CFU-GEMM cells differentiate into the megakaryocyte precursor cell the megakaryocyte CFU (CFU-Meg), under the influence of CFU-CSF and IL-3. Unlike other blood cells which undergo mitosis in the first few developmental stages, once the CFU-GEMM has differentiated into the CFU-Meg there is no further mitosis. The CFU-Meg then differentiates into the megakaryoblast.

Megakaryoblast

Cell is slightly basophilic, around 30µm and has a round nucleus that is non-lobed.

Promegakaryoblast

Cell is around 45µm with a larger cytoplasm and nucleus.

Megakaryocyte

Cells is around 50-70µm and responds to thrombopoietin (TPO) and undergoes endomitosis until the cell reaches around 64n. The size of the cell's cytoplasm and nucleus increases with the increase in the cell's number of chromosomes.

Thrombopoietin

Similarly to erythropoietin, thrombopoietin (TPO) regulates the production of thrombocytes by stimulating CFU-Meg to mature into megakaryocytes. TPO is mainly produced in the liver and acts via a negative feedback system. It binds to TPO receptors on both the megakaryocytes and platelets. Thus if these are in high concentrations in the blood plasma then the level of TPO is kept low, reducing the maturation of CFU-Meg and production of thrombocytes is reduced.

Thrombocyte formation

The megakaryocyte is a giant cell and pieces of it's cytoplasm and cell membrane bud off to form the thrombocytes. One megakaryocyte can produce up to 6000 thrombocytes.