Thrombopoiesis
Thrombopoiesis is the process of formation of thrombocytes (platelets).
Stages of Development
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 cytokines 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 develops 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
Cell 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 chromosome number.
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 is regulated by a negative feedback system. It binds to TPO receptors on both the megakaryocytes and platelets; 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 thrombocyte production.
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.