Platelet Production
Platelets are cytoplasmic fragments of megakaryocyte (precursor cells of platelets) that contain no nucleus. They number about 150,000 to 400,000/mm3 in the blood and range in diameter from 1-4 microns and a volume of 5-12 femtoliters (fl) 12; however, the average platelet is about 7.3 fl.
Platelet production is regulated to meet the demand for circulating platelets by growth factors. One of these growth factors is "thrombopoietin". Megakaryocytes mature by endomitosis, i.e., nuclear division without cell division. As the megakaryocytes mature, the cytoplasm becomes demarcated into platelet subunits. Platelets are released into the circulation through a process of megakaryocyte fragmentation, and have a life span of 7 -10 days.
Normally, two-thirds of the platelets released from the bone marrow stay in the general circulation. The remainder of the platelets are in a pool in the spleen that is freely exchangeable with circulating platelets. With progressive splenomegaly, a larger percentage of the body's platelets are pooled in the spleen and, as a result, peripheral thrombocytopenia may develop unless the bone marrow can increase platelet production sufficiently to compensate. Splenomegaly by itself does not lead to a shortened platelet survival.
In general, there is a direct relationship between the megakaryocyte mass in the bone marrow and the rate at which platelets are added to the circulation. This relationship breaks down when megakaryocyte development is faulty or when megakaryocytes are destroyed within the bone marrow. In these circumstances platelet delivery to the circulation is impaired, and thrombopoiesis is said to be ineffective. Ineffective thrombopoiesis is a frequent finding in patients with vitamin B12 or folate deficiency and is analogous to the production problem of red cells, which occurs in these conditions and gives rise to ineffective erythropoiesis.
When maximally stimulated, the bone marrow can increase platelet production 6-fold. However, when platelets are destroyed very rapidly, increased delivery to the peripheral blood will not occur for approximately five days.