Effects of Degree of Thrombocytopenia on Thrombocytopoietic Response

Rats were exchange transfused with platelet-poor blood to reduce the number of circulating platelets. When thrombocytopenia was moderate, the rate of platelet increase toward normal levels was relatively slow, whereas after severe thrombocytopenia an early slow rate of increase was followed by a rapid rate. It seems likely that the rapid platelet increase that begins about 36-48 hr after acute, severe thrombocytopenia is engendered by a ploidy shift among megakaryocytes and by an increased input of cells into the maturing population. The basis of the moderate increase in circulating platelets before 48 hr in severely thrombocytopenic rats and for about 5 days in moderately thrombocytopenic rats is less clear. Both peripheral changes in the age distribution of the platelet population and in platelet depots, as well as hemopoietic changes in the marrow, probably contribute.