Mild thrombocytosis is a common observation in patients receiving repeated phlebotomy for autologous transfusion. To clarify the mechanism of thrombocytosis after phlebotomy, we examined the morphological characteristics of newly circulating platelets and megakaryopoiesis in phlebotomized rats. Blood at a volume of 2% fo body weight was withdrawn from each rat three times at 5-day intervals. Fifteen days after the initial phlebotomy the platelet count in phlebotomized rats was higher than that in control rats. However, there was no difference in mean platelet volume or platelet distribution width.
IL-3-induced megakaryocyte colony formation was almost the same for the two groups. However, the number of megakaryocytes in phlebotomized rats was significantly higher, and the cells themselves were bigger. These findings indicate that some humoral factors related to megakaryocyte differentiation from late megakaryocyte progenitors, but not from immature multipotential progenitors, may be involved in the induction of increased platelet production during repeated phlebotomy.
