Effect of thrombopoietin on platelet size of mice.

Thrombopoietin (TSF)-rich kidney cell culture medium has been shown in previous studies to stimulate thrombocytopoiesis in mice, but platelet sizes were unaltered. In an attempt to clarify this discrepancy, mice were injected with potent TSF preparations and at daily intervals thereafter platelet sizes and % 35S incorporation into platelets were measured. TSF caused a substantial and significant increase in platelet sizes of recipient mice along with an elevated 35S uptake by platelets. The time of increased platelet size agreed with the time of elevated 35S incorporation into platelets, ie.e., both measurements showed the greatest response on day 2 after TSF injections; decreasing values were observed thereafter.