Electron microscopy autoradiographs showing cellular localization of ¹²⁵I-rmTPO within platelets, spleen, and sternum. Mice received a single IV bolus injection of ¹²⁵I-rmTPO and were killed 3 hours later. Sections of platelets, spleen, and sternum were fixed and processed for electron microscopy. Silver grains were found associated with platelets in (A) PRP and (B) spleen, and (C) megakaryocytes in sternum. The bar in the autoradiographs is equal to 1 μm.