Characterization young vs old platelets. (A) Representative field of AMC EV-megakaryocyte preparation on a hematocytometer before infusion. White arrows represent some of the visible platelet-size particles. Red arrows indicate large megakaryocyte-like cells, whereas a large cell with extended proplatelets/platelets is shown in the center (yellow arrow). Bar in left lower corner indicates size of the observed field. (B) Temporal profile of the percent of a circulating platelet population that are TO⁺. Thin gray horizontal bar is percent TO⁺ recipient murine platelets. Open squares are percent of infused human platelets that are TO⁺. Black squares are percent of derived platelets from infused AMC EV-megakaryocytes that are TO⁺, and gray squares are the same when infused into clodronate liposome-pretreated mice. Mean ± SEM are shown. N ≥ 5 studies per arm. (C) Representative temporal profiles of size as determined by forward scatter for TO⁺ (solid lines) and TO⁻ (dashed lines) platelets. Top row is recipient mice platelets after infused AMC EV-megakaryocytes. Second row is human platelets after infusion of donor platelets. Third and bottom row are human platelets derived from AMC EV-megakaryocytes without and with clodronate liposome pretreatment, respectively. (D) Graphs are percent of the maximum human platelet number (which occurred 1-hour after EV-megakaryocyte infusion) at each time point separated out by both TO staining and size. The AMC EV-megakaryocytes were infused into either mice that were untreated (top) or pretreated (bottom) with clodronate liposomes. Mean ± SEM are shown. N ≥ 5 studies per arm.