Megakaryocyte, proplatelet, and platelet cytoskeletal structure. Megakaryocytes from Pf4/Srf WT and KO mice were cultured in TPO containing medium for 4 days and subsequently plated on fibronectin-coated coverslips and stained for tubulin (green) and F-actin (phalloidin, red). 4,6′-diamidino-2-phenylindole was used to stain DNA. (A) Representative images of WT (left) and KO (right) megakaryocytes. (B) Representative images of WT (top) and KO (bottom) megakaryocytes forming proplatelets in culture. (C) Platelets from 3 Pf4/Srf WT and 3 Pf4/Srf KO mice were fixed, spun onto coverslips, and stained for tubulin and F-actin. (D) WT (n = 29) and KO (n = 34) platelet size were measured in micrometers. *P < .01.