Transmission electron micrographs of murine MKs, preplatelets, proplatelets, and platelets. MK cultures generated from murine fetal liver cells were fixed with 1.25% paraformaldehyde, 0.03% picric acid, and 2.5% glutaraldehyde in 0.1 M cacodylate buffer, pH 7.4, for 1 h, postfixed with 1% osmium tetroxide, dehydrated through a series of alcohols, infiltrated with propylene oxide, and embedded in epoxy resin. Ultrathin sections were stained and examined with an electron microscope (Tecnai G2 Spirit BioTWIN; FEI Company) at an accelerating voltage of 80 kV. Images were recorded with a charge-coupled device camera (2K; Advanced Microscopy Techniques) using digital acquisition and analysis software. (A) Overview of one MK showing multilobulated nucleus and IMS. (B) MK with a highly developed IMS. (C) Released preplatelets (#), proplatelets (*), and platelets (^). (D) Detailed view of platelets (bottom right) and an MK, highlighting its contents. N, nucleus; IMS, invaginated membrane system; G, granule; M, mitochondria; V, multivesicular body.