EYFP^ki MKs serve as reporters for the DMS. (A) Electron micrograph of a representative terminally differentiated MK, displaying abundant DMS that fills the cytoplasm except for the cell cortex. (B) MK derived from EYFP^ki mice, in which one GPIIb locus is replaced by modified (myristoyl-acceptor) EYFP cDNA, showing a fluorescent internal membrane system that corresponds to the morphology and distribution of the DMS. MK preparations centrifuged on cover slips were examined by fluorescence deconvolution microscopy; a central z-section is depicted. (C-E) Micrograph of a typical young EYFP^ki MK, with limited surface fluorescence; the cell is outlined (E) with CD61 antibody. (F-H) Costaining of advanced EYFP^ki MKs with Arp3 antibody (G) shows no correspondence between membranous EYFP signal and diffuse Arp3 staining. (I-K) Micrographs of EYFP^ki MKs in the process of elaborating proplatelets. Proplatelet membranes are contiguous with MK internal membranes, revealing directly that the latter provide the reservoir for proplatelet formation. (E) Peripheral blood platelets in EYFP^ki mice show surface fluorescence and further support a DMS origin for blood platelets. Scale bars: A, 5 μm; F-H, 10 μm; K, 3 μm; all others, 15 μm.