TPO treatment restored Mk maturation in GATA-1^low mice. Immunoelectron microscopy for von Willebrand factor of Mks from the spleen of untreated GATA-1^low mouse (A) and from a mutant mouse after 7 days of TPO treatment (B). The cytoplasm of the Mks from TPO-treated animals contains clearly recognizable platelet territories with numerous electron-dense granules, which, when analyzed at higher magnification, contain several von Willebrand factor–related gold particles (Bii). The rectangle in panels A and B indicates the region of the Mks shown at higher magnification in Aii and Bii, respectively. The arrows in Aii and Bii indicate von Willebrand factor–related gold particles. Magnification, × 4400 (Ai,Bi) and × 30 000 (Aii,Bii). Numerical aperture of the objective lenses: 0.1.