β1-tubulin^—/— platelets have abnormal marginal MT band structure. Confocal immunofluorescence with anti–α-tubulin antibody (A) and rapid-freeze electron microscopy (C) of wild-type (+/+) platelets illustrate the normal appearance and thickness of the marginal band. In β1-tubulin—/— platelets (B,D-F), the same structure is thinner, usually deformed, frequently broken (arrows in E-F) or kinked (B, arrows in D), and generally disorganized (F); these features are never encountered in wild-type platelets. Scale bar (C-F) = 0.2 μm. The exposure time for panel B is longer than for panel A to account for the difference in MT band thickness and to allow delineation of the structural anomalies. Both +/+ (G) and β1-tubulin^—/— (H) platelets show disassembled MTs at 4°C but regenerate a circumferential marginal band at 37°C. (I) The survival kinetics of transfused wild-type and β1-tubulin^—/— platelets in recipient mice are indistinguishable. CMFDA-labeled platelets (10⁸) were injected into +/+ recipients of the same genetic background, blood was collected at the indicated time points, and in vivo survival of the injected platelets (mean ± SD from 4 independent donors of each genotype) was assessed by flow cytometry. Original magnification for all immunofluorescence images, × 1000.