Ultrathin frozen sections of human (a) and Wistar rat (b) platelets isolated from fixed, whole blood and immunolabeled with monoclonal antibody to tubulin (a) or polyclonal antibody to Lyn (b). Tubulin is concentrated at the poles of the discoid platelet (arrows). Lyn is present on electron-dense cytoplasmic domains (small arrows) and is also concentrated at the poles of the discoid platelets (large arrows). G, granule; M, mitochondrion. Original magnification × 45,000.