Ultrathin frozen sections of human (a and b), Wistar rat (c and d), and mouse platelets (inset) immunolabeled with polyclonal antibodies to Fyn (a), Lck (b and c), and Fgr (inset). (d) Section of a Wistar rat platelet incubated with Lck peptide–absorbed anti-Lck antibodies. Arrows indicate label for these Src-related kinases on platelet plasma membranes and electron-dense cytoplasmic patches. Arrowheads indicate clusters of label closely associated with alpha granule (G) membranes. Asterisks indicate lumena of the SCCS system, a channel system in continuity with the outside space. (a) Original magnification × 43,000; (b) × 43,000; (c) × 50,000; (d) × 46,000; (inset) × 70,000.