P-selectin in resting and thrombin-activated Nbeal2^−/− platelets. High-resolution confocal laser immunofluorescence microscopy imaging of intracellular P-selectin membrane proteins in fixed resting platelets. Permeabilized cells were stained for α-tubulin (violet), P-selectin (red), and CD41/integrin α_IIb (green) and imaged (final magnification = ×150, Z stepping = 250 nm). Single-channel and merged mid-cell ZY/XY slices of representative individual platelets are shown (A-B). Resting platelets from WT (A) and Nbeal2^−/− (B) mice have similar flat, discoid morphology with a well-defined circumferential tubulin ring cytoskeleton. Both contain P-selectin, which defines compact looping structures of the α-granule secretory matrix typical of WT platelets that generally appear to be less orderly in Nbeal2^−/− platelets. Thrombin activated WT (C) and Nbeal2^−/− (D) platelets show characteristic activation-triggered changes in shape and surface mobilization of P-selectin.