Compound fusion of fibrinogen-labeled α granules is induced by 1 U/mL thrombin. Platelets were co-labeled with Oregon Green BAPTA-1 (orange, upper panel) and Alexa 647-conjugated fibrinogen (Fbg-647; red, middle panel). Platelets were washed before stimulation with thrombin (0.1 or 1 U/mL), and platelets were monitored in real time by fluorescence microscopy and differential interference contrast (DIC) microscopy (morphology, lower panel). (A) Representative live images were recorded before stimulation (unstimulated: R) and after stimulation at the indicated times (inset). The platelets were not stirred. Arrows indicate fibrinogen-positive spots that increased in size. Single platelets are surrounded by dotted circles. Scale bar, 2 µm. (B) Representative tracings of the time course of the changes in Oregon Green BAPTA-1 staining in platelets stimulated with 0.1 and 1 U/mL thrombin. The fluorescence is expressed as the normalized fluorescence intensity in arbitrary units (a.u.).