PECAM-1–deficient platelets display restricted cytoskeletal reorganization upon spreading on immobilized fibrinogen. (A-B) Washed PECAM-1^+/+ and PECAM-1^–/– platelets were allowed to adhere for 60 minutes at 37°C to a fibrinogen matrix. Adherent platelets were fixed and examined by scanning electron microscopy. Representative PECAM-1^+/+ and PECAM-1^–/– platelet scanning EM images are shown. (C) The number of filopodia per platelet was analyzed on scanning EM images from 8 random fields encompassing 100 platelets counted and expressed as a percentage of the total adherent platelets from PECAM-1^+/+ and PECAM-1^–/– platelets. Platelet filopodia were classified into 3 categories including 0, 1 to 3, and more than 3 filopodia. **P < .005; n = 4.