Delayed kinetics of clot retraction for PECAM-1–deficient platelets in the presence of normal integrin α_IIbβ₃ expression. (A) Photographs showing in vitro kinetics of clot retraction over a 20-hour time frame using platelet-rich plasma (PRP; normalized platelet counts) from wild-type and PECAM-1^–/– mice. Samples were treated with 2.5 U thrombin. Each photograph is representative of at least 3 experiments. (B) Serum volume was determined from wild-type and PECAM-1^–/– platelets undergoing clot retraction at 1, 2, 3, 4, and 20 hours following the addition of 2.5 U thrombin. Data are expressed as mean percentage serum volume at given time points. Results are representative of 3 independent experiments. *P < .05, n = 2. (C) The expression of surface markers on platelets was determined by staining with an isotype control (FITC-CD3), mAb 390 followed by anti–rat FITC for CD31, positive control FITC-CD44 mAb, FITC-CD9, and FITC-integrin β₃ mAb for both wild-type and PECAM-1^–/– platelets. FITC-labeled samples were analyzed on a FACSCalibur analyzer. Results are cumulative data derived from 3 independent experiments and presented as geometric mean fluorescence intensity (GMFI) ± SEM.