Compound exocytosis occurs in vivo during thrombus formation. (A) (i) TEM analysis of a murine mesenteric artery after superficial laser injury showing adherent platelets at the site of endothelial denudation. Scale bar, 5 µm. (ii) Higher magnification showing that the platelets contain individual granules but no multigranular compartments. Note the presence of δ granules (arrowheads) close to the plasma membrane (insert in ii). Scale bar, 1 µm. (B) (i) TEM analysis of a murine mesenteric artery after deep laser injury showing disruption of the vessel wall (arrows) and a gradient of activation of platelets. Scale bar, 20 µm. (ii) Higher magnification of the box illustrating the presence of fully degranulated platelets devoid of secretory organelles (arrowhead) in the deeper area and granule-containing platelets close to the lumen. Scale bar, 1 µm. Note the presence of multigranular compartments (*) in platelets located at the interface. (Insert in ii) Platelet containing a large intracellular structure (*). (C) Ultrastructure of an in vitro aggregate. Platelets were stimulated with 1 U/mL thrombin and fixed at the time point corresponding to maximal aggregation (3 minutes). (i) TEM analysis revealed a heterogeneous thrombus composed of degranulated platelets (arrowheads) at the periphery and granule-containing platelets in the center. Scale bar, 1 µm. (ii) Multigranular compartments were observed in both areas (*). Scale bar, 500 nm.