Running Title: Endothelium and prothrombinase function in vivo.

• Prothrombinase bound in the vicinity of vascular damage is distributed away from platelets and is largely found on activated endothelium. • The activated endothelium plays an unexpectedly important role in supporting prothrombinase assembly and function at the site of damage. For personal use only.on February 20, 2016. by guest www.bloodjournal.orgFrom The membrane-dependent interaction of factor Xa (FXa) with factor Va (FVa) forms prothrombinase and drives thrombin formation essential for hemostasis. Activated platelets are considered to provide the primary biologic surface to support prothrombinase function. However, the question of how other cell types may cooperate within the biological milieu to affect hemostatic plug formation remains unaddressed. We employed confocal fluorescence microscopy to image the distribution of site-specific fluorescent derivatives of FVa and FXa following laser injury in the mouse cremaster arteriole. These proteins bound to the injury site extending beyond the platelet mass to the surrounding endothelium. Although bound FVa and FXa may have been present on the platelet core at the nidus of the injury, bound proteins wer