Proposed role of spatial regulation of platelet α-granule secretion in hemostasis and maintenance of endothelial integrity. As a platelet under flow (left) encounters and initially adheres to a small site of vascular injury (middle), α-granules will redistribute and exocytose their contents primarily in regions beyond the geometric boundaries of the exposed subendothelial matrix. This spatially regulated α-granule secretion results in platelet “self-deposition” of adhesive matrix proteins such as fibrinogen and fibronectin, high expression of P-selectin beyond the matrix boundaries, and increased cell membrane to extend the overall platelet spreading area (right). Adhesion in those regions likely involves integrin α_IIbβ₃ redistribution and activation as well as P-selectin binding to PSGL-1 on the endothelial cell surface (zoomed in).