In situ thrombin generation on the cell surface modulates clot architecture in 3 dimensions. Clots were formed by incubating cells with recalcified normal, pooled PFP in the presence of 500μM (final) control peptide GRGESP (A-H) or GRGDSP (I-L). Three-dimensional projections show clot architecture in 10-μm stacks at (E-L) and above (A-D) the cell surface. Each image (146 μm × 146 μm, xy) is from 1 experiment, representative of at least 3 independent experiments. Darker areas show increased fibrin density. (M) Fibrin network density (± SD) of clots produced by fibroblasts (●), SMCs (♦), HUVECs (■), and TNFα-stimulated HUVECs (▴) was determined as described in “Methods” from at least 3 independent experiments. (N) Clots were formed in excised human saphenous vein segments as described in “Structural analysis of ex vivo clots by transmission electron microscopy.” Clots were then fixed, examined by transmission electron microscopy, and fibrin fibers (black dots) were quantified at (0 μm) and above (5 μm) from the cell surface (indicated by black rectangles). Original magnification, × 2000; bar represents 5 μm. The image is representative of 3 independent experiments.