Exposure of tissue factor (TF)-bearing cells to blood is the initial event in coagulation and intravascular thrombus formation. However, the mechanisms which determine thrombus growth remain poorly understood. To explore whether the procoagulant activity of vessel wall-bound cells regulates thrombus expansion, we studied in vitro spatial clot growth initiated by cultured human cells of different types in contact pathway-inhibited, non-flowing human plasma. Human aortic endothelial cells, smooth muscle cells, macrophages and lung fibroblasts differed in their ability to support thrombin generation in microplate assay with peaks of generated thrombin of 60 +/- 53 nmol L(-1), 135 +/- 57 nmol L(-1), 218 +/- 55 nmol L(-1) and 407 +/- 59 nmol L(-1) (mean +/- SD), respectively. Real-time videomicroscopy revealed the initiation and spatial growth phases of clot formation. Different procoagulant activity of cell monolayers was manifested as up to 4-fold difference in the lag times of clot formation. In contrast, the clot growth rate, which characterized propagation of clotting from the cell surface to plasma, was largely independent of cell type (< or = 30% difference). Experiments with factor VII (FVII)-, FVIII-, FX- or FXI-deficient plasmas and annexin V revealed that (i) cell surface-associated extrinsic Xase was critical for initiation of clotting; (ii) intrinsic Xase regulated only the growth phase; and (iii) the contribution of plasma phospholipid surfaces in the growth phase was predominant. We conclude that the role of TF-bearing initiator cells is limited to the initial stage of clot formation. The functioning of intrinsic Xase in plasma provides the primary mechanism of sustained and far-ranging propagation of coagulation leading to the physical expansion of a fibrin clot.