Cells and the activation of factor VII

Binding of factor VII to tissue factor (TF) present on or released from cells initiates TF-dependent coagulation in vivo. Earlier data obtained with relipidated TF suspensions led to the hypothesis that all factor VII bound to limiting TF sites exposed after tissue injury could be rapidly activated to VIIa/TF complexes by low concentrations of factor Xa, IXa, and VIIa. However, newer data on the activation of factor VII bound to TF expressed on cultured cell monolayers support the hypothesis that not all VII/TF complexes formed at a site of tissue injury are readily activated to VIIa/TF complexes during hemostasis. This is possibly related to why recombinant factor VIIa can bypass impaired IXa/VIIIa/phospholipid complex function in hemostasis.