* To whom correspondence should be addressed.
Received March 30, 2001; Revision received April 23, 2001
Inactivation of specific genes in mammals by gene targeting has accelerated our ability to determine gene function. Nearly all genes involved in the blood coagulation system have been knocked out in mice. Tissue factor (TF) is the main initiator of the coagulation system and functions as a cell surface receptor for coagulation factor VII (FVII). Knockout studies have shown that TF deficiency results in lethality around embryonic day (E) 8.5-10.5. The results suggest a role for TF in embryonic blood vessel development and maintenance of vascular integrity in the yolk sac. In addition, TF may be involved in the maintenance of the placental labyrinth. Factor X (FX) deficiency causes partial embryonic lethality between E11.5-12.5. FX-/- mice that were born died from fatal neonatal bleeding. In contrast, FVII deficiency is not embryonic lethal, but FVII-/- neonates died from hemorrhage within the first days after birth. The various lethal phenotypes of deficiencies of the different coagulation factors suggest involvement in processes beyond hemostasis. Both TF/FVIIa and FXa can trigger intracellular signaling events in certain cell types. Signaling by coagulation proteases and protease-activated receptors (PARs) may have important roles in embryonic development.
KEY WORDS: coagulation, gene targeting, tissue factor, factor VII, factor X, embryonic development, vasculogenesis