TISSUE factor, a member of the cytokine-receptor superfamily and high-
affinity receptor and cofactor for plasma factor VII/VIIa (ref, 1), is
the primary cellular initiator of blood coagulation. It is involved i
n thrombosis and inflammation associated with sepsis, atherosclerosis
and cancer(2), and can participate in other cellular processes includi
ng intracellular signalling(3), metastasis(4), tumour-associated angio
genesis(5), and embryogenesis(6). Here we report that inactivation of
the tissue factor gene (TF) results in abnormal circulation from yolk
sac to embryo beyond embryonic day 8.5, leading to embryo wasting and
death, Vitelline vessels from null mice were deficient in smooth-muscl
e a-actin-expressing mesenchymal cells, which participate in organizat
ion of the vessel wall, This implies that tissue factor has a role in
blood-vessel development.