We employed a novel mouse line that expresses low levels of human tissue factor (TF) in the absence of murine TF to analyze the role of TF in gestation. Low-TF female mice had a 14–18% incidence of fatal postpartum uterine hemorrhage, suggesting that TF plays an important role in uterine hemostasis. Low-TF female mice mated with low-TF male mice had a 42% incidence of fatal midgestational hemorrhage (n = 41), whereas no fatal midgestational hemorrhages were observed in low-TF female mice mated with wild-type male mice (n = 43). Placentas of low-TF embryos from both low-TF and normal (+/−) TF females were abnormal and contained numerous maternal blood pools in the labyrinth. Placentas of TF null embryos surviving beyond embryonic day 10.5 exhibited similar defects. The mouse maternal–embryonic placental barrier consists of four cellular layers (layers I, II, and III and endothelial cells), where layer I lines the maternal lacunae. Comparison of TF-deficient placentas with control placentas by immunohistochemical and ultrastructural analyses revealed thinning of layer I and a reduction in the number of cellular contacts of layer I trophoblasts spanning the maternal blood space between adjacent trabeculae. These structural changes in low-TF and TF null placentas result in enlarged maternal lacunae, as determined by morphometric analysis, and placental hemorrhage, which leads to midgestational death of low-TF female mice. This study demonstrated that TF is required for uterine hemostasis and revealed an unexpected role of TF in the maintenance of the placental labyrinth
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