Histologic analysis of TFPI^–/– placentas. E14.5 placentas from wild-type (A), as well as live (B) and dead (C) TFPI^–/– embryos were analyzed by H&E. Embryonic nucleated red blood cells (arrowhead) and maternal red blood cells (arrow) were observed in the wild-type placenta. The placenta of the live TFPI^–/– embryo contained thrombi within the labyrinthine zone (arrow). The labyrinthine zone of the placenta of the dead TFPI^–/– embryo was disrupted. Fibrin staining of placentas of TFPI^+/– (D, F, H) and the dead TFPI^–/– (E, G, I) embryos. TUNEL staining of apoptotic cells in placentas of TFPI^+/– (J, L) and the dead TFPI^–/– embryo (K, M). Arrowheads (D-E, J-K) indicate the trophoblast giant cell layer.Asterisks (D-E, J-K) indicate the labyrinthine zone. Arrows in panels L and M indicate TUNEL-positive cells. Original magnification, × 40 (D-E, J-K), × 400 (A-C, F-I, L-M), and × 1000 (inset in panel M showing nuclear condensation in a TUNEL-positive apoptotic cell).