Microscopic examination of yolk sac and placenta of E10.5 TFPI_K1(−/−) embryos. (A) Normal-appearing placenta and yolk sac from a normal appearing embryo. Nucleated embryonic erythrocytes are present in placenta (arrow) and yolk sac (arrowhead) vessels. (B) Yolk sac of a severely growth and developmentally retarded embryo displaying disruption and degeneration of yolk sac tissues. Arrowhead denotes blood cells within a broad space between endoderm and mesoderm layers, perhaps the microscopic equivalent of a macroscopic blood “lake” (Fig 2). (C) Placenta and yolk sac of modestly growth retarded embryo. Note lack of nucleated erythrocytes in placenta and yolk sac vasculature and the lateral expansion of yolk sac vessels (arrow). (D) Higher magnification of yolk sac shown in (C). Arrows point to thin acellular septae connecting the endodermal and mesodermal layers.