On the cover: Morphology of whole mouse embryos cultured in vitro in the presence or absence of soluble Frizzled-related protein-1 (sFRP-1), a Wnt signaling inhibitor. Conceptuses were harvested at 7.5 dpc (late primitive-streak stage) from pregnant mice. The trophoblast and Reichert membrane were completely removed, and the conceptuses were cultured in pooled rat serum with (left) or without (right) sFRP-1 (20μ/ml) for 24 hours, during the critical stage for early events in embryonic vascular development. The upper left panel demonstrates a representative embryo harvested at 7.5 dpc and cultured in rat serum, showing a normally developed embryo with 7 to 8 somites, comparable to a day 8.5 embryo in vivo. The embryo on the upper right was cultured in rat serum supplemented with sFRP-1. Somites are present, but the tail is foreshortened. PECAM staining of the yolk sacs (lower panels) demonstrated normal vascular patterning in embryos grown in rat serum alone (lower left) that was markedly disrupted in embryos when sFRP-1 was added to media (lower right). Intraembryonic vascular growth in the area of the aorta was also disrupted (not shown), indicating a critical role for Wnt signaling in vascular development during mouse embryogenesis.See page 1331.