Fig. 1. Mrj^-/- embryos fail to undergo chorioallantoic attachment because of defects within the chorionic trophoblast layer. (A,B) Expression of the ß-galactosidase (ß-gal; A; blue) and Mrj (B; red) proteins in E8.25 chorions. (C) Aggregation of a diploid Mrj^-/- embryo (blue) with a wild-type tetraploid embryo expressing the Egfp transgene (green) results in a genetically mosaic chorionic trophoblast, ectoplacental cone and trophoblast giant cell layer (Rossant and Cross, 2001). The chorionic mesothelium, allantois and embryo proper are almost exclusively composed of Mrj-deficient diploid cells. (D,E) Rescued chorioallantoic attachment in an Egfp:Mrj^-/- chimeric conceptus at E9.5. (E) Higher-magnification of boxed region in D. (F-H) Initiation of branching morphogenesis in the placental labyrinth layer (arrowheads) at E9.5 in Egfp:Mrj^+/+ (F) and Egfp:Mrj^-/- (H) chimeric placentas, but not in a Mrj^-/- chorion (G), which remains flat. al, allantois; ch, chorion; em, embryo; me, chorionic mesothelial layer; tr, chorionic trophoblast layer. Scale bars: 10 µm in A,B,F-H; 1 mm in D; 0.5 mm in E.