Fig. 1. Multiple defects of mesoderm development in Bmpr-MORE embryos. (A-E') Whole-mount views. (A,A') The no allantoic bud stage. Lateral view. Amnion and chorion were formed in control embryos at this stage (A, arrowheads). These tissues were not formed in Bmpr-MORE embryos (A', arrowhead). An acute curvature was observed (A', arrow). (B-C') The head-fold stage. (B,B') Lateral views. (C,C') Frontal views. Amnion and chorion were not formed in Bmpr-MORE embryos (B', arrowheads). Control embryos developed a typical heart crescent (C, arrowheads), whereas Bmpr-MORE embryos did not (C', arrowheads). (D-E') E8.5. (D) Dorsal view. Control embryos developed one column of somites on each side of the neural tube (arrowheads). (D') Ventral view. Mutant embryos developed multiple columns of somites on each side (D', arrowheads). White lines show approximate level of section for F,F'. (E,E') Lateral view. White lines show approximate level of section for G or G'. (F-H') Histological analyses (Hematoxylin and Eosin staining). (F,F') Frontal section. In control embryos, lateral plate mesoderm (LPM) developed between somites (so) (yellow arrowhead) and visceral yolk sac (vy) (black arrowhead). In mutant embryos, mesenchymal cells (blue asterisks) accumulated between somites (yellow arrowheads) and visceral yolk sac (black arrowhead) in mutants (E', see inset). Multiple somites developed to the lateral edge of the embryo (F', yellow arrowheads). (G,G') Transverse section. Between the somites (yellow arrowhead) and the visceral yolk sac (black arrowhead), LPM was observed in control embryos (blue arrowhead) (G). In Bmpr-MORE embryos, mesenchymal cell masses existed between the somites and the visceral yolk sac (G', blue asterisks). Somites developed as multiple rows (G', yellow arrowheads and inset). Heart (he) developed in the anterior region of control embryos (G, red arrowhead) but not in Bmpr-MORE embryos (G', red arrowhead). The anterior half of the mutant embryo consists of neural tissues. Somites develop only in the posterior half of the mutant embryo. (H,H') Sagittal section. Amnion (am) and allantois (al) were formed at this stage in mutant embryos (H', orange arrowheads). (I-L) Transverse section of control or Bmpr-MORE embryos carrying R26R loci after staining for ß-galactosidase activity. K and L are magnified images of I and J, respectively. Recombined (mutant) cells (blue) and heterozygous cells (pink) were distributed evenly among tissues such as somites (yellow arrowheads), LPM (blue arrowhead) or neural tube (nt). (M-P) Transverse section of control and Sox2Cre; Bmpr1a^flox/null embryos carrying R26R loci after staining for ß-galactosidase activity. Complete recombination of R26R was observed in the germ layers, namely in somites (O,P). Sox2Cre; Bmpr1a^flox/null embryos developed multiple columns of somites (N,P, arrowheads). Scale bars: 170 µm for A; 200 µm for A'; 250 µm for B-C',I,I',M,N; 500 µm for D-E',F-H',K,L,O,P.