Fig. 2. AER maturation, but not initiation, is impaired in Msx1^-/-; Msx2^-/- double-null embryos. (A,B) Initiation of the AER is not delayed in the double mutants as revealed by Fgf8 expression at the 14-somite stage. In both panels, arrowheads delimitate the forelimb territory. (C,D) At 10.5 dpc, the AER in the double-null embryo appears shorter and thicker than in the control. (E-H) At 11.5 dpc, the AER remains thicker than normal as revealed by Bmp7 expression (E,F) or histological analysis of transverse sections (G,H). (I,J) The level of Fgf4 expression in the AER at 10.5 dpc is lower in the double mutants. All panels display forelimbs and anterior is towards the right in A-F,I,J.