Fig. 1. Limb phenotype of Msx1^-/-; Msx2^-/- double-null embryos at 14.5 dpc. (A-F) Alcian blue-stained skeletal preparations. (A,B) Forelimb of a control (A) and a double homozygous (B) embryo. There is an overall reduction in size of the limb in the mutant and the radius is absent (asterisk). (C,D) Higher magnification of the autopods of the embryos shown in A and B, respectively. Only one phalange has developed on digits II to V in the double mutant (asterisk) and the interdigital webbing has not regressed. The arrow indicates the presence of an abnormal distal mesenchyme condensation between digit II and III. The metacarpal (m), the first (p1) and second (p2) phalanges are indicated only for digit III, with the missing phalange indicated by an asterisk for the double mutant. (E,F) Autopods of two polydactyl double mutant embryos. In both cases, six digit formations are present (arrowheads) and the anterior-most digit is longer than a normal thumb. (E) The radius is completely absent whereas it is almost normal in F. CI-IV, carpal I to IV; cc, central carpal; h, humerus; ic, intermedioradial carpal; r, radius; s, scapula; u, ulna; uc, ulnar carpal. (C-F) Anterior is towards the right.