Fig. 2. Skeletal defects of Six1^–/– fetuses revealed by Alizarin Red (skeleton) and Alcian Blue (cartilage) staining. (A,C,E,F) Six1^–/– fetuses, (B,D) normal littermates. (A-B) Ventral view and (C-D) lateral view of the trunk skeleton showing malformations including rib bifurcation, fusion of rib cartilage from adjacent ribs (arrows), truncated distal rib segments with defects in the attachment to the sternum (here, only two ribs reach the sternum (arrowhead), and disorganized ossification of the sternum. (E-F) Magnification of adjacent rib fusion and branching in two different Six1^–/– fetuses (anterior is left). (E) The cartilage segment of the fifth right rib splits up (arrowhead) before fusing with the forth right rib (asterisk). (F) The sixth to the ninth rib are fused (asterisk). The seventh rib forks at the distal part of the bone segment (arrowhead).