Fig. 2. Sirenomelia in Tsg^-/-;Bmp7^-/- and Tsg^+/–;Bmp7^-/- compound mutants. (A,B) Comparison of a wild-type neonate with a Tsg^-/-;Bmp7^-/- mutant displaying sirenomelia. The mutant appears normal up to the level of the hindlimbs which are fused. (C-F) Alcian Blue (cartilage) and Alizarin Red (bone) preparations of wild-type and Tsg^-/-;Bmp7^-/- and Tsg^+/–;Bmp7^-/- mutants. (C,D) Lateral view of the posterior region of a wild-type and Tsg^-/-;Bmp7^-/- skeleton. In the mutant, the tail is shorter and has fewer vertebrae. The single hindlimb contains two femurs (fe), two tibias (ti), a single foot and lacks both fibulae (fi). (E,F) Ventral views of the hindlimbs and pelvic region of a wild-type and Tsg^+/–;Bmp7^-/- neonate. The mutant hindlimb is formed by the fusion of two hindlimbs at the level of the ischium (is); two feet are present, fused at the level of the heel. Insets in D and F show higher magnification views of the feet of Tsg^-/-;Bmp7^-/- and Tsg^+/–;Bmp7^-/- neonates, respectively. Digits are indicated by numbers and digit duplications by letters. il, ilium; is, ischium; p, pubic bone; ps, pubic symphysis.