Fig. 2. Cyclopamine treatment blocks axolotl tail regeneration and a hedgehog agonist rescues the phenotype. (A-C) Control regenerates 4, 8 and 14 dpa. The blastema cells are surrounding the ependymal tube at 4 dpa (A). Cartilage has differentiated 8 dpa (B) and muscle 14 dpa (C). (D-F) Cyclopamine-treated tail regenerates 4, 8 and 14 dpa. The fin and the ependymal tube have grown, but a blastema is not visible (compare A and D). No cartilage has differentiated 8 dpa (E) and no muscle has formed 14 dpa (F). (G) Quantification of the lengths of control and cyclopamine-treated tail regenerates over time. The length of the regenerating spinal cord is taken as a measurement for overall tail regeneration. Data points represent the average length of three regenerates; error bars are standard deviations. (H-J) Cyclopamine-treated regenerate (H), cyclopamine and agonist-treated regenerate (I), and normal tail regenerate (J) 10 dpa. The tails in I and J are indistinguishable. (K) Quantification of regenerate lengths (as in G) in indicated conditions shows that 40 nmol/l of the agonist are sufficient to rescue the cyclopamine effect, whereas 4 nmol/l are not. The dashed line in A-F and H-J marks the amputation plane. Scale bars: 0.5 mm.