Fig. 2. Digit regeneration in vivo. (A) E17.5 autopod (stage 13), ventral view. The central 3 digits, digits 2, 3 and 4, were amputated at E14.5 and analyzed 3 days later. Note the regenerated digit tips (asterisk) are shorter than a non-amputated control digit tip (arrowhead). (B-F,H-L) In situ hybridization of regenerated digit tips 4 days (B-F) and 2 days (H-L) after amputation. (B-D,H-L) Frontal sections with distal toward the top of the image. (E-F) Sagittal sections with distal toward the top and dorsal to the left of the image. Sections in B-F show expression of Msx1 in the mesenchyme surrounding terminal phalanx (B), Msx2 in the apical epidermis (C), low levels of Bmp4 in the apical mesenchyme (D), Ihh in the forming terminal phalanx (E), and Hoxc13 in the nail organ (F). (G) Proximally amputated digits fail to mount a regeneration response and are negative for the expression of Bmp4. (H-L) In situ hybridization of 2 day regenerates showing expression of Msx1 (H), Msx2 (I), Bmp4 (J), Ihh (K) and Hoxc13 (L). The expression patterns of these marker genes in regenerating digits are largely similar to those of developing digits with the exception of Msx2, which displays an expanded mesenchymal domain.