Adventures in limb regeneration
Here is Hex's right front foot from the time it started to be more than a stub. The first two photos are awful — I then installed a light on the tank for a dramatic improvement in imaging performance! — but the weird little elf-boot curl the stub took on at first is discernible. Tetrapod limb development is supposed to require apoptosis to separate the fingers, so I was very interested in seeing whether Hex grew a paddle and then dissolved parts away to make digits. At the macro level, though, it looked a whole lot like little toes popping out of a foot and getting longer. This is why we need science.
I wondered about the developmental signals regulating axolotl limb regeneration, and it turns out that Wnt signaling is critical: if you block it (by using a virus to express a natural antagonist there), the limbs can fail to regenerate completely, or develop a single, Tim-Burtonesque, pointed toe. 
If Wnt signaling is only necessary to make a bunch of undifferentiated cells, what causes those cells to become a new foot? It's another massively important signaling molecule from embryogenesis, BMP. BMP is required for both cell division in the growing limb bud and appropriate apoptosis, as shown by overexpressing BMP and its antagonist Noggin in different limb buds . The images below are in situ hybridizations, which show in blue where axolotl BMP-2 is expressed in progressive stages of development: first all over, maybe regulating cell division, then where cells are being pared away to create toes — just like Hex's, which now look perfect. - KMP
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