Written by David M Gardiner
The best evidence is that all vertebrate embryos can regenerate the developing limb bud as embryos. This has been demonstrated directly in frog embryos, where it is relatively easy to do the surgical manipulations. In chick embryos, an amputated limb bud fails to regenerate because the epidermal covering fails to reform. If you graft a new apical epidermis, or provide the appropriate chemical signals that the epidermis normally produces (fibroblast growth factor), then the amputated limb bud will regenerate. Amputations of developing mammalian limb buds are very difficult because the embryo develops inside the uterus of the mother. These experiments have been done in mice, and as in the case with frogs, amputated limb structures can regenerate depending on the stage of development. In humans, the developing limb can get amputated at later stages by amniotic constriction bands, and these limbs fail to regenerate. Presumably, if the limb bud is damaged or amputated at very early stages they would regenerate as in the case with frogs.