Limb regeneration occurs after amputation of most larval and adult urodeles (newts and salamanders) and in the tadpole stages of anurans (frogs and toads). After metamorphosis in frogs and toads, regeneration is inhibited with the maximal response being the growth of a long spike. After metamorphosis in urodeles, regeneration is noticeably retarded, but still occurs. It is often said that all urodeles can regenerate their limbs, but this is not so (5), and in general, postmetamorphic regenerative ability declines with age and size.
A host of different species have been used over the past century for limb regeneration studies, including the European crested newt, Triturus cristatus, which is now a protected species. Today, however, four species predominate. One is the eastern spotted newt of North America, Notophthalmus viridescens, which is used as an adult. The adults of this species are common, adaptable to laboratory conditions, and can be purchased from commercial suppliers or collected in the wild. The spotted salamander, Ambystoma maculatum, is used at larval stages having been collected from ponds in the spring. The other two species, the ribbed newt, Pleurodeles waltl, and the axolotl, Ambystoma mexicanum, breed readily in captivity, and there are several established breeding colonies. These two offer several advantages—they breed more than once a year, the eggs can also be used for studies of early development, the larvae regenerate limbs remarkably rapidly, they are available in large numbers, and comparative studies can be performed between limb development and limb regeneration not only in the same species, but in the same animal. In the latter case, because the hindlimbs develop many days after the forelimbs, grafts can be exchanged between forelimb regeneration blastemas and hindlimb developing limb buds (6). The axolotl offers the peculiar advantage in having suppressed metamorphosis to retain its larval form and aquatic habit throughout life. Consequently, they have become worldwide curiosities not only for the very large size of the elderly adults, but also because by adding thyroxine to the water, they can be induced to metamorphose and transform into a different species! In addition, there is a very large axolotl colony established at Indiana University, Bloomington, IN (run by Susan T. Duhon, IU Axolotl Colony, Jordan Hall 407, Bloomington, IN 47405), which will supply eggs and larvae to anywhere in the world. They also keep all the available mutant lines, some of which are useful for regeneration research, e.g., Short toes (7).
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