Tissue regeneration is a fascinating biological subject. Many invertebrate organisms such as crabs, starfishes, sponges, jawless fish such as the lampreys and planarian flatworms can regenerate body parts. Among the vertebrates salamanders, especially the axolotl is a legendary study species for biologists. The axolotl is a highly endangered Mexican salamander with a fascinating ability to regenerate. Curiously, although both are amphibians salamanders can regenerate but frogs cannot. Comparison of such contrasting groups is particularly useful in understanding organ and tissue regeneration.
The Mexican axolotl has an extraordinary story for it is not only a cultural icon it is also a biomedical treasure. This fully aquatic salamander that rarely gets onto land is a valuable laboratory animal. It is also a highly sought after pet around the world. Unfortunately in the wild, the species is highly endangered facing risk of extinction. The polluted canals in the south of Mexico City is one of the last remaining habitats harboring the populations struggling to survive.
Dr. Susan Bryant of UC Irvine is one of the many researchers studying the regenerative capability of these amphibians. Why salamanders are so good in this while other amphibians such as the frogs can’t? Since 1920s the salamander embryo development have been studied with great detail and success. Salamander embryo development has played a pivotal role in the “Evo-Devo” studies which compare the developmental processes of organisms to infer the evolutionary ancestral relationships.
Salamanders are perfect organisms to study a curious evolutionary phenomenon called neoteny which is a technical term for retention of juvenile characters in adulthood. The axolotl is a neotenous salamander, retaining gills throughout its life. For instance birds living today are neotenized version of dinosaurs. The concept of neoteny also extends into human evolution. Biologists are now accumulating more evidence that human evolution is shaped for quite a bit based on retention of juvenile characteristics such as ability to digest milk (lactose tolerance) in adulthood or having larger brains by prolonged childhood period (leaving more time for brain to develop).
A striking detail in axolotl regeneration is that skin fibroblast cells appear to be aware of their location in the body. Grafting of cells from different parts confirmed this retention of positional identity. Biologists are keen to learn about how this molecular identity arises by studying gene expression patterns in single-cell resolution. In fact this has been achieved in a few model organisms such as the zebra fish and mouse where each cell in a developing embryo has been tracked and transcriptionally profiled. Now you may ask why fish or an amphibian like a salamander? Let’s not forget: Evolutionarily speaking all of us are tetrapods and contain a fish inside!
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