Famous, the Axolotl (Picture by Susan Crawford-Young)
Our favorite model animal is a salamander, called the axolotl (Ambystoma mexicanum). The axolotl is a big salamander, typically 20 centimeters as an adult, although they can often be a lot larger. Most of us have watched water dwelling frog tadpoles grow up, lose their tails and gills, and become air breathing adults. Most salamanders do the same thing. The axolotl is a neotenic salamander. Neotenic means it never goes through this metamorphosis to become a land dweller. The axolotl remains in the larval state, complete with external gills, even as a breeding adult. Neoteny is a recent adaptation in evolutionary time for the axolotl, and with a little artificial hormonal prodding in the laboratory, the axolotl can still transform into a regular looking terrestrial salamander although some will not survive the transformation. The axolotl can also cross breed and produce fertile offspring with its closest salamander relative, the tiger salamander (Ambystoma tigrinum) which does metamorphose naturally. The axolotl is a typical salamander, despite its fully aquatic life cycle.
During the evolution of higher species on earth, animals split into two broad groups, the invertebrates like insects and sea urchins and the vertebrates. All four legged vertebrates passed through, i.e., branched off from, an axolotl-like ancestor. Frogs went off on their own separate branch. Axolotls look very much like the earliest salamander fossils that have been found, so they haven’t changed much in appearance. Our common ancestor is likely to have had the many wonderful properties that are still available for study in axolotls. Axolotls, like all vertebrates including us, have a notochord. The notochord is a stiff rod of tissue that forms very early in embryogenesis and acts as a support structure underlying the region that will become the spine. The notochord preceded the spine in evolution. In more phylogenetically basal (more primitive) organisms, the notochord persists through the adult stage and it is the primary axial support running from the base of the brain all the way to the end of the tail. In most vertebrates, including us, the spine forms above the notochord and the spine takes over the function of main axial support. The notochord forms the nucleus pulosus where it acts as a shock absorbing material and separates the vertebrae.
The axolotl is native to Lake Xochimilco which once existed where Mexico City is now. Unfortunately, it is endangered, and possibly extinct, in the wild due to habitat destruction. Fortunately it has been used as a laboratory animal for over 100 years and is a popular pet and so it will likely not go extinct anytime soon, and if the day comes their habitat is restored, they can repatriated to their wild home. The axolotl takes two years of tending to go from an embryo to a fertile adult. The axolotl has a relatively huge egg. The egg is 2 mm in diameter, or 2000 µm, compared to 70 µm for a human egg. With the axolotl embryo’s large cells it has proven easy to watch every surface cell with a modest microscope.
Another one of the axolotl’s marvellous properties is that they have a tremendous capacity to regenerate. They are aggressive towards one another, and a large axolotl will bite off the leg of a small one. While the latter is slowed down and less of a competitor, it nevertheless regenerates its leg in 6-8 weeks. Jaws regenerate too. Skin wounds heal without scarring. If they lose their tail or a even part of their heart they will likewise regenerate, as does a severed spinal cord. A chunk of brain may be successfully transplanted and in a closely related salamander even memory has been transplanted. If you ever read Frank Herbert’s science fiction you may recall the Axlotl tanks. These were used to regenerate an entire new human clone from a cadaver or even a piece of a cadaver and no doubt he got the idea from the marvellous regenerative properties of the axolotl. Did he misspell the word on by accident or on purpose? It is a strange word. Axolotl is from the Nahuatl language of the Aztec civilization and so looks foreign to English eyes.
Axolotls make fascinating pets. Famous, the axolotl pictured above, is the pet of engineer, artist and teacher Susan Crawford-Young. In addition to her other talents she takes a good picture and she kindly gave us a picture of Famous which will be incorporated into our book, Embryogenesis Explained.