Amphibian and Reptile Body Part Regeneration

Amphibians and Reptiles are vertebrate animals known for their special ability of body part regeneration especially the tail. The process in which these animals regenerate the tail is called autotomy. According to Clause and Capaldi (2006) caudal autotomy is a strategy used by lizards in order to escape from a predator. Kaplan (2000) defined autotomy as the self-induced release of a particular body part. In land salamanders, it is the tail, digits and some segment of the limbs. The digits and appendages of a lizard do not regenerate. Lizard tail regeneration is considered as a useful model in the study of molecular basis of lymphangiogenesis to be able to develop cure for human lymphoedema (Daniels, et.al 2003). According to Chernoff et.al (2002) spinal cord regeneration is due to axonal regrowth, neurogenesis and glial responses. Regenerating lizard tails are potentially useful models for studying the molecular basis of lymphangiogenesis with a view to developing possible treatments for human lymphoedema.

Some amphibians that exhibit this amazing ability of autotomy and tail regeneration are Xenopus laevis commonly known as African clawed frog, Ambystoma mexicanum commonly known as axolotl, salamanders and other urodeles. According to Mochii, Taniguchi and Shikata (2007) the Xenopus tadpole has 3 major axial structures, these includes the spinal cord, notochord and myotomes. It takes 2 weeks for the tail to regenerate. The regenerated and normal tail has some differences in that it has an immature spinal cord and the muscle masses are not completely segmented. In contrast with urodele regeneration it was suggested that lineage-restricted stem cells derived from their own tissues is responsible for the reconstruction of the tail tissues. Another species exhibiting tail regeneration is the Ambystoma mexicanum commonly known as the axolotl.

According to Schnapp and Tamaka (2004), complex body structures such as the appendages, tail and spinal cord can be regenerated by axolotl throughout life. Maden and Hind (2003) stated that retinoic acid (RA) is responsible for the induction of tissues and organs regeneration in amphibians and mammals. According to their study, RA induces super-regeneration of urodele amphibian limb by respecifying its positional information. Urodele amphibians, salamanders and newts can regenerate injured spinal cord completely as adults in their whole life cycle. Both the gap replacement and caudal regeneration was brought about by the ependymal cells. New neurons and supporting axonal regrowth were also produced during spinal cord regeneration. (Chernoff, Stocum, Nye and Cameron 2003). According to Morrison Lööf, He and Simon (2005), in contrast to mammals, salamanders are capable of regenerating structures including appendages. It was stated that the regeneration of limbs depends largely on blastema formation which is responsible for the development of appendages. Before the occurrence of blastema formation dedifferentiation of skeletal muscles happens. In this study, new limb tissue formation is said to be caused by the satellite cell activation. The skeletal muscle tissue repair by mammalian myofiber mobilization of stem cells is a little similar to salamander satellite cells activation. Thus, mammalian tissue repair and limb regeneration share common molecular and cellular programs.

Like some amphibian species do, reptile species also exhibits body part regeneration. Reptiles exhibiting regeneration includes Great Plains Skink (Eumeces obsoletus), Five-lined skink (Eumeces fasciatus), Six-lined Racerunner (Cnemidophorus sexlineatus), Slender Glass Lizard (Ophisaurus attenuatus), Collared Lizard (Crotaphytus collaris), Cordylid Lizard (Cordylus melanotus melanotus) and Tuatara (Genus Sphenodon). According to Chapple and Swain (2002) lizards exhibit caudal autotomy  as an effective defense mechanism enabling them to escape during predatory attacks. However, Hoin the study conducted by McConnachie and Whiting (2003) about cost-associated with tail autotomy in cordylid lizard, it was discovered that the cost of tail autotomy was estimated 12% of the total body weight suggesting loss of energy. According to Fitch (2002), when he compared five lizard species in terms of adaptation degree in tail autotomy it was sequence by specialization. It was found out that Great Plains skink is the most specialized in that the new tail was conspicuously colored and adapted to flaunt it. The five-lined skink also has conspicuous color but the tail appeared was less developed. In six-lined racerunner the hatchling tail was conspicuously colored but no flaunting tail. The tail was easily broken and no special coloration in slender grass lizard. Lastly, in collared lizard there was no tail regeneration.

Among Vertebrates regeneration of limbs are very limited and only occurs in most fishes and salamanders. In larval frogs and toads tail regeneration may also take place. Few birds can undergo complex structure regeneration. On the other hand, mammals don’t have the ability to regenerate tails or limbs. Only peripheral appendages like deer’s antler and human liver undergoes this process. Strict polarity is followed by tissues in the process of regeneration emerging back to orientation of the body. (Science Centre Singapore 2008).  According to Price and Allen (2004) deer antlers are the only appendages of mammals which has the capacity of repeated rounds of regeneration. Every year, the antlers regrow from a blastema into structures of cartilage and bone used for display and fighting. The process of modified endochondral ossification results to longitudinal growth that in some species can grow 2 centimeters per day which shows the fastest organ growth rate in the animal kingdom.  Rinaldi (2005) in her article “The newt in us” stated that the regeneration ability of animals is based upon the local plasticity of their cells. It was demonstrated that the formation of blastema is caused by progenitor cell migration to the wound. Blastema then differentiates to be able to replace missing tissue. The removal up to the activation of plasticity in differentiated cells makes it possible for urodeles to regenerate.

According to Weia, Schubigerb, Harderc and Müllerc (2000) the differentiation potential of mammalian somatic stem cell is specific to only one tissue. Meaning to say, the differentiation potential of somatic stem cells are limited.  Tanaka (2003) stated that the destabilization of cell differentiation is triggered by amputation. Amputation also leads to the production of progenitor cells that proliferate extensively to recreate a perfect copy of the missing part by patterning themselves. The difference between an amphibian and mammal regeneration is that amphibians has higher adult tissue plasticity than mammals.

The discovery of the mechanism of amphibian regeneration by dedifferentiation is of big importance to regenerative medicine development. By understanding this mechanism, researchers from all over the world can discover chemicals that can induce regeneration of mammalian tissues and organs as well. Stocum (2004). Regenerative medicine is said to be an emerging field of biomedicine. The problem in this field is that the process of cell mobilization and integration  into functional tissue is not yet discovered (Mironov, Visconti and Markwald 2004). According to Brockes and Kumar (2005) regeneration of some complex structure like limbs is one of the pointers for regenerative medicine. It was stated that limb regeneration proceeds from the local blastema formation. From the viewpoint that blastema can regenerate autonomously as a self-organizing system in a linear dimension can be considered as a prospect for limb regeneration in mammals.

References

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