What is regeneration?
The process that makes genomes, cells, organisms and the ecosystem resilient to natural changes is referred to as the regeneration in biology. Also, it involves events that result in disturbance or damage. All species can regenerate, from bacteria to humans. Regeneration can either be complete where the new tissue is the same as the lost tissue or incomplete were after the necrotic tissue comes fibrosis. At its most basic stage, regeneration is mediated by the molecular processes of gene regulation. Regeneration in biology, however, mainly refers to the morphogenic processes that characterize the phenotypic plasticity of traits allowing multi-cellular organisms to repair and maintain the integrity of their physiological and morphological states. After the genetic stage, regeneration is controlled by asexual cellular processes. Regeneration is not the same as reproduction. For instance, hydra performs regeneration but reproduce by the method of budding.
Autonomy is the defensive function as the animal disengages a limb or tail to protect it from being captured. Cells move into action the moment the limb or the tail has been automized. Sometimes the shed limb can regenerate a new individual. Fishes and salamanders can regenerate limbs but in a partial manner whereby the tail regeneration takes place in larval frogs and toads (but not adults). The salamander’s limb or triton has the potential to grow severally after the amputation.
Some reptiles, such as the lizards, geckos and iguanas, can regenerate to a high degree after amputation. However, some reptiles such as the crocodiles, chelonians and snakes cannot regenerate lost body parts. Mostly, it happens as a defence mechanism when they drop off their tails and regenerate them after that. Therefore, if the predator catches the tail, it disconnects hence allowing them to escape.
AMPHIBIANS
DEFINING CHARACTERISTICS:
- Vertebrates
- Young have gills for breathing underwater
- The survive in both dry land and water
- Metamorphosis – There are few changes of the body shape, the diet and the lifestyle in the process of development.
- Ectothermic – The process relies on the warmth from the sunlight to become warm and physically active.
Amphibians refer to animals that live in both land and water. They are vertebrates and are also ectothermic; they depend on the sunlight to regulate their body heat to become active and gain warmth. That implies that Amphibians have to find a burrow or some other shade since they cannot cool down on their own else, they get too hot. Amphibians tend to be sluggish and do not move around much in cold weather.
Young amphibians, called larvae, look different from their parents., a process called metamorphosis. They undergo a change in body shape, diet, and lifestyle as they develop. Taking a case study of a frog, they start as a tadpole with gills so that they can breathe underwater and a tail to help them swim. As the tadpoles grow, they develop lungs, legs, and a different mouth. Its eyes also change position, and it loses its tail. At this point, it is an adult frog, and now instead of living in water, they start spending most of its time hopping on land.
Amphibians have three modern orders which are the Anura (the frogs and toads), Urodela (the salamanders), and Apoda (the caecilians). There are approximately 8,000 of amphibian species, of which nearly 90% are frogs. ( Zweifel, Richard G. “Urodela.” AccessScience, McGraw-Hill Education, 2014). The smallest known amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amanuensis) with a length of just 7.7 mm (0.30 in). The largest living amphibian is the 1.8 m (5 ft 11 in) South China giant salamander (Andrias Sligo), but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus from the middle Permian of Brazil. The study of reptiles and amphibians is known as the herpetology, while the study of the amphibians only is referred to as the batrachology.
Urodela
Different types of salamanders exist, though the axolotl (Ambystoma Mexicanum) and three species of newts (Notophthalmus viridescens, Eastern red-spotted newt; Cynops phyrrogaster, Japanese fire-belly newt; and Pleurodeles waltl, Iberian ribbed newt),are the most commonly used one for research.All the mentioned three types of salamanders could regenerate even though in a remarkably similar way but differently.
For example, axtolts cannot regenerate more body parts like the newts. Newts can regenerate the lens of their eyes throughout their entire life. However, axolotls can only regenerate eye lens only in their first two weeks after hatching (Sousounis et al., 2014). In Cynops, however, lens regeneration does not decline with age or with the number of lens removal/regeneration cycles. Another example is lens regeneration in newts, which is dependent on iris pigmented epithelial cells that dedifferentiate and proliferate, and the subsequent transdifferentiation of a subset of these cells into a new lens. Also, when newts undergo limb regeneration, multinucleated muscle cells break up into mononucleate (single nucleus) progeny, which subsequently re-enter the cell cycle and contribute to the new appendage. In contrast, the axolotl does not exhibit muscle dedifferentiation during limb regeneration; instead, new muscle fibres seem to be entirely derived from the activation of a resident Pax7-expressing stem cell population (Pax7 is a transcription factor that plays a role in myogenesis through regulation of muscle precursor cells proliferation. It can bind to DNA as a heterodimer with PAX3) (Fei et al., 2017; SandovalGuzmán et al., 2014; Wang and Simon, 2016).
All salamander do not have exact same lifecycle notophthalmus and cynops have a more complicated life cycle, having both aquatic and terrestrial phases. Therefore, it is difficult for them to breed in the laboratory. In addition, their regeneration time is relatively longer. Axolotl is fully aquatic paedomorphic animal which implies that it keeps larval features like the external gills in its entire life span. Therefore, it is easy to keep and maintain Axolotls in a laboratory conditions and make them breed captivity as they provide offspring in a season-independent manner (Khattak et al., 2014).
Axolotl is the best model for regeneration studies due to the above-mentioned factors. This has also been mainly facilitated by the feasibility of germline(In other words, they are the cells that form the egg, sperm and the fertilized egg) transgenesis (It is the experiment that involves inserting a foreign gene into the genome of an organism) in axolotls, which has enabled germline mutagenesis and Cre-loxP( A site-specific recombinase technology, used to carry out deletions, insertions, translocations and inversions at specific sites in the DNA of cells.) reporter-mediated lineage tracking In addition, although the axolotl genome is gigantic (32 Gb), it is now assembled and annotated with impressive contiguity.
Genome
The genomes of salamanders are remarkably huge between 14 and 120 GB. How salamander genomes became gigantic is subject to discussion but recent sequencing data have given for information and light to the researchers (Sun and Mueller, 2014; Elewa et al., 2017; Nowoshilow et al., 2018). These data indicate that there is a inconsistent expansion of repetitive sequences – predominantly long terminal repeat (LTR) retrotransposons ( a type of genetic component that copy and paste themselves into different genomic locations by converting RNA back into DNA through the process reverse transcription using an RNA transposition intermediate.) – contributes significantly to salamander genome gigantism. Repeated elements are often located in introns whose median size in the axolotl is on average an order of magnitude longer than introns in the human genome. In addition, intergenic regions in the axolotl genome are an order of magnitude longer than those in other vertebrates. Analyses of salamander genomes have also provided clues about the genes that function during regenerative processes. As noted in the Introduction, development and regeneration are two tightly interlinked processes. For example, genes responsible for patterning and morphogenesis are re-activated during limb regeneration, although their precise regulation is not a complete recapitulation of embryonic development (Stocum, 2017). Although major signalling components of the Wnt and Hedgehog signalling pathways are present in the axolotl, a surprising finding was that Pax3 is absent in the axolotl genome (Nowoshilow et al., 2018). Loss-of-function experiments in axolotls, using TALEN(Transcription activator-like effector nucleases are restriction enzymes that can be engineered to cut specific sequences of DNA) and CRISPR/Cas9 ( When a virus infects a microbial cell, the microbe employs a special CRISPR-associated nuclease (Cas9) to chop off a piece of the viral DNA. The nuclease is directed to its target sequence by a short RNA fragment known as a guide RNA (gRNA), which is complementary to the target segment of the viral genome. The snipped DNA fragment may then be stored between the palindromic CRISPR sequences to retain a genetic memory for disabling future infections from the same viral strain) genome editing, indicate that the paralogue Pax7 takes on the role that Pax3 performs in other vertebrates, as Pax7 axolotl mutants have major developmental abnormalities and lack limb muscle (Nowoshilow et al., 2018). In sharp contrast, the Pleurodeles genome harbours both Pax3 and Pax7. Loss of Pax3 in Pleurodeles leads to severe abnormalities, including skeletal muscle agenesis, again as occurs in other vertebrates (Elewa et al., 2017). Despite Pax7 being essential for successful skeletal muscle regeneration in mammals (Kuang et al., 2006), Pax7 loss of function in Pleurodeles does not cause any major regeneration phenotype. This finding might indicate that, in the absence of Pax7, skeletal muscle regeneration is fuelled by dedifferentiation of myofibers in Pleurodeles (Elewa et al., 2017).