Tail regeneration in the Paleozoic tetrapodMicrobrachis pelikaniand comparison with extant salamanders and squamates

Effects of life history strategies and habitats on limb regeneration in plethodontid salamanders

BACKGROUND

Salamanders are the only tetrapods that exhibit the ability to fully regenerate limbs. The axolotl, a neotenic salamander, has become the model organism for regeneration research. Great advances have been made providing a detailed understanding of the morphological and molecular processes involved in limb regeneration. However, it remains largely unknown how limb regeneration varies across salamanders and how factors like variable life histories, ecologies, and limb functions have influenced and shaped regenerative capacities throughout evolution.

RESULTS

This study focuses on six species of plethodontid salamanders representing distinct life histories and habitats. Specimens were examined for regeneration ability after bite injuries as well as after controlled amputations. Morphological investigations revealed great regenerative abilities in all investigated species and frequent anatomical limb anomalies. Correlations were observed with respect to speed of regeneration and habitat.

CONCLUSIONS

Investigating regeneration in non-model salamander taxa is essential for disentangling shared features of the regeneration process versus those that may be more taxon-specific. Gaining insights into variable aspects of regeneration under natural conditions and after conspecific biting rather than controlled amputations adds important new datapoints for understanding the evolutionary framework of regeneration and provides a broader context for interpreting findings made in the model organism axolotl.

The evolutionary origin and mechanism of chordate tail regeneration. An ancient tale?

Chordate tail regeneration represents the remarkable ability of some chordates to partially or completely regenerate a significant portion of their primary body axis. In this review we will discuss the chordate regenerative ability, what is known about the cellular sources which contribute to the regenerating tail, how various structures such as the spinal cord and vertebral column are re-established, and how scaling of the regenerating tail is regulated. Finally, we propose that tail regeneration is evolutionarily conserved and is fundamentally different from tail development however the origin and mechanism of this process remain elusive.

Nature-Inspired Scarless Healing: Guiding Biomaterials Design for Advanced Therapies

The use of biomaterials in the treatment of skin wounds has been steadily increasing over the last two decades. The key to the successful application of biomaterials in scar reduction is the up-to-date knowledge of the actors involved in accelerated healing and the cellular factors that can be implemented in bioinspired materials. Natural models of scarless healing such as oral mucosa, fetal skin and the skin of amphibians, fish, and reptiles are a great source of information. By investigating their microenvironments, cellular factors, and inflammatory self-regulatory systems, a general model of scarless healing can be defined. This review introduces the basic and current concepts of skin wound healing and focuses on providing a detailed overview of the main processes of accelerated healing without scarring. The article outlines the common features and key points that develop and promote scar-free healing. The tissues and healing processes of the selected natural models are described individually (tissue organization, structural components, ratios of cellular factors such as Collagen and transforming growth factor and their mechanisms of regulation of inflammation and scar overgrowth). A comparative work of each natural model concerning healing in human skin is included in the discussion. Finally, the patterns identified through the analysis of each model and their differences from normal healing are presented to facilitate the knowledge for the implementation of new treatments.

Regeneration in anamniotes was replaced by regengrow and scarring in amniotes after land colonization and the evolution of terrestrial biological cycles

An evolutionary hypothesis explaining failure of regeneration among vertebrates is presented. Regeneration derives from postembryonic processes present during the life cycles of fish and amphibians that include larval and metamorphic phases with broad organ reorganizations. Developmental programs imprinted in their genomes are re-utilized with variations also in adults for regeneration. When vertebrates colonized land adopting the amniotic egg, some genes driving larval changes, and metamorphosis were lost and new genes evolved, further limiting regeneration. These included neural inhibitors for maintaining complex nervous systems, behavior and various levels of intelligence, and adaptive immune cells. The latter, that in anamniotes are executioners of metamorphic reorganization, became intolerant to embryonic-oncofetal-antigens impeding organ regeneration, a process that requires de-differentiation of adult cells and/or expansion of stem cells where these early antigens are formed. The evolution of terrestrial lifecycles produced vertebrates with complex bodies but no longer capable to regenerate their organs, mainly repaired by regengrow. Efforts of regenerative medicine to improve healing in humans should determine the diverse developmental pathways evolved between anamniotes and amniotes before attempting genetic manipulations such as the introduction of "anamniote regenerative genes" in amniotes. This operation may determine alteration in amniote developmental programs leading to teratomes, cancer, or death.

Salamander-like tail regeneration in the West African lungfish

Salamanders, frog tadpoles and diverse lizards have the remarkable ability to regenerate tails. Palaeontological data suggest that this capacity is plesiomorphic, yet when the developmental and genetic architecture of tail regeneration arose is poorly understood. Here, we show morphological and molecular hallmarks of tetrapod tail regeneration in the West African lungfish Protopterus annectens, a living representative of the sister group of tetrapods. As in salamanders, lungfish tail regeneration occurs via the formation of a proliferative blastema and restores original structures, including muscle, skeleton and spinal cord. In contrast with lizards and similar to salamanders and frogs, lungfish regenerate spinal cord neurons and reconstitute dorsoventral patterning of the tail. Similar to salamander and frog tadpoles, Shh is required for lungfish tail regeneration. Through RNA-seq analysis of uninjured and regenerating tail blastema, we show that the genetic programme deployed during lungfish tail regeneration maintains extensive overlap with that of tetrapods, with the upregulation of genes and signalling pathways previously implicated in amphibian and lizard tail regeneration. Furthermore, the lungfish tail blastema showed marked upregulation of genes encoding post-transcriptional RNA processing components and transposon-derived genes. Our results show that the developmental processes and genetic programme of tetrapod tail regeneration were present at least near the base of the sarcopterygian clade and establish the lungfish as a valuable research system for regenerative biology.

Conflicting evidence for the use of caudal autotomy in mesosaurs

The early Permian mesosaurs were the first amniotes to re-invade aquatic environments. One of their most controversial and puzzling features is their distinctive caudal anatomy, which has been suggested as a mechanism to facilitate caudal autotomy. Several researchers have described putative fracture planes in mesosaur caudal vertebrae — unossified regions in the middle of caudal vertebral centra — that in many extant squamates allow the tail to separate and the animal to escape predation. However, the reports of fracture planes in mesosaurs have never been closely investigated beyond preliminary descriptions, which has prompted scepticism. Here, using numerous vertebral series, histology, and X-ray computed tomography, we provide a detailed account of fracture planes in all three species of mesosaurs. Given the importance of the tail for propulsion in many other aquatic reptiles, the identification of fracture planes in mesosaurs has important implications for their aquatic locomotion. Despite mesosaurs apparently having the ability to autotomize their tail, it is unlikely that they actually made use of this behaviour due to a lack of predation pressure and no record of autotomized tails in articulated specimens. We suggest that the presence of fracture planes in mesosaurs is an evolutionary relic and could represent a synapomorphy for an as-yet undetermined terrestrial clade of Palaeozoic amniotes that includes the earliest radiation of secondarily aquatic tetrapods.