Immunolocalization of lix1 in the regenerating tail of lizard indicates that the protein is mainly present in the nervous tissue

PURPOSE

Lizard regeneration derives from the re-activation of a number of developmental genes after tail amputation. Among genes with the highest expression, as indicated from the transcriptome, is lix1 which functional role is not known.

METHOD

An antibody that cross-reacts with the lizard Podarcis muralis lix1 has been utilized to detect by immunofluorescence the sites of localization of the protein in the regenerating tail.

RESULTS

Lix1-protein is almost exclusively localized in the regenerating spinal cord (ependyma) and nerves growing into the blastema, in sparse blastema cells but is undetectable in other tissues.

CONCLUSIONS

Since the spinal cord is essential to stimulate tail regeneration it is hypothesized that the lix1 protein is part of the signaling or growing factors produced from the regenerating spinal cord that are needed for tail regeneration of the lizard tail.

Embryonic Development and Histological Analysis of Skeletal Muscles of Tenuidactylus caspius (Eichwald, 1831) Lizards (Reptilia: Squamata)

During embryonic development of the Caspian thin-toed gecko migration, formation of myoblast and myosatellite cells occurs in the cranial-distal direction. Somite formation begins in the body part close to the skull and ends in the tail. The time of separation of somites from the proximal mesoderm depends on the temperature of the air and the substrate. Myoblast cells reach their targets and are connected, and the membranes in the area of their contact are destroyed. Myoblast’s fusion creates myosymplasts. The intermediate stage is observed after the formation of small myosymplasts. After that, the chain shape of myosymplasts are transformed into an intermediate plaque form. At this intermediate stage, the number of a nucleus is small, the shape of the nucleus differs from each other, and the location of the nucleus varies. Afterward, the connection of the intermediate forms with each other and with myoblasts forms a rounded shape, where the initial development of myotubules takes place. A fully formed myotubular and myosatellite cells are surrounded by a basal membrane and shape a muscle fiber. The skeletal muscles of the adult Caspian thin-toed gecko are mainly composed of white fibers. Thus, it allows the gecko to move very fast in a short time. Due to the small number of mitochondria in the myotubes, oxygen gas demand is decreased and the body is prevented from overheating.

At What Cost? Trade-Offs and Influences on Energetic Investment in Tail Regeneration in Lizards Following Autotomy

Caudal autotomy, the ability to shed a portion of the tail, is a widespread defence strategy among lizards. Following caudal autotomy, and during regeneration, lizards face both short- and long-term costs associated with the physical loss of the tail and the energy required for regeneration. As such, the speed at which the individual regenerates its tail (regeneration rate) should reflect the fitness priorities of the individual. However, multiple factors influence the regeneration rate in lizards, making inter-specific comparisons difficult and hindering broader scale investigations. We review regeneration rates for lizards and tuatara from the published literature, discuss how species' fitness priorities and regeneration rates are influenced by specific, life history and environmental factors, and provide recommendations for future research. Regeneration rates varied extensively (0-4.3 mm/day) across the 56 species from 14 family groups. Species-specific factors, influencing regeneration rates, varied based on the type of fracture plane, age, sex, reproductive season, and longevity. Environmental factors including temperature, photoperiod, nutrition, and stress also affected regeneration rates, as did the method of autotomy induction, and the position of the tail also influenced regeneration rates for lizards. Additionally, regeneration could alter an individual's behaviour, growth, and reproductive output, but this varied depending on the species.

Spinal ganglia and peripheral nerves innervating the regenerating tail and muscles of lizards

The present review summarizes available information on the contribution of regenerating nerves to the process of regeneration in the tail of lizards. From the last three segments of the spinal cord and ganglia proximal to the regenerating tail, motor, sensory somatic and autonomous nerves regenerate and richly innervate the growing blastema. However, experimental studies have indicated that peripheral nerves are not essential for stimulating the regeneration of the tail that instead is mainly sustained by the interaction of the apical ependyma with the wound epidermis. Ganglion neurons innervating the regenerating blastema increase their size and some satellite cells multiply but no ganglion neurons are regenerated. Numerous Schwann cells proliferate to keep pace with nerve regeneration, and they form myelin starting from 3 to 4 weeks of tail regeneration. The hypertrophic ganglion neurons synthesize growth factors and signaling proteins such as FGFs and Wnts that are transported into the regenerating blastema through the regenerating nerves. Nerves form synaptic-like contacts with mesenchymal cells or fibroblasts at the tip of the regenerating blastema but not synaptic boutons. These terminals may discharge stimulating factors that favor cell proliferation but this is not experimentally demonstrated. Most of the innervation is directed to differentiating muscles where nerve endings form cholinergic motor-plates. Transcriptome data on the regenerating blastema-cone detect up-regulation of various genes coding for ionic channels, neurotransmitter receptors and signaling proteins. The latter suggests that the neurotrophic stimulation may control cell proliferation but is most directed to the functionality of regenerating muscles.

Future Tail Tales: A Forward-Looking, Integrative Perspective on Tail Research

Synopsis Tails are a defining characteristic of chordates and show enormous diversity in function and shape. Although chordate tails share a common evolutionary and genetic-developmental origin, tails are extremely versatile in morphology and function. For example, tails can be short or long, thin or thick, and feathered or spiked, and they can be used for propulsion, communication, or balancing, and they mediate in predator-prey outcomes. Depending on the species of animal the tail is attached to, it can have extraordinarily multi-functional purposes. Despite its morphological diversity and broad functional roles, tails have not received similar scientific attention as, for example, the paired appendages such as legs or fins. This forward-looking review article is a first step toward interdisciplinary scientific synthesis in tail research. We discuss the importance of tail research in relation to five topics: (1) evolution and development, (2) regeneration, (3) functional morphology, (4) sensorimotor control, and (5) computational and physical models. Within each of these areas, we highlight areas of research and combinations of long-standing and new experimental approaches to move the field of tail research forward. To best advance a holistic understanding of tail evolution and function, it is imperative to embrace an interdisciplinary approach, re-integrating traditionally siloed fields around discussions on tail-related research.

Review: Regeneration of the tail in lizards appears regulated by a balanced expression of oncogenes and tumor suppressors

BACKGROUND

Tail regeneration in lizards is the only case of large multi-tissue organ regeneration in amniotes.

METHODS

The present Review summarizes numerous immunolocalization and gene-expression studies indicating that after tail amputation in lizards the stump is covered in 7-10 days by the migration of keratinocytes. This allows the accumulation of mesenchymal-fibroblasts underneath the wound epidermis and forms a regenerative blastema and a new tail.

RESULTS

During migration keratinocytes transit from a compact epidermis into relatively free keratinocytes in a process of "Epithelial Mesenchymal Transition" (EMT). While EMT has been implicated in carcinogenesis no malignant transformation is observed during these cell movements in the regenerative blastema. Immunolabeling for E-cadherin and snail shows that these proteins are present in the cytoplasm and nuclei of migrating keratinocytes. The basal layer of the wound epithelium of the apical blastema express onco-proteins (wnt2b, egfr, c-myc, fgfs, fgfr, rhov, etc.) and tumor suppressors (p53/63, fat2, ephr, apc, retinoblastoma, arhgap28 etc.). This suggests that their balanced action regulates proliferation of the blastema.

CONCLUSIONS

While apical epidermis and mesenchyme are kept under a tight proliferative control, in more proximal regions of the regenerating tail the expression of tumor-suppressors triggers the differentiation of numerous tissues, forming the large myomeres, axial cartilage, simple spinal cord and nerves, new scales, arteries and veins, fat deposits, dermis and other connective tissues. Understanding gene expression patterns of developmental pathways activated during tail regeneration in lizards is useful for cancer research and for future attempts to induce organ regeneration in other amniotes including humans.

Regeneration in Reptiles Generally and the New Zealand Tuatara in Particular as a Model to Analyse Organ Regrowth in Amniotes: A Review

The ability to repair injuries among reptiles, i.e., ectothermic amniotes, is similar to that of mammals with some noteworthy exceptions. While large wounds in turtles and crocodilians are repaired through scarring, the reparative capacity involving the tail derives from a combined process of wound healing and somatic growth, the latter being continuous in reptiles. When the tail is injured in juvenile crocodilians, turtles and tortoises as well as the tuatara (Rhynchocephalia: Sphenodon punctatus, Gray 1842), the wound is repaired in these reptiles and some muscle and connective tissue and large amounts of cartilage are regenerated during normal growth. This process, here indicated as “regengrow”, can take years to produce tails with similar lengths of the originals and results in only apparently regenerated replacements. These new tails contain a cartilaginous axis and very small (turtle and crocodilians) to substantial (e.g., in tuatara) muscle mass, while most of the tail is formed by an irregular dense connective tissue containing numerous fat cells and sparse nerves. Tail regengrow in the tuatara is a long process that initially resembles that of lizards (the latter being part of the sister group Squamata within the Lepidosauria) with the formation of an axial ependymal tube isolated within a cartilaginous cylinder and surrounded by an irregular fat-rich connective tissue, some muscle bundles, and neogenic scales. Cell proliferation is active in the apical regenerative blastema, but much reduced cell proliferation continues in older regenerated tails, where it occurs mostly in the axial cartilage and scale epidermis of the new tail, but less commonly in the regenerated spinal cord, muscles, and connective tissues. The higher tissue regeneration of Sphenodon and other lepidosaurians provides useful information for attempts to improve organ regeneration in endothermic amniotes.

The Review of the Autotomy of Agamid Lizards with Considerations about the Types of Autotomy and Regeneration

We present a review of the data on the intervertebral autotomy and regeneration of agamid lizards based on an analysis of information obtained over a 35-year period after the publication of thorough reviews (Arnold, 1984, 1988 and Bellairs, Bryant, 1985). It is supplemented by our own studies of 869 specimens of agamid lizards (Sauria, Agamidae) stored in the herpetological collections of the Zoological Institute of the Russian Academy of Sciences (St. Petersburg, Russia) and the Zoological Museum of the Moscow State University (Moscow, Russia), represented by 31 species of 16 genera. The manifestations of the ability for autotomy and regeneration in phylogenetic lineages within the family—Leiolepidinae, Amphibolurinae, Agaminae, Draconinae—are considered. A comparative morphological analysis of the structure of the caudal vertebrae was carried out using the Computer Microtomography Methods (micro-CT) in the following ecomorphological types of agama: (1) with developed abilities to caudal autotomy and regeneration, (2) with the ability to caudal autotomy but without regeneration and (3) without the ability to autotomy. The phenomenon of intervertebral autotomy (urotomy) in snakes is considered too. Possible ways of evolution of the ability to caudal autotomy as a defense strategy against predators are discussed in the phylogenetic context.