Lizard Tail Research Articles

"A tale of tail": A case of lizard tail abuse.

"A tale of tail": A case of lizard tail abuse.

Fibroblast growth factor-2 signaling modulates matrix reorganization and cell cycle turnover rate in the regenerating tail of Hemidactylus flaviviridis

Lizards restore their lost tail by the recruitment of multipotent cells which are selectively differentiated into varied cell types so as to sculpt a new tail. The precise coordination of the events involved in this complex process requires crosstalk between many signaling molecules and differential regulation of several mediators that facilitate the achievements of various milestones of regeneration. Fibroblast growth factor-2 is one such signaling molecule which activates a number of intracellular signaling pathways. Herein, the regulatory role of FGF2 during tail regeneration in Hemidactylus flaviviridis was investigated. Upon inhibition of FGFR using SU5402, the FGF2 levels were found to be significantly reduced at both transcript and protein level. Further, the compromised levels of the gelatinases, namely MMP2 and MMP9 in the tail tissues of treated lizards indicate that FGF2 regulates the activity of these enzymes perhaps to facilitate the recruitment of multipotent mesenchymal cells (blastema). The in vivo 5BrdU incorporation assay showed a lower cell proliferation rate in FGF2 signal inhibited animals during all the proliferative stages of regeneration studied. This observation was substantiated by decreased levels of PCNA in treated group. Moreover, from the combined results of Caspase-3 localization and its expression levels in the regenerates of control and SU5402 treated lizards it can be deduced that FGF2 signal regulates apoptosis as well during early stages of regeneration. Overall, the current study indicates beyond doubt that FGF2 signaling plays a pivotal role in orchestrating the matrix reorganization and cell cycle turnover during lizard tail regeneration.

The vertebrate tail: a gene playground for evolution.

The tail of all vertebrates, regardless of size and anatomical detail, derive from a post-anal extension of the embryo known as the tail bud. Formation, growth and differentiation of this structure are closely associated with the activity of a group of cells that derive from the axial progenitors that build the spinal cord and the muscle-skeletal case of the trunk. Gdf11 activity switches the development of these progenitors from a trunk to a tail bud mode by changing the regulatory network that controls their growth and differentiation potential. Recent work in the mouse indicates that the tail bud regulatory network relies on the interconnected activities of the Lin28/let-7 axis and the Hox13 genes. As this network is likely to be conserved in other mammals, it is possible that the final length and anatomical composition of the adult tail result from the balance between the progenitor-promoting and -repressing activities provided by those genes. This balance might also determine the functional characteristics of the adult tail. Particularly relevant is its regeneration potential, intimately linked to the spinal cord. In mammals, known for their complete inability to regenerate the tail, the spinal cord is removed from the embryonic tail at late stages of development through a Hox13-dependent mechanism. In contrast, the tail of salamanders and lizards keep a functional spinal cord that actively guides the tail's regeneration process. I will argue that the distinct molecular networks controlling tail bud development provided a collection of readily accessible gene networks that were co-opted and combined during evolution either to end the active life of those progenitors or to make them generate the wide diversity of tail shapes and sizes observed among vertebrates.

Cerebrospinal fluid-contacting neurons in the regenerating spinal cord of lizards and amphibians are likely mechanoreceptors.

During spinal cord (SC) regeneration in the tail of amphibians and lizards, small neurons in contact with the central canal and cerebrospinal fluid (CSF) are formed. The present review summarizes previous and recent studies that have characterized most of these neurons as cerebrospinal fluid-contacting neurons (CSFCNs), especially in the regenerating caudal SC of lizards. CSFCNs form tufts of stereocilia immersed in the CSF, secrete exosomes, and are often in contact with a secreted protein-rod indicated as Reissner fiber. Ultrastructural, autoradiographic, immunohistochemical, and behavioral studies strongly indicate that most of these cells are mechanoreceptors that differentiate from ependymal cells within 20-30 days after SC amputation. Numerous CSFCNs are gamma amino-butyric acid (GABA)-ergic, uptake amino acids, receive few synaptic boutons, and contain neurofilaments, fibroblast growth factor (FGFs), and other signaling proteins, the latter likely secreted into the central canal. Similar neurons are formed in the SC of the tuatara (Sphenodon puctatus), anurans, and urodeles during tail regeneration. In lizard, most of their projection remains in the SC close to the regenerated tail, but they form synapses with neurons that receive descending nerves from the brainstem, including vestibular nuclei. CSFCNs, aside a possible neurosecretory activity, might sense liquor movements for maintenance of balance, a role that is supported from recent studies on other caudate vertebrates. The regeneration of these cells also in the nervous system of other vertebrates remains unknown.

Regeneration in the Podarcis bocagei model organism: a comprehensive immune-/histochemical analysis of the tail

Regeneration is the process of regrowth of an injured/missing tissue/organ from the residual tissue. One outstanding example is the regeneration of tail in lizards in which they can completely regenerate without the formation of scaring or fibrotic tissue. This study presents an analysis of the main differences between a mature regenerated tail and the original tail of the lizard Podarcis bocagei by comparing histological biomarkers for support structures, morphology and pigmentation, vasculature and energy storage. Matured regenerated and original tails were studied with different histological staining’s and immunohistochemistry to highlight the various tissue components. We found differences in the morphological features scalation pattern and pigmentation, as well as in the support structures: muscle organization, collagen and calcification. Vasculature was remodeled. Energy storage was restored with regeneration maintaining therefore the main metabolic function. We demonstrate that the regenerated tail does not lose its main functions like locomotion and energy metabolism. The present findings may open a new window of research with implications in regenerative medicine.

Gekkonidae, Lizard tail extracts elicit apoptotic response against non-small cell lung cancer via inhibiting Akt signaling

The genes of Gekkonidae, a lizard, are known to be very similar to human genes. According to previous research, lizard extracts inhibit angiogenesis and show anticancer activity against various cancers. In this regard, this study assessed whether lizard tail extracts (LTE) cause anticancer activity against lung cancer in mouse and human lung cancer cell lines. The results showed that LTE exhibited anticancer activity against lung cancer in vitro and in vivo. In vitro, cell viability and proliferation decreased in two lung cancer cell lines, while annexin V and single-stranded DNA both increased, showing apoptotic activity caused by LTE. We also found that LTE induced apoptosis in a caspase-3/7 cascade-dependent manner and inhibited the phosphorylation of Akt by participating in the PI3k/Akt pathway. In vivo, LTE decreased tumor volume in LLC bearing mice. Our results demonstrate the potential of LTE as a natural-derived anticancer drug to overcome the chemotherapy side effects during cancer treatment and contribute to the discovery of candidate substances.

Stimulation of regenerative blastema formation in lizards as a model to analyze limb regeneration in amniotes.

The hypothesis here presented tries to explain why organ regeneration is present in fish and amphibians (anamniotes) but is absent in reptiles, birds and mammals (amniotes). Anamniotes possess complex life cycles including larvae and metamorphosis stages, the latter representing a physiological form of organ destruction and regeneration coded in their genome that can be reactivated in adults in the form of regeneration. Part of the genome for larvae and metamorphosis phases was likely lost in amniotes with the evolution of direct development, the potentiation of the immune system and the increase in complexity of the nervous system. These events consequently determined incapability for organ regeneration in extant amniotes with the exception of the lizard tail. This likely derives from the evolution of a mechanism of immunosuppression that allows the regeneration of the tail although the complete morphogenetic plane of tail embryogenesis is lost. The lizard model of imperfect but outstanding organ regeneration indicates the possibility to improve organ regeneration also in other amniotes. In fact, the induction of a blastema in the amputated lizard limb has stimulated the formation of short limbs containing cartilaginous bones of the femur, tibia and fibula, and these experiments foster some hope for future attempts to induce limb and digit regeneration also in mammals.

Lizard Tail Extracts Exert Protective Effect Against Oxidative Stress-Induced Cellular Senescence in Human Fibroblasts.

Lizards are the evolutionarily closest animals to humans among the self-renewable species. Recent reports show that lizard tail extracts (LTE) inhibit the proliferation and angiogenesis of cancer cells but do not show any toxicity against human fibroblast cells. Nevertheless, few scientific studies investigated the effects of LTE on the treatment of skin diseases, especially oxidative stress aging. Therefore, we explored the effect of LTE on the anti-aging activity of human fibroblasts. We confirmed the anti-aging effect of LTE by SA-β-galactosidase staining. In addition, the hydrogen peroxide-induced reactive oxygen species (ROS) were decreased by the LTE, as measured by staining with the 2',7'-dichlorofluorescein diacetate reagent. We performed Western blot analysis to examine the signaling pathways. In conclusion, the LTE can prevent cellular senescence through the suppression of ROS and the downregulation of p21.

Macro and Microanatomical Structures of Lizard’s Regenerate Tail (Mabouya multifasciata Kuhl, 1820)

Lizards are animals that can do autotomy. Research on the autotomy of lizard’s tail has been carried out, but few have examined about axial skeletons that focus on vertebrae caudales or arranged muscles. That is the background of this research. This research is expected to be a database for further research and as a comparison between animals that can perform other autotomies. This research intend to determine the macro and micro structures of the anatomical axial skeleton lizard’s regenerate tail and to know the micro structures of anatomy muscular segmentation lizard’s regenerate tail. The used methods are X-Ray, Alizarin Red S and Alcian Blue, Paraffin method with Hematoxylin-Eosin and Mallory Triple Strain staining. The result showed that the lizard's regenerate tail was darker than the original. On observations using X-Ray and Alizarin staining showed that the original tail is seen broad and have a Processus. The original lizard's tail is composed of bones, because it is red which shows perfectly calcified bones. While the regenerate of the lizard's tail is composed of bones in the form of long pipes which are red-colored because they have calcified from outside. Muscular segmentation of the original lizard's tail when viewed transversely has irregular segments. While the lizard's regenerate tail has 15 muscle segments. Muscles are composed of a collection of myotubes that become myotomes, each of myotomes confined by the myoseptum.

Immunohistochemical localization of a proto‐cadherin fat tumour‐suppressor homolog in the regenerating tail of lizard suggests a role in apical growth control

AbstractThe present immunohistochemical and western blotting study evaluates the localization of a proto‐cadherin which gene is overexpressed in the regenerating blastema of the lizard Podarcis muralis. Bioinformatic analysis suggests that the antibody recognizes FAT1/2 proteins. Western blot indicates a main band around 50 kDa, a likely fragment derived from the original membrane‐bound large protein. Immunofluorescence shows main labelling in differentiating wound keratinocytes, lower in ependyma, mesenchyme and extracellular matrix of the blastema. The apical epidermal peg contains keratinocytes with labelled peripheral cytoplasm, as confirmed using ultrastructural immunogold that also reveals most labelling located along the cell surface of mesenchymal cells. Myoblasts and differentiating myotubes of regenerating muscles are less intensely labelled. The regenerating cartilaginous tube contains sparse labelled chondroblasts, especially in external and internal perichondria. In regenerating scales, differentiating beta‐cells appear immunofluorescent mainly along the cell perimeter. In more differentiated muscle, cartilage and connective tissues of the new tail, the labelling lowers or disappears. The observations indicate that FAT1/2 proto‐cadherins are present in the apical blastema where an intense remodelling takes place for the growth of the new tail but where also a tight control of cell division and migration is active and may regulate potential tumorigenic process.

Microscopical observations on the regenerating tail in the tuatara Sphenodon punctatus indicate a tendency to scarring, but also influence from somatic growth.

The process of tail regeneration in the tuatara (Sphenodon punctatus) is not entirely known. Similarity to and differences from lizard tail regenerations are indicated in the present histological and ultrastructural study. Regeneration is influenced by the animal's age and ambient temperature, but in comparison to that of lizards it is very slow and tends to produce outgrowths that do not reach the length of the original tail. Although microscopically similar to lizard blastemas, the mesenchyme rapidly gives rise to a dense connective tissue that contains few muscle bundles, nerves, and fat cells. The unsegmented cartilaginous tube forming the axial skeleton is not calcified after 5 months of regeneration, but calcification in the inner region of the cartilage, present after 10 months, increases thereafter. Amyelinic and myelinic peripheral nerves are seen within the regenerating tails of 2-3 mm in length and the spinal cord forms an ependymal tube inside a cartilaginous casing. Tissues of the original tail, like muscles, vertebrae and the adipose mass, are largely replaced by dense connective tissue that occupies most of the volume of the new tail at 5 and 10 months of regeneration. It is unknown whether the differentiation of the dense connective tissue is caused by the relatively low temperature that this species lives under or stems from a genetic predisposition toward scarring as with most other amniotes. Increases of muscle and adipose tissues seen in older regenerated tails derive from somatic growth of the new tail in the years following tail loss and not from a rapid regeneration process like that in lizards.

Immunodetection of ephrin receptors in the regenerating tail of the lizard Podarcis muralis suggests stimulation of differentiation and muscle segmentation.

Ephrin receptors are the most common tyrosine kinase effectors operating during development. Ephrin receptor genes are reported to be up-regulated in the regenerating tail of the Podarcis muralis lizard. Thus, in the current study, we investigated immunolocalization of ephrin receptors in the Podarcis muralis tail during regeneration. Weak immunolabelled bands for ephrin receptors were detected at 15–17 kDa, with a stronger band also detected at 60–65 kDa. Labelled cells and nuclei were seen in the basal layer of the apical wound epidermis and ependyma, two key tissues stimulating tail regeneration. Strong nuclear and cytoplasmic labelling were present in the segmental muscles of the regenerating tail, sparse blood vessels, and perichondrium of regenerating cartilage. The immunolocalization of ephrin receptors in muscle that gives rise to large portions of new tail tissue was correlated with their segmentation. This study suggests that the high localization of ephrin receptors in differentiating epidermis, ependyma, muscle, and cartilaginous cells is connected to the regulation of cell proliferation through the activation of programs for cell differentiation in the proximal regions of the regenerating tail. The lower immunolabelling of ephrin receptors in the apical blastema, where signaling proteins stimulating cell proliferation are instead present, helps maintain the continuous growth of this region.

Immunolocalization of Matrix Metalloproteinases in regenerating lizard tail suggests that an intense remodelling activity allows for apical tail growth

AbstractRegeneration of the tail in lizards suggests that a continuous remodelling process is active in the developing tissues. The present immunological study has detected Matrix Metalloproteinases (MMPs), previously indicated from a transcriptome study, in the lizard Podarcis muralis. Immunoblots show a main labelled band for MMPs around 68 kDa, possibly a MMP16‐like. Immunofluorescence shows that MMPs are mainly present in the central apical region of the mesenchymal blastema, in the ependyma of regenerating spinal cord, and in the basal layer and basement membrane‐region of the apical wound epidermis. The immune‐labelling decreases or disappears in differentiating tissues present in proximal regions of the regenerating tail such as the epidermis and dermis, muscles, inner connective tissues and in the axial cartilaginous tube. The prevalent localization of MMPs in the central mesenchyme of the blastema indicates a continuous remodelling of the extracellular matrix that actively contributes to the progressive distal growth of the apical blastema into a new tail. The study stresses that in regenerating organs of amniotes, a massive growth can only occur where connective tissues are maintained loose by the continuous action of extracellular matrix proteases in concert with the high production of hyaluronate and consequent tissue hydration.

Immunolocalization of serpins in the regenerating tail of lizard suggests a role for epidermal and neural barrier formation

Tail regeneration in lizard is stimulated from the apical epidermis and spinal cord, the principal sources of growth factors and signaling proteins that sustain regeneration. Immunolabeling shows that serpins (serine protease inhibitors), which genes are among those most up-regulated during tail regeneration, are prevalently immunolocalized in the regenerating epidermis and ependyma. Western blot detects main protein bands extracted from regenerating tail at 25-27 and 48-52 kDa. The former band may correspond to p27 serpin, a degraded immunogenic fragment of serpin detected in psoriasis and cancer. This suggests that also in lizard the degradation of these proteins occurs but is uncertain whether it is physiological with some function or the fragments derive from the extractive process. In the regenerating epidermis serpins are mainly accumulated in pre-corneous and corneous layers (alpha-layers), and also in the forming oberhautchen and hard beta-layer. In the tail tip serpin-immunolabeling is mainly seen in the ependymal tube and with lower intensity in blastema (mesenchymal) cells. Aside the control on endogenous proteases for the formation of a protective corneous barrier, serpins may also form a protective anti-microbial barrier for the ependyma. The protection of the epidermis and ependyma allows these tissue to continue the stimulation for tail regeneration.

Differences in neural stem cell identity and differentiation capacity drive divergent regenerative outcomes in lizards and salamanders

While lizards and salamanders both exhibit the ability to regenerate amputated tails, the outcomes achieved by each are markedly different. Salamanders, such as Ambystoma mexicanum, regenerate nearly identical copies of original tails. Regenerated lizard tails, however, exhibit important morphological differences compared with originals. Some of these differences concern dorsoventral patterning of regenerated skeletal and spinal cord tissues; regenerated salamander tail tissues exhibit dorsoventral patterning, while regrown lizard tissues do not. Additionally, regenerated lizard tails lack characteristically roof plate-associated structures, such as dorsal root ganglia. We hypothesized that differences in neural stem cells (NSCs) found in the ependyma of regenerated spinal cords account for these divergent regenerative outcomes. Through a combination of immunofluorescent staining, RT-PCR, hedgehog regulation, and transcriptome analysis, we analyzed NSC-dependent tail regeneration. Both salamander and lizard Sox2+ NSCs form neurospheres in culture. While salamander neurospheres exhibit default roof plate identity, lizard neurospheres exhibit default floor plate. Hedgehog signaling regulates dorsalization/ventralization of salamander, but not lizard, NSCs. Examination of NSC differentiation potential in vitro showed that salamander NSCs are capable of neural differentiation into multiple lineages, whereas lizard NSCs are not, which was confirmed by in vivo spinal cord transplantations. Finally, salamander NSCs xenogeneically transplanted into regenerating lizard tail spinal cords were influenced by native lizard NSC hedgehog signals, which favored salamander NSC floor plate differentiation. These findings suggest that NSCs in regenerated lizard and salamander spinal cords are distinct cell populations, and these differences contribute to the vastly different outcomes observed in tail regeneration.

Immunodetection of High Mobility Group Proteins in the regenerating tail of lizard mainly indicates activation for cell proliferation

AbstractImmunodetection of High Mobility Group Proteins (HMGs) in the regenerating tail of lizard indicates activation for cell proliferation. Acta Zoologica (Stockolm). High Mobility Group Proteins (HMGs) are involved in chromatin assembling and control of transcription, especially during development. Transcriptome data indicate that HMGs are abundantly expressed in the early regenerating tail of lizards but their cellular localization remains unknown. Protein bands at 60–62 and 28–30 kDa are detected in western blots, more intense in the regenerating blastema. Immunodetection of HMGs in regenerating tail of the lizard Podarcis muralis indicates that these proteins are mainly localized in tissues where cell proliferation is as active as the apical wound epidermis, ependyma of the spinal cord and pro‐muscle aggregates. Few immunolabelled cells were seen in the regenerating cartilage and growing myomeres or nerves. Only in the wound epidermis labelled cells show a prevalent nuclear labelling while in other tissues a cytoplasmic labelling is prevalent. Only sparse immunolabelled cells are observed in the apical mesenchymal blastema and in the derived connective tissues formed in the mature regions of the regenerating tail, also destined to form adipose cells. The study suggests that the tissues with the highest immunolabelling correspond to those containing more proliferating cells, and that HMGs are mainly activated in these cells to promote cell division for the growth of the new tail.

Ultrastructural analysis of early regenerating lizard tail suggests that a process of dedifferentiation is involved in the formation of the regenerative blastema.

The formation of the regenerating tail blastema of lizards occurs by the multiplication of stem cells but also some dedifferentiation from adult cells may take place after tail loss by autotomy, as it is suggested in the present study. Using 5BrdU-immunocytochemistry and transmission electron microscopy it is shown that part of the damaged tissues undergo progressive cytological de-differentiation (cell reprogramming). This occurs for muscle, fibrocytes, chondrocytes, adipocytes, and cells derived from the spinal cord during the initial 3-8 days post-autotomy of the tail in the wall lizard Podarcis muralis. Dedifferentiating cells loose most endoplasmic reticulum, sarcomeres in myocells, lipid droplets in adipocytes, extracellular matrix in chondrocytes. Numerous cytoplasmic vesicles are formed, perhaps reflecting an initial sufferance of dedifferentiating cells. These cells are not dying because they incorporate 5BrdU and proliferate. Nuclei of small fibrocytes present in the dermis and inter-muscle connective tissues, initially heterochromatic, become euchromatic and their cytoplasm increases in volume although the endoplasmic reticulum remains limited, as it is typical for mesenchymal cells. The present study, supported by previous transcriptome and 5BrdU-labeling data, and from recent tracing studies, suggests that aside stem cells present in different tissues of the tail, also cell dedifferentiation occurs in the injured tail of lizards. The relative contribution between de-differentiation and stem cells for the formation of the regenerating lizard blastema likely depends from the extension of the trauma.

Temporal distribution of 5BrdU‐labelled cells suggests that most injured tissues contribute proliferating cells for the regeneration of the tail and limb in lizard

AbstractAfter tail and limb amputation in lizard, injection of 5BrdU for 6 days produces immunolabelled cells in most tissues of tail and limb stumps. After further 8 and 16 days, and 14 and 22 days of regeneration, numerous 5BrdU‐labelled cells are detected in regenerating tail and limb, derived from most stump tissues. In tail blastema cone at 14 days, sparse‐labelled cells remain in proximal dermis, muscles, cartilaginous tube and external layers of wound epidermis but are numerous in the blastema. In apical regions at 22 days of regeneration, labelled mesenchymal cells are sparse, while the apical wound epidermis contains numerous labelled cells in suprabasal and external layers, indicating cell accumulation from more proximal epidermis. Cell proliferation dilutes the label, and keratinocytes take 8 days to migrate into corneous layers. In healing limbs, labelled cells remain sparse from 14 to 22 days of regeneration in wound epidermis and repairing tissues and little labelling dilution occurs indicating low cell proliferation for local tissue repair but not distal growth. Labelled cells are present in epidermis, intermuscle and peri‐nerve connectives, bone periosteum, cartilaginous callus and sparse fibroblasts, leading to the formation of a scarring outgrowth. Resident stem cells and dedifferentiation occur when stump tissues are damaged.

Development of unidentified dna-specific hif 1α gene of lizard (hemidactylus platyurus) which plays a role in tissue regeneration process

Development of unidentified specific gene is essential to analyze the availability these genes in biological process. Identification unidentified specific DNA of HIF 1α genes is important to analyze their contribution in tissue regeneration process in lizard tail (Hemidactylus platyurus). Bioinformatics and PCR techniques are relatively an easier method to identify an unidentified gene. The most widely used method is BLAST (Basic Local Alignment Sequence Tools) method for alignment the sequences from the other organism. BLAST technique is online software from website https://blast.ncbi.nlm.nih.gov/Blast.cgi that capable to generate the similar sequences from closest kinship to distant kindship. Gecko japonicus is a species that it has closest kinship with H. platyurus. Comparing HIF 1 α gene sequence of G. japonicus with the other species used multiple alignment methods from Mega7 software. Conserved base areas were identified using Clustal IX method. Primary DNA of HIF 1 α gene was design by Primer3 software. HIF 1α gene of lizard (H. platyurus) was successfully amplified using a real-time PCR machine by primary DNA that we had designed from Gecko japonicus. Identification unidentified gene of HIF 1a lizard has been done successfully with multiple alignment method. The study was conducted by analyzing during the growth of tail on day 1, 3, 5, 7, 10, 13 and 17 of lizard tail after autotomy. Process amplification of HIF 1α gene was described by CT value in real time PCR machine. HIF 1α expression of gene is quantified by Livak formula. Chi-square statistic test is 0.000 which means that there is a different expression of HIF 1 α gene in every growth day treatment.

Protein expression pattern and analysis of differentially expressed peptides during various stages of tail regeneration in Hemidactylus flaviviridis

Epimorphic regeneration is a process allowing the animal to regain its lost structure which depends on the resident pluripotent stem cells as well as de-differentiation of existing cells to form multi-potent stem cells. Many studies have been done to understand the appendage regeneration mechanism. The animal model used since decades is an urodele amphibian the axolotl. However, this ability is also seen in some members of reptiles, mainly lizards which on autotomy of tail regain the same by forming a replica of its lost tail. Lizards being closer to mammals are of greater interest and cannot be neglected. Hence, a stage specific protein profiling was undertaken in order to find the peptides playing a major role in epimorphosis. 2-DGE being basic tool for creating a protein profile was used. With advent of newer modern technology, label-free analysis which uses MS/MS was also performed. The study reports the peptides involved in apoptosis, inflammation and ECM remodelling across the stages of lizard tail regeneration for the first time. Apart from these peptides, structural protein, enzymes involved in metabolism have also been highlighted in the current study to give a bigger picture of the processes and the specific peptides required for tail regeneration.