Appendage regeneration in anamniotes utilizes genes active during larval-metamorphic stages that have been lost or altered in amniotes: The case for studying lizard tail regeneration

Progressive modifications during evolution involving epigenetic changes have determined loss of regeneration mainly in terrestrial animals: A hypothesis

In order to address a biological explanation for the different regenerative abilities present among animals, a new evolutionary speculation is presented. It is hypothesized that epigenetic mechanisms have lowered or erased regeneration during the evolution of terrestrial invertebrates and vertebrates. The hypothesis indicates that a broad regeneration can only occur in marine or freshwater conditions, and that life on land does not allow for high regeneration. This is due to the physical, chemical and microbial conditions present in the terrestrial environment with respect to those of the aquatic environment. The present speculation provides examples of hypothetic evolutionary animal lineages that colonized the land, such as parasitic annelids, terrestrial mollusks, arthropods and amniotes. These are the animals where regeneration is limited or absent and their injuries are only repaired through limited healing or scarring. It is submitted that this loss derived from changes in the developmental gene pathways sustaining regeneration in the aquatic environment but that cannot be expressed on land. Once regeneration was erased in terrestrial species, re-adaptation to freshwater niches could not reactivate the previously altered gene pathways that determined regeneration. Therefore a broad regeneration was no longer possible or became limited and heteromorphic in the derived, extant animals. Only in few cases extensive healing abilities or regengrow, a healing process where regeneration overlaps with somatic growth, have evolved among arthropods and amniotes. The present paper is an extension of previous speculations trying to explain in biological terms the different regenerative abilities present among metazoans.

Loss of the ability to regenerate body appendages in vertebrates: from side effects of evolutionary innovations to gene loss

The ability to regenerate large body appendages is an ancestral trait of vertebrates, which varies across different animal groups. While anamniotes (fish and amphibians) commonly possess this ability, it is notably restricted in amniotes (reptiles, birds, and mammals). In this review, we explore the factors contributing to the loss of regenerative capabilities in amniotes. First, we analyse the potential negative impacts on appendage regeneration caused by four evolutionary innovations: advanced immunity, skin keratinization, whole-body endothermy, and increased body size. These innovations emerged as amniotes transitioned to terrestrial habitats and were correlated with a decline in regeneration capability. Second, we examine the role played by the loss of regeneration-related enhancers and genes initiated by these innovations in the fixation of an inability to regenerate body appendages at the genomic level. We propose that following the cessation of regenerative capacity, the loss of highly specific regeneration enhancers could represent an evolutionarily neutral event. Consequently, the loss of such enhancers might promptly follow the suppression of regeneration as a side effect of evolutionary innovations. By contrast, the loss of regeneration-related genes, due to their pleiotropic functions, would only take place if such loss was accompanied by additional evolutionary innovations that compensated for the loss of pleiotropic functions unrelated to regeneration, which would remain even after participation of these genes in regeneration was lost. Through a review of the literature, we provide evidence that, in many cases, the loss in amniotes of genes associated with body appendage regeneration in anamniotes was significantly delayed relative to the time when regenerative capability was lost. We hypothesise that this delay may be attributed to the necessity for evolutionary restructuring of developmental mechanisms to create conditions where the loss of these genes was a beneficial innovation for the organism. Experimental investigation of the downregulation of genes involved in the regeneration of body appendages in anamniotes but absent in amniotes offers a promising avenue to uncover evolutionary innovations that emerged from the loss of these genes. We propose that the vast majority of regeneration-related genes lost in amniotes (about 150 in humans) may be involved in regulating the early stages of limb and tail regeneration in anamniotes. Disruption of this stage, rather than the late stage, may not interfere with the mechanisms of limb and tail bud development during embryogenesis, as these mechanisms share similarities with those operating in the late stage of regeneration. Consequently, the most promising approach to restoring regeneration in humans may involve creating analogs of embryonic limb buds using stem cell-based tissue-engineering methods, followed by their transfer to the amputation stump. Due to the loss of many genes required specifically during the early stage of regeneration, this approach may be more effective than attempting to induce both early and late stages of regeneration directly in the stump itself.

Putative epithelial-mesenchymal transitions during salamander limb regeneration: Current perspectives and future investigations

Previous studies have implicated epithelial-mesenchymal transition (EMT) in salamander limb regeneration. In this review, we describe putative roles for EMT during each stage of limb regeneration in axolotls and other salamanders. We hypothesize that EMT and EMT-like gene expression programs may regulate three main cellular processes during limb regeneration: (1) keratinocyte migration during wound closure; (2) transient invasion of the stump by epithelial cells undergoing EMT; and (3) use of EMT-like programs by non-epithelial blastemal progenitor cells to escape the confines of their niches. Finally, we propose nontraditional roles for EMT during limb regeneration that warrant further investigation, including alternative EMT regulators, stem cell activation, and fibrosis induced by aberrant EMT.

Mechanisms and translational applications of regeneration in limbs: From renewable animals to humans

BACKGROUND

The regenerative capacity of organisms declines throughout evolution, and mammals lack the ability to regenerate limbs after injury. Past approaches to achieving successful restoration through pharmacological intervention, tissue engineering, and cell therapies have faced significant challenges.

OBJECTIVES

This review aims to provide an overview of the current understanding of the mechanisms behind animal limb regeneration and the successful translation of these mechanisms for human tissue regeneration.

RESULTS

Particular attention was paid to the Mexican axolotl (Ambystoma mexicanum), the only adult tetrapod capable of limb regeneration. We will explore fundamental questions surrounding limb regeneration, such as how amputation initiates regeneration, how the limb knows when to stop and which parts to regenerate, and how these findings can apply to mammalian systems.

CONCLUSIONS

Given the urgent need for regenerative therapies to treat conditions like diabetic foot ulcers and trauma survivors, this review provides valuable insights and ideas for researchers, clinicians, and biomedical engineers seeking to facilitate the regeneration process or elicit full regeneration from partial regeneration events.

Dynamic Alternative Polyadenylation during Litopenaeus Vannamei Metamorphosis Development

As an important mechanism in the post-transcriptional regulation of eukaryotic gene expression, alternative polyadenylation (APA) plays a key role in biological processes such as cell proliferation and differentiation. However, the role and dynamic pattern of APA during Litopenaeus vannamei metamorphosis are poorly understood. Here, RNA-seq data covering from the embryo to the maturation (16 time points) of L. vannamei were utilized. We identified 247 differentially expressed APA events between early and adult stages, and through fuzzy mean clustering analysis, we discovered five dynamic APA patterns. Among them, the gradual elongation of the 3'UTR is the major APA pattern that changes over time, and its genes are enriched in the pathways of protein and energy metabolism. Finally, we constructed mRNA-miRNA and PPI networks and detected several central miRNAs that may regulate L. vannamei development. Our results revealed the complex APA mechanisms in L. vannamei metamorphosis, shedding new light on post-transcriptional regulation of crustacean metamorphosis.

Multiple factors influence claw characteristics in oribatid mites (Acari)

Claws, as nature's multifaceted instruments, play fundamental roles across the animal kingdom, aiding in prey capture and enabling movement across diverse terrains. Claw features often reflect the ecologies of the respective taxa and thus can provide important insights into the different lifestyles. This study explores the claw morphology of monodactylous oribatid mites through geometric morphometrics, analyzing 559 specimens from 49 species across various ecosystems. The research identifies distinct claw characteristics associated with specific habitats, revealing a significant correlation between claw morphology and the mites' environmental adaptations. Littoral mites exhibit notably larger claws compared to terrestrial counterparts, with aquatic and semiaquatic species presenting intermediate traits. The analysis shows an inverse relationship between claw curvature and sharpness, differing from patterns observed in larger animals. A trend of increasing claw bluntness with body size in terrestrial mites echoes biomechanical constraints seen in larger species. The study also observes consistent claw shapes within oribatid superfamilies, suggesting a potential, albeit muted, phylogenetic influence alongside environmental factors. These findings reveal how ecological, evolutionary, and functional aspects influence claw morphology in oribatid mites, enhancing our knowledge of arthropod biology and potentially inspiring biomimetic advances in material science and engineering.

Regeneration Abilities among Extant Animals Depend on Their Evolutionary History and Life Cycles

The present brief manuscript summarizes the main points supporting recently proposed hypotheses explaining the different distributions of regenerative capacity among invertebrates and vertebrates. The new hypotheses are based on the evolution of regeneration from marine animals to the terrestrial animals derived from them. These speculations suggest that animals that were initially capable of broad regeneration in the sea underwent epigenetic modifications during terrestrial adaptation that determined the loss of their regenerative abilities in sub-aerial conditions. These changes derived from the requirements of life on land that include variable dry and UV-exposed conditions. Terrestrial conditions do not allow for organ regeneration, especially in arthropods and amniotes. Nematodes, the other main metazoan group unable of regeneration, instead evolved eutely (a fixed number of body cells), a process which is incompatible with regeneration. All these changes involved gene loss, modification and new gene interactions within the genomes of terrestrial adapting animals that gave rise to sophisticated invertebrates and vertebrates adapted to living on land but with low cellular plasticity.

Single-cell analysis of lizard blastema fibroblasts reveals phagocyte-dependent activation of Hedgehog-responsive chondrogenesis

Lizards cannot naturally regenerate limbs but are the closest known relatives of mammals capable of epimorphic tail regrowth. However, the mechanisms regulating lizard blastema formation and chondrogenesis remain unclear. Here, single-cell RNA sequencing analysis of regenerating lizard tails identifies fibroblast and phagocyte populations linked to cartilage formation. Pseudotime trajectory analyses suggest spp1+-activated fibroblasts as blastema cell sources, with subsets exhibiting sulf1 expression and chondrogenic potential. Tail blastema, but not limb, fibroblasts express sulf1 and form cartilage under Hedgehog signaling regulation. Depletion of phagocytes inhibits blastema formation, but treatment with pericytic phagocyte-conditioned media rescues blastema chondrogenesis and cartilage formation in amputated limbs. The results indicate a hierarchy of phagocyte-induced fibroblast gene activations during lizard blastema formation, culminating in sulf1+ pro-chondrogenic populations singularly responsive to Hedgehog signaling. These properties distinguish lizard blastema cells from homeostatic and injury-stimulated fibroblasts and indicate potential actionable targets for inducing regeneration in other species, including humans.

Regeneration among animals: An evolutionary hypothesis related to aquatic versus terrestrial environment

The present hypothesis tries to explain animal regeneration in relation to the life cycles and environment of different animals. Regeneration is a basic phenomenon present since the origin of life in the sea, as testimonial in lower or more complex extant marine animals. Aquatic animals that evolved an indirect development, forming larvae and transiting into the adult stage through metamorphosis, use gene networks present in their genome for these transformations. In case of injury or organ loss as adults, they can re-utilize most or part of the gene networks previously activated during larval growth and metamorphosis. In contrast, terrestrial animals that evolved life cycles with the elimination of larvae and metamorphosis for the adaptation to land conditions lost some of the genes implicated in these post-developmental processes and consequently also the ability to regenerate. Few arthropods and lizards are capable to form hydrated regenerative blastemas with a similar consistence of embryonic tissues. The present hypothesis submits that regeneration cannot be activated in the dry land environment and consequently was largely or completely abolished in terrestrial animals. After injury or organ loss, nematodes, most arthropods and terrestrial vertebrates can only form scars or a limited healing or regengrow in juveniles. This is a process where somatic growth is superimposed to wound healing so that the apparent regeneration derives from the combination from both processes. When full growth is terminated these terrestrial animals can only heal by scarring.

Regeneration or Scarring Derive from Specific Evolutionary Environmental Adaptations of the Life Cycles in Different Animals

The ability to heal or even regenerate large injuries in different animals derives from the evolution of their specific life cycles during geological times. The present, new hypothesis tries to explain the distribution of organ regeneration among animals. Only invertebrates and vertebrates that include larval and intense metamorphic transformations can broadly regenerate as adults. Basically, regeneration competent animals are aquatic while terrestrial species have largely or completely lost most of the regeneration ability. Although genomes of terrestrial species still contain numerous genes that in aquatic species allow a broad regeneration ("regenerative genes"), the evolution of terrestrial species has variably modified the genetic networks linking these genes to the others that evolved during land adaptation, resulting in the inhibition of regeneration. Loss of regeneration took place by the elimination of intermediate larval phases and metamorphic transformations in the life cycles of land invertebrates and vertebrates. Once the evolution along a specific lineage generated species that could no longer regenerate, this outcome could not change anymore. It is therefore likely that what we learn from regenerative species will explain their mechanisms of regeneration but cannot or only partly be applied to non-regenerative species. Attempts to introduce "regenerative genes" in non-regenerative species most likely would disorder the entire genetic networks of the latter, determining death, teratomas and cancer. This awareness indicates the difficulty to introduce regenerative genes and their activation pathways in species that evolved genetic networks suppressing organ regeneration. Organ regeneration in non-regenerating animals such as humans should move to bio-engineering interventions in addition to "localized regenerative gene therapies" in order to replace lost tissues or organs.

Activation of cell adhesion molecules and Snail during epithelial to mesenchymal transition prior to formation of the regenerative tail blastema in lizards

After few days from tail amputation in lizards the stump is covered with mesenchymal cells accumulated underneath a wound epidermis and forms a regenerative blastema. During migration, some keratinocytes transit from a compact epidermis into relatively free keratinocytes in a process of "epithelial to mesenchymal transition" (EMT). EMT is also induced after damaging the regenerating epidermis by cauterization, whereas keratinocytes detach and migrate as mesenchymal-like cells among the superficial blastema cells and reconstruct a wound epidermis after about a week from the damage. In normal amputation or after cauterization, no malignant transformation is observed during the transition and migration of keratinocytes. Immunolabeling for markers of EMT confirms the histological description and shows a unique pattern of expression for l-CAM (E-cadherin), N-CAM, and SNAIL-1 and -2 (SLUG). These proteins are present in the cytoplasm and nuclei of migrating keratinocytes. It is hypothesized that the nuclear labeling for E-cadherin coupled to cytoplasmic SNAIL-labeling is somehow related to an initially regulated EMT. After the migrating keratinocytes have reached confluence over the stump, they reverse into a "mesenchymal to epithelial transition" (MET) forming the wound epidermis. The basal layers of the apical wound epidermis of the blastema show some nuclear E-cadherin labeling, while the tail regenerates. It is hypothesized that, together with other tumor suppressors proteins, the apical epidermis and mesenchyme are kept under a tight proliferative control, while in proximal regions the prevalent effect of tumor suppressors determine the differentiation of the new tail tissues.

Immunolocalization of tumor suppressors arhgap28 and retinoblastoma in the lizard Podarcis muralis suggests that they contribute to the regulated regeneration of the tail

Tail regeneration in lizards is an outstanding and unique postembryonic morphogenetic process. This developmental process is regulated by poorly known factors, but recent studies have suggested that it derives from a balanced activity between oncoproteins and tumor suppressors. Transcriptome and expression data have indicated that arhgap28 and retinoblastoma proteins are among the main tumor suppressors activated during tail regeneration. However, their cellular localization is not known. Therefore, in the present immunohistochemical study, two proteins have been detected in various tissues at the beginning of their differentiation. Both proteins are present especially in the new scales, axial cartilage, and muscle bundles of the regenerating tail, the main tissues forming the new tail. Sparse or occasionally labeled cells are observed in the blastema, but intense labeling is seen in the basal layers of the wound (regenerating) epidermis and in external differentiating epidermal layers. Numerous keratinocytes also show a nuclear localization for both proteins, suggesting that the latter may activate a gene program for tissue differentiation after the inhibition of cell multiplication. Based on microscopic, molecular, experimental, and in vitro studies, a hypothesis on the "inhibition of contact" among the apical cells of the blastema and those of proximal differentiating tissues is proposed to explain the permanence of an active blastema only at the apex of the regenerating tail without tail growth can degenerate into a tumorigenic outgrowth.

Introduction to the Study on Regeneration in Lizards as an Amniote Model of Organ Regeneration

Initial observations on the regeneration of the tail in lizards were recorded in brief notes by Aristotle over 2000 years ago, as reported in his book, History of Animals (cited from [...].

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Review. Limb regeneration in lizards under natural and experimental conditions with considerations on the induction of appendages regeneration in amniotes

BACKGROUND

Study on the failure of limb regeneration in lizards evidences the difficult problems met from amniotes to regenerate organs. Contrary to the tail, limb loss in terrestrial environment is generally fatal and no selection for its regeneration occurred during lizard evolution.

METHODS

Experimentally amputated limbs were fixed and embedded for microscopy.

RESULTS

After limb loss an intense inflammatory reaction occurs and immune cells are recruited underneath a wound epidermis, forming a vascularized granulation tissue. The regenerating epidermis takes 2-3 weeks to cover the limb stump since degenerating long bones must be excised first while a dense connective tissue is formed and no limb growth occurs. Cell proliferation occurs in granulation tissues and wound epidermis during the initial 2-3 weeks of wound healing but disappears later determining the arrest of growth. Transcriptome data indicates that the limb, contrary to the tail, activates numerous genes involved in inflammation, immunity and fibroplasia while down-regulates some proliferative and most myogenic genes. Attempts to stimulate limb regeneration, by implants of nervous tissues or growth factors such as FGFs only maintain proliferation for few weeks but eventually the scarring program prevails and only short outgrowths missing of autopodial elements are regenerated.

CONCLUSIONS

While lizard limbs show the typical scarring outcome of mammals, the comparison of genes activated in the regenerating tail has allowed identifying key genes implicated in organ regeneration in amniotes.

Growth associated protein 43 and neurofilament immunolabeling in the transected lumbar spinal cord of lizard indicates limited axonal regeneration

Previous cytological studies on the transected lumbar spinal cord of lizards have shown the presence of differentiating glial cells, few neurons and axons in the bridge region between the proximal and distal stumps of the spinal cord in some cases. A limited number of axons (20–50) can cross the bridge and re-connect the caudal stump of the spinal cord with small neurons located in the rostral stump of the spinal cord. This axonal regeneration appears to be related to the recovery of hind-limb movements after initial paralysis. The present study extends previous studies and shows that after transection of the lumbar spinal cord in lizards, a glial-connective tissue bridge that reconnects the rostral and caudal stumps of the interrupted spinal cord is formed at 11–34 days post-injury. Following an initial paralysis some recovery of hindlimb movements occurs within 1–3 months post-injury. Immunohistochemical and ultrastructural analysis for a growth associated protein 43 (GAP-43) of 48–50 kDa shows that sparse GAP-43 positive axons are present in the proximal stump of the spinal cord but their number decreased in the bridge at 11–34 days post-transection. Few immunolabeled axons with a neurofilament protein of 200–220 kDa were seen in the bridge at 11–22 days post-transection but their number increased at 34 days and 3 months post-amputation in lizards that have recovered some hindlimb movements. Numerous neurons in the rostral and caudal stumps of the spinal cord were also labeled for GAP43, a cytoplasmic protein that is trans-located into their axonal growth cones. This indicates that GAP-43 biosynthesis is related to axonal regeneration and sprouting from neurons that were damaged by the transection. Taken together, previous studies that utilized tract-tracing technique to label the present observations confirm that a limited axonal re-connection of the transected spinal cord occurs 1–3 months post-injury in lizards. The few regenerating-sprouting axons within the bridge reconnect the caudal with the rostral stumps of the spinal cord, and likely contribute to activate the neural circuits that sustain the limited but important recovery of hind-limb movements after initial paralysis. The surgical procedures utilized in the study followed the regulations on animal care and experimental procedures under the Italian Guidelines (art. 5, DL 116/92).

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.

An introduction to an evolutionary tail: EvoDevo, structure and function of post-anal appendages

Although tails are common and versatile appendages that contribute to evolutionary success of animals in a broad range of ways, a scientific synthesis on the topic had yet to be initiated. For our Society for Integrative and Comparative Biology (SICB) symposium we brought together researchers from different areas of expertise (e.g., robotosists, biomechanists, functional morphologists, and evolutionary and developmental biologists), to highlight their research but also to emphasize the interdisciplinary nature of this topic. The four main themes that emerged based on the research presented in this symposium are: 1) How do we define a tail? 2) Development and regeneration inform evolutionary origins of tails, 3) Identifying key characteristics highlights functional morphology of tails, 4) Tail multi-functionality leads to the development of bioinspired technology. We discuss the research provided within this symposium, in light of these four themes. We showcase the broad diversity of current tail research and lay an important foundational framework for future interdisciplinary research on tails with this timely symposium.

Myeloid cell diversification during regenerative inflammation: Lessons from skeletal muscle

Understanding the mechanisms of tissue and organ regeneration in adult animals and humans is of great interest from a basic biology as well as a medical, therapeutical point of view. It is increasingly clear that the relatively limited ability to regenerate tissues and organs in mammals as oppose to lower vertebrates is the consequence of evolutionary trade-offs and changes during development and aging. Thus, the coordinated interaction of the immune system, particularly the innate part of it, and the injured, degenerated parenchymal tissues such as skeletal muscle, liver, lung, or kidney shape physiological and also pathological processes. In this review, we provide an overview of how morphologically and functionally complete (ad integrum) regeneration is achieved using skeletal muscle as a model. We will review recent advances about the differentiation, activation, and subtype specification of circulating monocyte to resolution or repair-type macrophages during the process we term regenerative inflammation, resulting in complete restoration of skeletal muscle in murine models of toxin-induced injury.

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.

From Autonomy to Integration, From Integration to Dynamically Balanced Integrated Co-existence: Non-aging as the Third Stage of Development

Reversible senescence at the cellular level emerged together with tissue specialization in Metazoans. However, this reversibility (ability to permanently rejuvenate) through recapitulation of early stages of development, was originally a part of ontogenesis, since the pressure of integrativeness was not dominant. The complication of specialization in phylogenesis narrowed this "freedom of maneuver", gradually "truncating" remorphogenesis to local epimorphosis and further up to the complete disappearance of remorphogenesis from the ontogenesis repertoire. This evolutionary trend transformed cellular senescence into organismal aging and any recapitulation of autonomy into carcinogenesis. The crown of specialization, Homo sapiens, completed this post-unicellular stage of development, while in the genome all the potential for the next stage of development, which can be called the stage of balanced coexistence of autonomous and integrative dominants within a single whole. Here, completing the substantiation of the new section of developmental biology, we propose to call it Developmental Biogerontology.

Appendage Regeneration in Vertebrates: What Makes This Possible?

The ability to regenerate amputated or injured tissues and organs is a fascinating property shared by several invertebrates and, interestingly, some vertebrates. The mechanism of evolutionary loss of regeneration in mammals is not understood, yet from the biomedical and clinical point of view, it would be very beneficial to be able, at least partially, to restore that capability. The current availability of new experimental tools, facilitating the comparative study of models with high regenerative ability, provides a powerful instrument to unveil what is needed for a successful regeneration. The present review provides an updated overview of multiple aspects of appendage regeneration in three vertebrates: lizard, salamander, and zebrafish. The deep investigation of this process points to common mechanisms, including the relevance of Wnt/β-catenin and FGF signaling for the restoration of a functional appendage. We discuss the formation and cellular origin of the blastema and the identification of epigenetic and cellular changes and molecular pathways shared by vertebrates capable of regeneration. Understanding the similarities, being aware of the differences of the processes, during lizard, salamander, and zebrafish regeneration can provide a useful guide for supporting effective regenerative strategies in mammals.

Study of Natural Longlife Juvenility and Tissue Regeneration in Caudate Amphibians and Potential Application of Resulting Data in Biomedicine

The review considers the molecular, cellular, organismal, and ontogenetic properties of Urodela that exhibit the highest regenerative abilities among tetrapods. The genome specifics and the expression of genes associated with cell plasticity are analyzed. The simplification of tissue structure is shown using the examples of the sensory retina and brain in mature Urodela. Cells of these and some other tissues are ready to initiate proliferation and manifest the plasticity of their phenotype as well as the correct integration into the pre-existing or de novo forming tissue structure. Without excluding other factors that determine regeneration, the pedomorphosis and juvenile properties, identified on different levels of Urodele amphibians, are assumed to be the main explanation for their high regenerative abilities. These properties, being fundamental for tissue regeneration, have been lost by amniotes. Experiments aimed at mammalian cell rejuvenation currently use various approaches. They include, in particular, methods that use secretomes from regenerating tissues of caudate amphibians and fish for inducing regenerative responses of cells. Such an approach, along with those developed on the basis of knowledge about the molecular and genetic nature and age dependence of regeneration, may become one more step in the development of regenerative medicine.