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Axolotls continue to grow after sexual maturity. Live images of a hatchling axolotl (∼3 weeks old), young juvenile (∼3 months old), late juvenile (∼5 months old), sexually mature adult (∼1 year old), and a 3-year-old adult show the dramatic increase in size over time.
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Loss of regenerative capacity is a normal part of aging. However, some organisms, such as the Mexican axolotl, retain striking regenerative capacity throughout their lives. Moreover, the development of age-related diseases is rare in this organism. In this review, we will explore how axolotls are used as a model system to study regenerative process...
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Context 1
... maximum life expectancy of an axolotl is estimated to be up to 25 years in captivity. Despite the lack of age-related research in this species, there are observable changes in the body structure of the animal with time. One of these changes is size; the axolotl is an indefinitely growing animal, continuing to increase in size throughout its life (Fig. 1). Another is in the composition of its skeleton; during the larval stages of life, the axolotl skeleton is highly cartilaginous, and this cartilage is replaced by bone as the animal ages. Changes in tissue composition are also evident, such as the limb dermal layer thickening as the animal ages (Fig. 2). Behavior also changes with age. ...
Context 2
... a number of factors. (1) The rate of regeneration slows as the axolotl increases in age, from weeks in larval animals to months in sexually mature adults. This may be a consequence of the animal's size, with smaller animals having less extremity girth and mass relative to adult animals leading to faster wound closure and tissue restoration rates (Fig. 1). (2) With age, the skin of the axolotl thickens and loses flexibility, possibly making it more difficult to form a wound epithelium, as well as increase the likelihood of generating insufficient positional interactions for the generation of a complete limb structure at the site of injury [19]. Additional factors, such as circulating ...
Citations
... While humans struggle with limited neuro-repair, some organisms have remarkable neuro-regenerative abilities. One of these organisms is known as the axolotl, a type of aquatic salamander native to Mexico which is famous for its impressive regenerative capabilities [2][3][4]. Axolotls can regenerate entire limbs, including the nerves, spinal cord, and other tissues [5]. One of the most striking features of axolotls is their capacity to regenerate portions of the spinal cord. ...
Despite significant improvements in the comprehension of neuro-regeneration, restoring nerve injury in humans continues to pose a substantial therapeutic difficulty. In the peripheral nervous system (PNS), the nerve regeneration process after injury relies on Schwann cells. These cells play a crucial role in regulating and releasing different extracellular matrix proteins, including laminin and fibronectin, which are essential for facilitating nerve regeneration. However, during regeneration, the nerve is required to regenerate for a long distance and, subsequently, loses its capacity to facilitate regeneration during this progression. Meanwhile, it has been noted that nerve regeneration has limited capabilities in the central nervous system (CNS) compared to in the PNS. The CNS contains factors that impede the regeneration of axons following injury to the axons. The presence of glial scar formation results from this unfavourable condition, where glial cells accumulate at the injury site, generating a physical and chemical barrier that hinders the regeneration of neurons. In contrast to humans, several species, such as axolotls, polychaetes, and planarians, possess the ability to regenerate their neural systems following amputation. This ability is based on the vast amount of pluripotent stem cells that have the remarkable capacity to differentiate and develop into any cell within their body. Although humans also possess these cells, their numbers are extremely limited. Examining the molecular pathways exhibited by these organisms has the potential to offer a foundational understanding of the human regeneration process. This review provides a concise overview of the molecular pathways involved in axolotl, polychaete, and planarian neuro-regeneration. It has the potential to offer a new perspective on therapeutic approaches for neuro-regeneration in humans.
... In contrast, lizard limb amputation results in wound healing and blastema formation but they are unable to regenerate the missing limb structure [23,24]. Animals like axolotls and salamanders can regenerate a variety of tissues and complex structures into adulthood, whereas frogs undergo regenerative and non-regenerative stages [18,25]. These are all in contrast to mammals that have a more limited regenerative capacity where they can regenerate organs like the liver and pancreas, but not the heart, CNS, or limbs. ...
... In contrast, lizard limb amputation results in wound healing and blastema form they are unable to regenerate the missing limb structure [23,24]. Animals like ax salamanders can regenerate a variety of tissues and complex structures into a whereas frogs undergo regenerative and non-regenerative stages [18,25]. Thes contrast to mammals that have a more limited regenerative capacity where th generate organs like the liver and pancreas, but not the heart, CNS, or limbs. ...
This review explores the regenerative capacity of Xenopus laevis, focusing on tail regeneration, as a model to uncover cellular, molecular, and developmental mechanisms underlying tissue repair. X. laevis tadpoles provide unique insights into regenerative biology due to their regeneration-competent and -incompetent stages and ability to regrow complex structures in the tail, including the spinal cord, muscle, and skin, after amputation. The review delves into the roles of key signaling pathways, such as those involving reactive oxygen species (ROS) and signaling molecules like BMPs and FGFs, in orchestrating cellular responses during regeneration. It also examines how mechanotransduction, epigenetic regulation, and metabolic shifts influence tissue restoration. Comparisons of regenerative capacity with other species shed light on the evolutionary loss of regenerative abilities and underscore X. laevis as an invaluable model for understanding the constraints of tissue repair in higher organisms. This comprehensive review synthesizes recent findings, suggesting future directions for exploring regeneration mechanisms, with potential implications for advancing regenerative medicine.
... Neural regeneration capacity widely varies across animal species. The regeneration of nervous systems range from the growth of innervated limbs in salamanders [1,2] or recovery from spinal cord injuries [3,4] in certain vertebrates to full nervous system regeneration in Hydra [5][6][7] or flatworms [8,9]. Mammals, on the other hand, exhibit limited regenerative abilities, along with a complex immune response that slows neural regrowth [10]. ...
Understanding how neural circuits are regenerated following injury is a fundamental question in neuroscience. Hydra is a powerful model for studying this process because it has a simple neural circuit structure, significant and reproducible regenerative abilities, and established methods for creating transgenics with cell-type-specific expression. While Hydra is a long-standing model for regeneration and development, little is known about how neural activity and behavior is restored following significant injury. In this study, we ask if regenerating neurons terminally differentiate prior to reforming functional neural circuits, or if neural circuits regenerate first and then guide the constituent naive cells toward their terminal fate. To address this question, we developed a dual-expression transgenic Hydra line that expresses a cell-type-specific red fluorescent protein (tdTomato) in ec5 peduncle neurons, and a calcium indicator (GCaMP7s) in all neurons. With this transgenic line, we can simultaneously record neural activity and track the reappearance of the terminally-differentiated ec5 neurons. Using SCAPE (Swept Confocally Aligned Planar Excitation) microscopy, we monitored both calcium activity and expression of tdTomato-positive neurons in 3D with single-cell resolution during regeneration of Hydra’s aboral end. The synchronized neural activity associated with a regenerated neural circuit was observed approximately 4 to 8 hours after expression of tdTomato in ec5 neurons. These data suggest that regenerating ec5 neurons undergo terminal differentiation prior to re-establishing their functional role in the nervous system. The combination of dynamic imaging of neural activity and gene expression during regeneration make Hydra a powerful model system for understanding the key molecular and functional processes involved in neural regeneration following injury.
... For example, adult forms of neotropical electric fishes of South America (Teleostei: Gymnotiformes), can regenerate an injured brain, injured spinal cord and amputated body parts, and can do so repeatedly, after each injury (Unguez, 2013). Among amphibians, which are evolutionarily on a higher level, axolotl Ambystoma mexicanum can regenerate many parts of its body, but it cannot reproduce asexually (Vieira et al., 2020). In reptiles, which are evolutionarily on an even higher level, the ability to regenerate is significantly reduced compared to the organisms already mentioned (Alibardi & Meyer-Rochow, 2021). ...
This new theory of aging explains that aging and death due to aging are due to five factors, and also explains how these factors are interconnected and jointly lead to aging and death of the organism, pointing to many facts that strongly support it. The first factor is the harmful changes that occur in cellular structures. The second factor is the cessation of cell division in adult organisms, which leads to the inability to restore cellular structures. The third factor is the feature that cells do not die due to the accumulation of harmful changes that occur in the cells during the life of the organism. The fourth factor is the inability of stem cells to regenerate tissue by replacing such cells with new ones, because somatic cells do not die and there are no signals that stimulate the proliferation of stem cells and their differentiation into new ones that would replace dead cells. The fifth factor is that all cells die suddenly, due to the cessation of one of the vital functions of the organism, and not gradually during life, due to a decrease in the functionality of cells caused by the introduction of harmful changes in cellular structures, which would allow stem cells to regenerate tissues and keep the body young. Also, to show that this aging theory is valid, the theory gives its view of the evolution of five factors, which according to this theory lead to aging, which gives strong support to this theory.
... The fish species present in cellular senescence studies are Nothbranchius furzeri, which has a short lifespan (~13 weeks) but is easy to reproduce and can be kept at relatively high population densities, allowing for rapid expansion, thus providing an easy and inexpensive animal model [136,158]. Among the long-lived Fishes, the Rockfishes are used because they present a variation in and a large number of species, and in their case, it was discovered that there was a positive correlation between depth and longevity, and the longest-lived species do not seem to be subject to reproductive senescence [140,159]. ...
Cellular senescence is a permanent condition of cell cycle arrest caused by a progressive shortening of telomeres defined as replicative senescence. Stem cells may also undergo an accelerated senescence response known as premature senescence, distinct from telomere shortening, as a response to different stress agents. Various treatment protocols have been developed based on epigenetic changes in Tcells throughout senescence, using different drugs and antioxidants, senolytic vaccines, or the reprogramming of somatic senescent cells using Yamanaka factors. Even with all the recent advancements, it is still unknown how different epigenetic modifications interact with genetic profiles and how other factors such as microbiota physiological conditions, psychological states, and diet influence the interaction between genetic and epigenetic pathways. The aim of this review is to highlight the new epigenetic modifications that are involved in stem cell senescence. Here, we review recent senescence-related epigenetic alterations such as DNA methylation, chromatin remodeling, histone modification, RNA modification, and non-coding RNA regulation outlining new possible targets for the therapy of aging-related diseases. The advantages and disadvantages of the animal models used in the study of cellular senescence are also briefly presented.
... Regeneration, the process of restoring the function of organs and tissues after damage, is a critical area of research in regenerative biology [1,2]. A key model organism in this field is the axolotl, known for its extraordinary regenerative abilities, particularly in limb regeneration [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. However, despite extensive in vivo studies on blastema, there is currently no established cell line that retains the essential characteristics of blastema cells for in vitro research. ...
Regenerative biology is a pivotal field aimed at understanding and harnessing the ability of organisms to restore damaged tissues and organs. The axolotl (Ambystoma mexicanum) is a key model organism renowned for its exceptional regenerative capabilities, particularly in limb regeneration. However, current in vitro research is hampered by the lack of well-defined axolotl blastema cell lines and unreliable primary culture protocols. To address these challenges, we present a novel, robust, gel-free 3D culture system for axolotl blastema cells. This system overcomes limitations of 2D culture methods by enabling the formation of spheroid structures that closely mimic the in vivo environment. Using this protocol, we observed that spheroids derived from axolotl blastema tissues retained key regenerative markers, including PRRX1, and exhibited stable expression of crucial blastema markers through extended culture periods. Our approach facilitates the study of cellular and molecular mechanisms underlying limb regeneration in axolotls and provides a valuable tool for drug testing and regenerative research. The development of this 3D culture system represents a significant advancement in regenerative biology, offering a more consistent and reliable model for exploring the regenerative potential of axolotl cells.
... Among the amphibians used in the study of cellular senescence are the neotenic salamander (Proteus anguinus) which has an average lifespan of 68 years [154], the common mudpuppy (Necturus maculosus) which has an estimated lifespan of 34 years [155] and the axolotl (Ambystoma mexicanum) which lives about 20 years [156]. These species stand out, in addition to the increased longevity in relation to their body size, by an extremely low incidence of cancer and a unique capacity for regeneration (having the ability to regenerate certain wounded tissues, the tail or even limbs) [138]. ...
Cellular senescence is a permanent condition of cell cycle arrest caused by a progressive shorten-ing of telomeres defined as replicative senescence. Stem cells may also undergo an accelerated se-nescence response, distinct from telomere shortening, known as premature senescence as a re-sponse to different stress agents. Various treatment protocols have been developed based on epi-genetic changes in cells throughout senescence, using different drugs and antioxidants, senolytic vaccines, or reprogramming of somatic senescent cells using Yamanaka factors. Even with all the recent advancements, it is still unknown how different epigenetic modifications interact with ge-netic profile and how other factors such as microbiota physiological conditions, psychological states and diet influence the interaction between genetic and epigenetic pathways. The aim of this review is to highlight the new epigenetic modifications that are involved in stem cell senescence. Here, we review recent senescence-related epigenetic alterations such as DNA methylation, chro-matin remodeling, histone modification, RNA modification, and non-coding RNA regulation outlining new possible targets for therapy of aging-related diseases. The advantages and disad-vantages of the animal models used in the study of cellular senescence are also briefly presented.
... For example, decreased keratinization was observed in the blastema of regenerating limbs of tadpoles of the frog P. maculatus, accompanied by downregulation of several cytoskeletal keratins, compared with the blastema of non-regenerating limbs of froglets of the same species (Mahapatra et al., 2023). A correlation between reduced skin elasticity and decreased regeneration effectiveness was observed between young and old axolotls (Vieira, Wells & McCusker, 2020). With age, axolotl skin thickens and loses flexibility, possibly making it more difficult to form a wound epithelium and to ensure the secretion of signalling molecules necessary for the generation of a complete limb structure at the site of injury . ...
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.
... In most organisms, the regenerative capacity is higher during early development and the turnover potential of tissues and organs decreases during ageing (Rando & Jones, 2021). However, adult individuals from some species have greater capacity to regenerate than others, and some organisms, such as teleost fish and salamanders (Mexican axolotl), which are known to possess regenerative competence even throughout adulthood (Ninov & Yun, 2015;Vieira et al., 2020), whereas frogs lose their regenerative abilities after metamorphosis (Lust & Tanaka, 2019). ...
... Regenerative growth helps in the formation of a functional-structural pattern and is stably resolved in the formation for new tissue. However, it is correlated with greater tumour progression and metastasis in tumourigenesis, resulting in a poor clinical prognosis (Vieira et al., 2020). The mechanisms that produce such different results in apparently similar environments remain unknown. ...
Homeostasis constitutes a key concept in physiology and refers to self‐regulating processes that maintain internal stability when adjusting to changing external conditions. It diminishes internal entropy constituting a driving force behind evolution. Natural selection might act on homeostatic regulatory mechanisms and control mechanisms including homeodynamics, allostasis, hormesis and homeorhesis, where different stable stationary states are reached. Regeneration is under homeostatic control through hormesis. Damage to tissues initiates a response to restore the impaired equilibrium caused by mild stress using cell proliferation, cell differentiation and cell death to recover structure and function. Repair is a homeorhetic change leading to a new stable stationary state with decreased functionality and fibrotic scarring without reconstruction of the 3‐D pattern. Mechanisms determining entrance of the tissue or organ to regeneration or repair include the balance between innate and adaptive immune cells in relation to cell plasticity and stromal stem cell responses, and redox balance. The regenerative and reparative capacities vary in different species, distinct tissues and organs, and at different stages of development including ageing. Many cell signals and pathways play crucial roles determining regeneration or repair by regulating protein synthesis, cellular growth, inflammation, proliferation, autophagy, lysosomal function, metabolism and metalloproteinase cell signalling. Attempts to favour the entrance of damaged tissues to regeneration in those with low proliferative rates have been made; however, there are evolutionary constraint mechanisms leading to poor proliferation of stem cells in unfavourable environments or tumour development. More research is required to better understand the regulatory processes of these mechanisms. image
... The importance of the immune system in regulating pro-regenerative responses in axolotl has been well documented during limb regeneration, where injury elicits an immediate wound healing response while in mammals promotes a strong innate immune response that triggers, in later phases of the injury, a fibrotic scarring program that limits the ability of the damaged tissue to regenerate [51]; additionally, several genes that contribute to the regeneration of amputated limbs in axolotls have been already identified, providing information about how regeneration could be elicited in species with limited tissue repair [13,52]. Therefore, the identification of genes and molecular programs that are specifically regulated in the injured spinal cord of the axolotl (and that are also conserved in mammals) could provide information for the study of potential regenerative therapies in humans affected by this condition. ...
Background:
Traumatic spinal cord injury (SCI) is a disabling condition that affects millions of people around the world. Currently, no clinical treatment can restore spinal cord function. Comparison of molecular responses in regenerating to non-regenerating vertebrates can shed light on neural restoration. The axolotl (Ambystoma mexicanum) is an amphibian that regenerates regions of the brain or spinal cord after damage.
Methods:
In this study, we compared the transcriptomes after SCI at acute (1-2 days after SCI) and sub-acute (6-7 days post-SCI) periods through the analysis of RNA-seq public datasets from axolotl and non-regenerating rodents.
Results:
Genes related to wound healing and immune responses were upregulated in axolotls, rats, and mice after SCI; however, the immune-related processes were more prevalent in rodents. In the acute phase of SCI in the axolotl, the molecular pathways and genes associated with early development were upregulated, while processes related to neuronal function were downregulated. Importantly, the downregulation of processes related to sensorial and motor functions was observed only in rodents. This analysis also revealed that genes related to pluripotency, cytoskeleton rearrangement, and transposable elements (e.g., Sox2, Krt5, and LOC100130764) were among the most upregulated in the axolotl. Finally, gene regulatory networks in axolotls revealed the early activation of genes related to neurogenesis, including Atf3/4 and Foxa2.
Conclusions:
Immune-related processes are upregulated shortly after SCI in axolotls and rodents; however, a strong immune response is more noticeable in rodents. Genes related to early development and neurogenesis are upregulated beginning in the acute stage of SCI in axolotls, while the loss of motor and sensory functions is detected only in rodents during the sub-acute period of SCI. The approach employed in this study might be useful for designing and establishing regenerative therapies after SCI in mammals, including humans.