Vasculature in pre-blastema and nerve-dependent blastema stages of regenerating forelimbs of the adult newt, Notophthalmus viridescens

The vascularization, innervation and myogenesis of early regenerated tail in Gekko japonicus

Many species of lizards are capable of tail regeneration. There has been increased interest in the study of lizard tail regeneration in recent years as it is an amenable regeneration model for amniotes. In this study, Gekko japonicus was used as a model to investigate the initiation of vascularization, innervation and myogenesis during tail regeneration. We found that angiogenesis and axon regeneration occurred almost simultaneously within 4 days post amputation. The results showed that the endothelial cells of the original vasculature proliferated and extended into the blastema as capillary vessels, which inter-connected to form a capillary network. The nerve fibers innervated the regenerated tissue from the original spinal cord and dorsal root ganglia, and the fiber bundles increased during 14 days. Regenerating muscle tissues emerged 2 weeks after amputation. PAX3 and PAX7 expression were detected during myogenesis, with PAX7 showing a continuous increase in expression from day 3 until the day 14, whereas PAX3 reached a peak level on day 10 day post amputation, and then declined quickly to level as normal control on day 14. PCNA and PAX3 double-positive satellite cells were observed in the original rostral tissues, indicating the involvement of satellite cell proliferation during tail regeneration. Taken together, these data suggest that tail regeneration in Gekko japonicus involved rapid angiogenesis from the beginning to the day 10 and followed by capillary remodeling. The innervation of regenerated tail was significant on day 4 and increased gradually during regeneration, while the regenerated muscle tissues was obvious on day 14 after amputation.

A comparative perspective on lung and gill regeneration

The ability to continuously grow and regenerate the gills throughout life is a remarkable property of fish and amphibians. Considering that gill regeneration was first described over one century ago, it is surprising that the underlying mechanisms of cell and tissue replacement in the gills remain poorly understood. By contrast, the mammalian lung is a largely quiescent organ in adults but is capable of facultative regeneration following injury. In the course of the past decade, it has been recognized that lungs contain a population of stem or progenitor cells with an extensive ability to restore tissue; however, despite recent advances in regenerative biology of the lung, the signaling pathways that underlie regeneration are poorly understood. In this Review, we discuss the common evolutionary and embryological origins shared by gills and mammalian lungs. These are evident in homologies in tissue structure, cell populations, cellular function and genetic pathways. An integration of the literature on gill and lung regeneration in vertebrates is presented using a comparative approach in order to outline the challenges that remain in these areas, and to highlight the importance of using aquatic vertebrates as model organisms. The study of gill regeneration in fish and amphibians, which have a high regenerative potential and for which genetic tools are widely available, represents a unique opportunity to uncover common signaling mechanisms that may be important for regeneration of respiratory organs in all vertebrates. This may lead to new advances in tissue repair following lung disease.

Nerves in cancer

The contribution of nerves to the pathogenesis of malignancies has emerged as an important component of the tumour microenvironment. Recent studies have shown that peripheral nerves (sympathetic, parasympathetic and sensory) interact with tumour and stromal cells to promote the initiation and progression of a variety of solid and haematological malignancies. Furthermore, new evidence suggests that cancers may reactivate nerve-dependent developmental and regenerative processes to promote their growth and survival. Here we review emerging concepts and discuss the therapeutic implications of manipulating nerves and neural signalling for the prevention and treatment of cancer.

Evolution of epimorphosis in mammals

Mammalian epimorphic regeneration is rare and digit tip regeneration in mice is the best-studied model for a multi-tissue regenerative event that involves blastema formation. Digit tip regeneration parallels human fingertip regeneration, thus understanding the details of this response can provide insight into developing strategies to expand the potential of human regeneration. Following amputation, the digit stump undergoes a strong histolytic response involving osteoclast-mediated bone degradation that is spatially and temporally linked to the expansion of blastema osteoprogenitor cells. Blastemal differentiation occurs via direct intramembranous ossification. Although robust, digit regeneration is imperfect: The amputated cortical bone is replaced with woven bone and there is excessive bone regeneration restricted to the dorsal-ventral axis. Ontogenetic and phylogenetic analysis of digit regeneration in amphibians and mammals raise the possibility that mammalian blastema is a product of convergent evolution and we hypothesize that digit tip regeneration evolved from a nonregenerative precondition. A model is proposed in which the mammalian blastema evolved in part from an adaptation of two bone repair strategies (the bone remodeling cycle and fracture healing) both of which are conserved across tetrapod vertebrates. The view that epimorphic regeneration evolved in mammals from a nonregenerative precondition is supported by recent studies demonstrating that complex regenerative responses can be induced from a number of different nonregenerative amputation wounds by specific modification of the healing response.

Common themes in tetrapod appendage regeneration: a cellular perspective

Complete and perfect regeneration of appendages is a process that has fascinated and perplexed biologists for centuries. Some tetrapods possess amazing regenerative abilities, but the regenerative abilities of others are exceedingly limited. The reasons underlying these differences have largely remained mysterious. A great deal has been learned about the morphological events that accompany successful appendage regeneration, and a handful of experimental manipulations can be reliably applied to block the process. However, only in the last decade has the goal of attaining a thorough molecular and cellular biological understanding of appendage regeneration in tetrapods become within reach. Advances in molecular and genetic tools for interrogating these remarkable events are now allowing for unprecedented access to the fundamental biology at work in appendage regeneration in a variety of species. This information will be critical for integrating the large body of detailed observations from previous centuries with a modern understanding of how cells sense and respond to severe injury and loss of body parts. Understanding commonalities between regenerative modes across diverse species is likely to illuminate the most important aspects of complex tissue regeneration.

Inhibition of Vascular Endothelial Growth Factor Receptor Decreases Regenerative Angiogenesis in Axolotls

Angiogenesis is crucial for tissue growth and repair in mammals, and is chiefly regulated by vascular endothelial growth factor (VEGF) signaling. We evaluated the effect of chemical inhibition of VEGF receptor signaling in animals with superior regenerative ability, axolotl salamanders, to determine the impact on vascularization and regenerative outgrowth. Following tail amputation, treated animals (100 nM PTK787) and controls were examined microscopically and measured over the month-long period of regeneration. Treatment with VEGFR inhibitor decreased regenerative angiogenesis; drug-treated animals had lower vascular densities in the regenerating tail than untreated animals. This decrease in neovascularization, however, was not associated with a decrease in regenerative outgrowth or with morphological abnormalities in the regrown tail. Avascular but otherwise anatomically normal regenerative outgrowth over 1 mm beyond the amputation plane was observed. The results suggest that in this highly regenerative species, significant early tissue regeneration is possible in the absence of a well-developed vasculature. This research sets the groundwork for establishing a system for the chemical manipulation of angiogenesis within the highly regenerative axolotl model, contributing to a better understanding of the role of the microvasculature within strongly proliferative yet well-regulated environments. Anat Rec, 300:2273-2280, 2017. © 2017 Wiley Periodicals, Inc.

Nerve Dependence: From Regeneration to Cancer

Nerve dependence has long been described in animal regeneration, where the outgrowth of axons is necessary to the reconstitution of lost body parts and tissue remodeling in various species. Recent discoveries have demonstrated that denervation can suppress tumor growth and metastasis, pointing to nerve dependence in cancer. Regeneration and cancer share similarities in regard to the stimulatory role of nerves, and there are indications that the stem cell compartment is a preferred target of innervation. Thus, the neurobiology of cancer is an emerging discipline that opens new perspectives in oncology.

Blood vessel formation during tail regeneration in the leopard gecko (Eublepharis macularius): The blastema is not avascular

Unique among amniotes, many lizards are able to self-detach (autotomize) their tail and then regenerate a replacement. Tail regeneration involves the formation of a blastema, an accumulation of proliferating cells at the site of autotomy. Over time, cells of the blastema give rise to most of the tissues in the replacement tail. In non-amniotes capable of regenerating (such as urodeles and some teleost fish), the blastema is reported to be essentially avascular until tissue differentiation takes place. For tail regenerating lizards less is known. Here, we investigate neovascularization during tail regeneration in the leopard gecko (Eublepharis macularius). We demonstrate that the gecko tail blastema is not an avascular structure. Beginning with the onset of regenerative outgrowth, structurally mature (mural cell supported) blood vessels are found within the blastema. Although the pattern of blood vessel distribution in the regenerate tail differs from that of the original, a hierarchical network is established, with vessels of varying luminal diameters and wall thicknesses. Using immunostaining, we determine that blastema outgrowth and tissue differentiation is characterized by a dynamic interplay between the pro-angiogenic protein vascular endothelial growth factor (VEGF) and the anti-angiogenic protein thrombospondin-1 (TSP-1). VEGF-expression is initially widespread, but diminishes as tissues differentiate. In contrast, TSP-1 expression is initially restricted but becomes more abundant as VEGF-expression wanes. We predict that variation in the neovascular response observed between different regeneration-competent species likely relates to the volume of the blastema. J. Morphol. 278:380-389, 2017. © 2017 Wiley Periodicals, Inc.

Angiogenesis is inhibitory for mammalian digit regeneration

The regenerating mouse digit tip is a unique model for investigating blastema formation and epimorphic regeneration in mammals. The blastema is characteristically avascular and we previously reported that blastema expression of a known anti‐angiogenic factor gene, Pedf, correlated with a successful regenerative response (Yu, L., Han, M., Yan, M., Lee, E. C., Lee, J. & Muneoka, K. (2010). BMP signaling induces digit regeneration in neonatal mice. Development, 137, 551–559). Here we show that during regeneration Vegfa transcripts are not detected in the blastema but are expressed at the onset of differentiation. Treating the amputation wound with vascular endothelial growth factor enhances angiogenesis but inhibits regeneration. We next tested bone morphogenetic protein 9 (BMP9), another known mediator of angiogenesis, and found that BMP9 is also a potent inhibitor of digit tip regeneration. BMP9 induces Vegfa expression in the digit stump suggesting that regenerative failure is mediated by enhanced angiogenesis. Finally, we show that BMP9 inhibition of regeneration is completely rescued by treatment with pigment epithelium‐derived factor. These studies show that precocious angiogenesis is inhibitory for regeneration, and provide compelling evidence that the regulation of angiogenesis is a critical factor in designing therapies aimed at stimulating mammalian regeneration.

Positional information is reprogrammed in blastema cells of the regenerating limb of the axolotl (Ambystoma mexicanum)

The regenerating region of an amputated salamander limb, known as the blastema, has the amazing capacity to replace exactly the missing structures. By grafting cells from different stages and regions of blastemas induced to form on donor animals expressing Green Fluorescent Protein (GFP), to non-GFP host animals, we have determined that the cells from early stage blastemas, as well as cells at the tip of late stage blastemas are developmentally labile such that their positional identity is reprogrammed by interactions with more proximal cells with stable positional information. In contrast, cells from the adjacent, more proximal stump tissues as well as the basal region of late bud blastemas are positionally stable, and thus form ectopic limb structures when grafted. Finally, we have found that a nerve is required to maintain the blastema cells in a positionally labile state, thus indicating a role for reprogramming cues in the blastema microenvironment.

Pseudotyped retroviruses for infecting axolotl in vivo and in vitro

Axolotls are poised to become the premiere model system for studying vertebrate appendage regeneration. However, very few molecular tools exist for studying crucial cell lineage relationships over regeneration or for robust and sustained misexpression of genetic elements to test their function. Furthermore, targeting specific cell types will be necessary to understand how regeneration of the diverse tissues within the limb is accomplished. We report that pseudotyped, replication-incompetent retroviruses can be used in axolotls to permanently express markers or genetic elements for functional study. These viruses, when modified by changing their coat protein, can infect axolotl cells only when they have been experimentally manipulated to express the receptor for that coat protein, thus allowing for the possibility of targeting specific cell types. Using viral vectors, we have found that progenitor populations for many different cell types within the blastema are present at all stages of limb regeneration, although their relative proportions change with time.

Denervation affects regenerative responses in MRL/MpJ and repair in C57BL/6 ear wounds

The MRL/MpJ mouse displays the rare ability amongst mammals to heal injured ear tissue without scarring. Numerous studies have shown that the formation of a blastema-like structure leads to subsequent tissue regeneration in this model, indicating many parallels with amphibian limb regeneration and mammalian embryogenesis. We have recently shown that the MRL/MpJ mouse also possesses an enhanced capacity for peripheral nerve regeneration within the ear wound. Indeed, nerves are vital for the initial phase of blastema formation in the amphibian limb. In this study we investigated the capacity for wound regeneration in a denervated ear. The left ears of MRL/MpJ mice and C57BL/6 (a control strain known to have a poorer regenerative capacity) were surgically denervated at the base via an incision and nerve transection, immediately followed by a 2-mm ear punch wound. Immunohistochemical analysis showed a lack of neurofilament expression in the denervated ear wound. Histology revealed that denervation prevented blastema formation and chrondrogenesis, and also severely hindered normal healing, with disrupted re-epithelialisation, increasing wound size and progressive necrosis towards the ear tip. Denervation of the ear obliterated the regenerative capacity of the MRL/MpJ mouse, and also had a severe negative effect on the ear wound repair mechanisms of the C57BL/6 strain. These data suggest that innervation may be important not only for regeneration but also for normal wound repair processes.

A novel amniote model of epimorphic regeneration: the leopard gecko, Eublepharis macularius

Background

Epimorphic regeneration results in the restoration of lost tissues and structures from an aggregation of proliferating cells known as a blastema. Among amniotes the most striking example of epimorphic regeneration comes from tail regenerating lizards. Although tail regeneration is often studied in the context of ecological costs and benefits, details of the sequence of tissue-level events are lacking. Here we investigate the anatomical and histological events that characterize tail regeneration in the leopard gecko, Eublepharis macularius.

Results

Tail structure and tissue composition were examined at multiple days following tail loss, revealing a conserved pattern of regeneration. Removal of the tail results in a consistent series of morphological and histological events. Tail loss is followed by a latent period of wound healing with no visible signs of regenerative outgrowth. During this latent period basal cells of the epidermis proliferate and gradually cover the wound. An additional aggregation of proliferating cells accumulates adjacent to the distal tip of the severed spinal cord marking the first appearance of the blastema. Continued growth of the blastema is matched by the initiation of angiogenesis, followed by the re-development of peripheral axons and the ependymal tube of the spinal cord. Skeletal tissue differentiation, corresponding with the expression of Sox9, and muscle re-development are delayed until tail outgrowth is well underway.

Conclusions

We demonstrate that tail regeneration in lizards involves a highly conserved sequence of events permitting the establishment of a staging table. We show that tail loss is followed by a latent period of scar-free healing of the wound site, and that regeneration is blastema-mediated. We conclude that the major events of epimorphic regeneration are highly conserved across vertebrates and that a comparative approach is an invaluable biomedical tool for ongoing regenerative research.

Maintenance of blastemal proliferation by functionally diverse epidermis in regenerating zebrafish fins

Appendage regeneration in salamanders and fish occurs through formation and maintenance of a mass of progenitor tissue called the blastema. A dedicated epidermis overlays the blastema and is required for its proliferation and patterning, yet this interaction is poorly understood. Here, we identified molecularly and functionally distinct compartments within the basal epidermal layer during zebrafish fin regeneration. Proximal epidermal subtypes express the transcription factor lef1 and the blastemal mitogen shh, while distal subtypes express the Fgf target gene pea3 and wnt5b, an inhibitor of blastemal proliferation. Ectopic overexpression of wnt5b reduced shh expression, while pharmacologic introduction of a Hh pathway agonist partially rescued blastemal proliferation during wnt5b overexpression. Loss- and gain-of-function approaches indicate that Fgf signaling promotes shh expression in proximal epidermis, while Fgf/Ras signaling restricts shh expression from distal epidermis through induction of pea3 expression and maintenance of wnt5b. Thus, the fin wound epidermis spatially confines Hh signaling through the activity of Fgf and Wnt pathways, impacting blastemal proliferation during regenerative outgrowth.

Characterizing regeneration in the vertebrate ear

We have previously shown that MRL/MpJ mice have a capacity for regeneration instead of scar formation following an ear punch wound. Understanding the differences that occur between scar-free regeneration or repair with scarring will have great impact upon advances in skin tissue engineering. A key question that remains unanswered in the MRL/MpJ mouse model is whether regeneration was restricted to the ear or whether it extended to the skin. A histological analysis was conducted up to 4 months post-wounding, not only with 2-mm punch wounds to the ear but also to the skin on the backs of the same animals. MRL/MpJ mouse ear wounds regenerate faster than control strains, with enhanced blastema formation, a markedly thickened tip epithelium and reduced scarring. Interestingly, in the excisional back wounds, none of these regenerative features was observed and both the C57BL/6 control and MRL/MpJ mice healed with scarring. This review gives an insight into how this regenerative capacity may be due to evolutionary processes as well as ear anatomy. The ear is thin and surrounded on both sides by epithelia, and the dorsal skin is devoid of cartilage and under greater tensile strain. Analysis of apoptosis during ear regeneration is also discussed, assessing the role and expression of various members of the Bcl-2 family of proteins. Ongoing studies are focusing on de novo cartilage development in the regenerating ear, as well as understanding the role of downstream signalling cascades in the process. Identification of such signals could lead to their manipulation and use in a novel tissue-engineered skin substitute with scar-free integration.

Location of injury influences the mechanisms of both regeneration and repair within the MRL/MpJ mouse

The adult MRL/MpJ mouse regenerates all differentiated structures after through-and-through ear punch wounding in a scar-free process. We investigated whether this regenerative capacity was also shown by skin wounds. Dorsal skin wounds were created, harvested and archived from the same animals (MRL/MpJ and C57BL/6 mice) that received through-and-through ear punch wounds. Re-epithelialization was complete in dorsal wounds in both strains by day 5 and extensive granulation tissue was present by day 14 post-wounding. By day 21, wounds from both strains contained dense amounts of collagen that healed with a scar. The average wound area, as well as alpha-smooth muscle actin expression and macrophage influx were investigated during dorsal skin wound healing and did not significantly differ between strains. Thus, MRL/MpJ mice regenerate ear wounds in a scar-free manner, but heal dorsal skin wounds by simple repair with scar formation. A significant conclusion can be drawn from these data; mechanisms of regeneration and repair can occur within the same animal, potentially utilizing similar molecules and signalling pathways that subtly diverge dependent upon the microenvironment of the injury.

Cellular electroporation induces dedifferentiation in intact newt limbs

Newts have the remarkable ability to regenerate lost appendages including their forelimbs, hindlimbs, and tails. Following amputation of an appendage, the wound is rapidly closed by the migration of epithelial cells from the proximal epidermis. Internal cells just proximal to the amputation plane begin to dedifferentiate to form a pool of proliferating progenitor cells known as the regeneration blastema. We show that dedifferentiation of internal appendage cells can be initiated in the absence of amputation by applying an electric field sufficient to induce cellular electroporation, but not necrosis or apoptosis. The time course for dedifferentiation following electroporation is similar to that observed following amputation with evidence of dedifferentiation beginning at about 5 days postelectroporation and continuing for 2 to 3 weeks. Microarray analyses, real-time RT-PCR, and in situ hybridization show that changes in early gene expression are similar following amputation or electroporation. We conclude that the application of an electric field sufficient to induce transient electroporation of cell membranes induces a dedifferentiation response that is virtually indistinguishable from the response that occurs following amputation of newt appendages. This discovery allows dedifferentiation to be studied in the absence of wound healing and may aid in identifying genes required for cellular plasticity.

Tissue inhibitor of metalloproteinase 1 regulates matrix metalloproteinase activity during newt limb regeneration

Matrix metalloproteinase (MMP) activity is important for newt limb regeneration. In most biological processes that require MMP function, MMP activity is tightly controlled by a variety of mechanisms, including the coexpression of natural inhibitors. Here, we show that gene expression of one such inhibitor, tissue inhibitor of metalloproteinase 1 (NvTIMP1), is upregulated during the wound healing and dedifferentiation stages of regeneration when several MMPs are at their maximal expression levels. Newt MMPs and NvTIMP1 also exhibit similar spatial expression patterns during the early stages of limb regeneration. NvTIMP1 inhibits the proteolytic activity of regeneration-related newt MMPs and, like human TIMP1, can induce a weak mitogenic response in certain cell types. These results suggest that NvTIMP1 may be functioning primarily to maintain optimal levels of MMP activity during the early stages of limb regeneration, while possibly serving a secondary role as a mitogen.

Tail regeneration and ependymal outgrowth in the adult newt, Notophthalmus viridescens, are adversely affected by experimentally produced ischemia

Spinal axons of the adult newt will regenerate when the spinal cord is severed or when the tail is amputated. Ischemia and associated hypoxia have been correlated with poor central nervous system regeneration in mammals. To test the effects of ischemia on newt spinal cord regeneration, the spinal cord and major blood vessels of the newt tail were severed 2 cm caudal to the cloaca as a primary injury. This primary injury severely reduced circulation in the caudal direction for 7 days; by day 8, circulation was largely restored. After various periods of time after primary injury, tails were amputated 1 cm caudal to the primary injury (in the area of ischemia) and tested for regeneration. If the tail was amputated within 5 days of the primary injury, regeneration did not occur. If amputation was 7 days or longer after the primary injury, a regenerative response occurred. Histology showed that in the non-regenerating tails the spinal cord and associated ependyma, known to be important to tail regeneration, had degenerated in the rostral direction. Such degeneration was prevented when tails were first amputated and allowed to form blastemas before the primary injury. The data indicate that the first 5-7 days of blastema formation are particularly sensitive to compromised blood flow (ischemia/hypoxia). It follows that mechanisms must be present in the adult newt to reduce ischemia to a minimum and thus allow ependymal outgrowth and tail regeneration.

Some principles of regeneration in mammalian systems

This article presents some general principles underlying regenerative phenomena in vertebrates, starting with the epimorphic regeneration of the amphibian limb and continuing with tissue and organ regeneration in mammals. Epimorphic regeneration following limb amputation involves wound healing, followed shortly by a phase of dedifferentiation that leads to the formation of a regeneration blastema. Up to the point of blastema formation, dedifferentiation is guided by unique regenerative pathways, but the overall developmental controls underlying limb formation from the blastema generally recapitulate those of embryonic limb development. Damaged mammalian tissues do not form a blastema. At the cellular level, differentiation follows a pattern close to that seen in the embryo, but at the level of the tissue and organ, regeneration is strongly influenced by conditions inherent in the local environment. In some mammalian systems, such as the liver, parenchymal cells contribute progeny to the regenerate. In others, e.g., skeletal muscle and bone, tissue-specific progenitor cells constitute the main source of regenerating cells. The substrate on which regeneration occurs plays a very important role in determining the course of regeneration. Epimorphic regeneration usually produces an exact replica of the structure that was lost, but in mammalian tissue regeneration the form of the regenerate is largely determined by the mechanical environment acting on the regenerating tissue, and it is normally an imperfect replica of the original. In organ hypertophy, such as that occurring after hepatic resection, the remaining liver mass enlarges, but there is no attempt to restore the original form.

Harnessing wound healing and regeneration for tissue engineering

Biomedical science has made major advances in understanding how cells grow into functioning tissue and the signalling mechanisms used to achieve this are slowly being dissected. Tissue engineering is the application of that knowledge to the building or repairing of organs, including skin, the largest organ in the body. Generally, engineered tissue is a combination of living cells and a supporting matrix. Besides serving as burn coverings, engineered skin substitutes can help patients with diabetic foot ulcers. Today, most of these ulcers are treated with an approach that includes antibiotics, glucose control, special shoes and frequent cleaning and bandaging. The results of such treatments are often disappointing and ineffectual, and scarring remains a major problem, mechanically, cosmetically and psychologically. Within our group we are attempting to address this by investigating novel approaches to skin tissue engineering. We are identifying novel therapeutic manipulations to improve the degree of integration between a tissue engineered dermal construct and the host by both molecular manipulation of growth factors but also by understanding and harnessing mechanisms of regenerative biology. For the purpose of this summary, we will concentrate primarily on the latter of these two approaches in that we have identified a novel mouse mutant that completely and perfectly regenerates skin and cartilaginous components following ear injury. This experimental animal will allow us to characterize not only novel genes involved in the regeneration process but also to utilize cells from such animals in artificial skin equivalents to assess their behaviour compared with normal cells. This approach should allow us to create a tissue-engineered substitute, which more closely resembles the normal regional microanatomy and physiology of the skin, allowing better integration to the host with minimal or no scarring.

Vascular supplies differ in regenerating and nonregenerating amputated rodent digits

Bone regenerates following amputation through the level of the nail, but bone is capped following amputation through more proximal levels. Because osteogenesis requires an ample blood supply, we postulated that a restricted vascular supply might be correlated with restricted regenerative ability at proximal levels. More than 40 rats and mice were injected with ink or resin to visualize vascular supplies of intact, regenerating, and nonregenerating rat and mouse digits. Ink-injected specimens were viewed as histological sections or cleared whole mounts. Partially digested resin casts were viewed using scanning electron microscopy. Contrary to our hypothesis, prior to amputation, proximal sites are more vascular than distal sites. At both proximal and distal levels, endosteal and periosteal vascular systems are evident. However, in proximal phalanges, additional subcutaneous and dermal layers encircle the bone. Beneath the distal nail, these layers are absent, and a single layer of vessels provides both periosteal and cutaneous supplies. After amputation at both levels, new vessels sprout profusely in osteogenic areas of both endosteum and periosteum. However, at proximal levels, the additional hypodermal and dermal vessels contribute to a vascular plexus that, paradoxically, may impair bone regrowth by contributing to the formation of dermal scar rather than bone.

Responses to amputation of denervated ambystoma limbs containing aneurogenic limb grafts

The developing neural tubes and associated neural crest cells were removed from stage 30 Ambystoma maculatum embryos to obtain larvae with aneurogenic forelimbs. Forelimbs were allowed to develop to late 3 digit or early 4 digit stages. Limbs amputated through the mid radius-ulna regenerated typically in the aneurogenic condition. Experiments were designed to test whether grafts of aneurogenic limb tissues would rescue denervated host limb stumps into a regeneration response. In Experiment 1, aneurogenic limbs were removed at the body wall and grafted under the dorsal skin of the distal end of amputated forelimbs of control, normally innervated limbs of locally collected Ambystoma maculatum or axolotl (Ambystoma mexicanum) larvae. In Experiment 1, at the time of grafting or 1, 2, 3, 4, 5, 7, or 8 days after grafting, aneurogenic limbs were amputated level with the original host stump. At 7 and 8 days, this amputation included removing the host blastema adjacent to the graft. The host limb was denervated either one day after grafting or on the day of graft amputation. These chimeric limbs only infrequently exhibited delayed blastema formation. Thus, not only did the graft not rescue the host, denervated limb, but the aneurogenic limb tissues themselves could not mount a regeneration response. In Experiment 2, the grafted aneurogenic limb was amputated through its mid-stylopodium at 3, 4, 5, 7, or 8 days after grafting. By 7 and 8 days after grafting, the host limb stump exhibited blastema formation even with the graft extending out from under the dorsal skin. The host limb was denervated at the time of graft amputation. When graft limbs of Experiment 2 were amputated and host limbs were denervated on days 3, 4, or 5, host regeneration did not progress and graft regeneration did not occur. But, when graft limbs were amputated on days 7 or 8 with concomitant denervation of the host limb, regeneration of the host continued and graft regeneration occurred. Thus, regeneration of the graft was correlated with acquisition of nerve-independence by the host limb blastema. In Experiment 3, aneurogenic limbs were grafted with minimal injury to the dorsal skin of neurogenic hosts. When neurogenic host limbs were denervated and the aneurogenic limbs were amputated through the radius/ulna, regeneration of the aneurogenic limb occurred if the neurogenic limb host was not amputated, but did not occur if the neurogenic limb host was amputated. Results of Experiment 3 indicate that the inhibition of aneurogenic graft limb regeneration on a denervated host limb is correlated with substantial injury to the host limb. In Experiment 4, aneurogenic forelimbs were amputated through the mid-radius ulna and pieces of either peripheral nerve, muscle, blood vessel, or cartilage were grafted into the distal limb stump or under the body skin immediately adjacent to the limb at the body wall. In most cases, peripheral nerve inhibited regeneration, blood vessel tissue sometimes inhibited, but other tissues had no effect on regeneration. Taken together, the results suggest: (1) Aneurogenic limb tissues do not produce the neurotrophic factor and do not need it for regeneration, and (2) there is a regeneration-inhibiting factor produced by the nerve-dependent limb stump/blastema after denervation that prevents regeneration of aneurogenic limbs.

Regeneration of the ear after wounding in different mouse strains is dependent on the severity of wound trauma

The replacement and restoration of tissue mass after organ damage or injury in adult higher vertebrates is critical to the architecture and function of the organ. If replacement occurs with scar tissue, this often results in adverse effects on function and growth as well as an undesirable cosmetic appearance. However, certain mammals, such as the MRL/MpJ mouse, have shown a restricted capacity for regeneration, rather than scar tissue formation, after an excisional ear punch wound. To investigate the changes in tissue architecture leading to ear wound closure, initial ear wounding studies with a 2-mm clinical biopsy punch were performed on MRL/MpJ mice, by using C57BL/6 mice as a nonregenerative control strain. In contrast to previously reported studies on mouse ear regeneration, we observed that C57BL/6 mice in fact showed a limited regenerative capacity. One explanation for this difference could be attributed to the method of wounding used; both previous studies on mouse ear regeneration used a thumb punch, whereas our approach was to use a clinical biopsy punch. This approach led us to further investigate whether the severity of trauma applied influenced the rate of wound healing. We, therefore, compared the effects of the sharp clinical biopsy punch with that of a cruder thumb punch, and introduced a third strain of mouse, Balb/c, known to be a slow-healing strain. A new method to quantify ear punch hole closure was developed and a histologic investigation conducted up to 4 months after wounding. Image analysis data showed a reduction in original ear wound area of 85% in MRL/MpJ mice at 4 weeks and of 91.7% over 4 months by using a biopsy punch. In contrast, the crude thumb punch methodology resulted in an increase in wound area of up to 58% in Balb/c ears; thought to be due to increased necrosis of the wound site. All biopsy-punched wound areas plateaued in healing between days 28 and 112. Only 5 of 80 MRL/MpJ mouse ears showed no residual holes macroscopically after 28 days. Histologically, all strains of mice healed their ear wounds in a similar manner involving re-epithelialization, blastema-like formation, dermal extension, blood vessel formation, chondrogenesis, folliculogenesis, and skeletal muscle and fat differentiation. However, all regenerative features were more pronounced and accelerated in MRL/MpJ mice when compared with C57BL/6 and Balb/c biopsy-punched mouse ears.

Expression and biological effect of urodele fibroblast growth factor 1: relationship to limb regeneration

Fibroblast growth factors (FGFs) have been previously implicated in urodele limb regeneration. Here, we examined expression of FGF-1 by blastema cells and neurons and investigated its involvement in wound epithelial formation and function and in the trophic effect of nerves. Neurons innervating the limb and blastema cells in vivo and in vitro expressed the FGF-1 gene. The peptide was present in blastemas in vivo. Wound epithelium thickened when recombinant newt FGF-1 was provided on heparin-coated beads, demonstrating that the FGF-1 was biologically active and that the wound epithelium is a possible target tissue of FGF. FGF-1 did not stimulate accessory limb formation. FGF-1 was as effective as 10% fetal bovine serum in maintaining proliferative activity of blastema cells in vitro but was unable to maintain growth of denervated, nerve-dependent stage blastemas when provided on beads or by injection. FGF-1 had a strong stimulating effect on blastema cell accumulation and proliferation of limbs inserted into the body cavity that were devoid of an apical epithelial cap (AEC). These results show that FGF-1 can signal wound epithelium cap formation and/or function and can stimulate mesenchyme accumulation/proliferation in the absence of the AEC but that FGF-1 is not directly involved in the neural effect on blastema growth.