The influence of fundamental traits on mechanisms controlling appendage regeneration

One of the most compelling questions in evolutionary biology is why some animals can regenerate injured structures while others cannot. Appendage regeneration appears to be common when viewed across the metazoan phylogeny, yet this ability has been lost in many taxa to varying degrees. Within species, the capacity for regeneration also can vary ontogenetically among individuals. Here we argue that appendage regeneration along the secondary body axis may be constrained by fundamental traits such as body size, aging, life stage, and growth pattern. Studies of the molecular mechanisms affecting regeneration have been conducted primarily with small organisms at early life stages. Such investigations disregard the dramatic shifts in morphology and physiology that organisms undergo as they age, grow, and mature. To help explain interspecific and intraspecific constraints on regeneration, we link particular fundamental traits to specific molecular mechanisms that control regeneration. We present a new synthesis for how these fundamental traits may affect the molecular mechanisms of regeneration at the tissue, cellular, and genomic levels of biological organization. Future studies that explore regeneration in organisms across a broad phylogenetic scale, and within an ontogenetic framework, will help elucidate the proximate mechanisms that modulate regeneration and may reveal new biomedical applications for use in regenerative medicine.     

Morphological,cellular and molecular characterization of posterior regeneration in the marine annelid Platynereis dumerilii

Regeneration, the ability to restore body parts after an injury or an amputation, is a widespread but highly variable and complex phenomenon in animals. While having fascinated scientists for centuries, fundamental questions about the cellular basis of animal regeneration as well as its evolutionary history remain largely unanswered. Here, we present a study of regeneration of the marine annelid Platynereis dumerilii, an emerging comparative developmental biology model, which, like many other annelids, displays important regenerative abilities. When P. dumerilii worms are amputated, they are able to regenerate the posteriormost differentiated part of their body and a stem cell-rich growth zone that allows the production of new segments replacing the amputated ones. We show that posterior regeneration is a rapid process that follows a well reproducible path and timeline, going through specific stages that we thoroughly defined. Wound healing is achieved one day after amputation and a regeneration blastema forms one day later. At this time point, some tissue specification already occurs, and a functional posterior growth zone is re-established as early as three days after amputation. Regeneration timing is only influenced, in a minor manner, by worm size. Comparable regenerative abilities are found for amputations performed at different positions along the antero-posterior axis of the worm, except when amputation planes are very close to the pharynx. Regenerative abilities persist upon repeated amputations without important alterations of the process. We also show that intense cell proliferation occurs during regeneration and that cell divisions are required for regeneration to proceed normally. Finally, 5-ethynyl-2’-deoxyuridine (EdU) pulse and chase experiments suggest that blastemal cells mostly derive from the segment immediately abutting the amputation plane. The detailed characterization of P. dumerilii posterior body regeneration presented in this article provides the foundation for future mechanistic and comparative studies of regeneration in this species.     

A Comparative Perspective on Brain Regeneration in Amphibians and Teleost Fish

Regeneration of lost cells in the central nervous system, especially the brain, is present to varying degrees in different species. In mammals, neuronal cell death often leads to glial cell hypertrophy, restricted proliferation, and formation of a gliotic scar, which prevents neuronal regeneration. Conversely, amphibians such as frogs and salamanders and teleost fish possess the astonishing capacity to regenerate lost cells in several regions of their brains. While frogs lose their regenerative abilities after metamorphosis, teleost fish and salamanders are known to possess regenerative competence even throughout adulthood. In the last decades, substantial progress has been made in our understanding of the cellular and molecular mechanisms of brain regeneration in amphibians and fish. But how similar are the means of brain regeneration in these different species? In this review, we provide an overview of common and distinct aspects of brain regeneration in frog, salamander, and teleost fish species: from the origin of regenerated cells to the functional recovery of behaviors.     

The zebrafish as a model of heart regeneration

Regeneration is a complex biological process by which animals can restore the shape, structure and function of body parts lost after injury, or after experimental amputation. Only a few species of vertebrates display the capacity to regenerate body parts during adulthood. In the case of the heart, newts display a remarkable ability to regenerate large portions of myocardium after amputation, although the mechanisms underlying this process have not been addressed. Recently, it has been shown that adult zebrafish can also regenerate their hearts, thus offering new possibilities for experimentally approaching this fascinating biological phenomenon. The first insights into heart regeneration gained by studying this model organism are reviewed here.     

Regulation and phylogeny of skeletal muscle regeneration

One of the most fascinating questions in regenerative biology is why some animals can regenerate injured structures while others cannot. Skeletal muscle has a remarkable capacity to regenerate even after repeated traumas, yet limited information is available on muscle repair mechanisms and how they have evolved. For decades, the main focus in the study of muscle regeneration was on muscle stem cells, however, their interaction with their progeny and stromal cells is only starting to emerge, and this is crucial for successful repair and re-establishment of homeostasis after injury. In addition, numerous murine injury models are used to investigate the regeneration process, and some can lead to discrepancies in observed phenotypes. This review addresses these issues and provides an overview of some of the main regulatory cellular and molecular players involved in skeletal muscle repair.     

'Open minded' cells: how cells can change fate

It has long intrigued researchers why some but not all organisms can regenerate missing body parts. Plants are remarkable in that they can regenerate the entire organism from a small piece of tissue, or even a single cell. Epigenetic mechanisms that control chromatin organization are now known to regulate the cellular plasticity and reprogramming necessary for regeneration. Interestingly, although animals and plants have evolved different strategies and mechanisms to control developmental processes, they have maintained many similarities in the way they regulate chromatin organization. Given that plants can rapidly switch fate, we propose that an understanding of the mechanisms regulating this process in plant cells could provide a new perspective on cellular dedifferentiation in animals.     

Regeneration in vertebrates

One way or another, all species possess the ability to regenerate damaged tissues. The degree of regeneration, however, varies considerably among tissues within a body and among species, with urodeles being the most spectacular. Such differences in regenerative capacity are indicative of specific mechanisms that control the different types of regeneration. In this review the different types of regeneration in vertebrates and their basic characteristics are presented. The major cellular events, such as dedifferentiation and transdifferentiation, which allow complex organ and body part regeneration, are discussed and common molecular mechanisms are pinpointed.     

Mechanisms of muscle dedifferentiation during regeneration

For many years people have known that amphibians have an amazing ability to regenerate lost body parts. In contrast humans have limited regeneration capacity and even simple wound healing results in scarring. Despite more than a century of scientific inquiry, this remarkable phenomenon remains poorly understood. Recent research has begun to provide insight into how this unique process that is now fully accepted to occur via the reversal of cell differentiation is executed at the molecular level. As more and more is known about regeneration and dedifferentiation we can begin to address the question: if given the right signals could mammals also regenerate body structures?     

EvoRegen in animals: Time to uncover deep conservation or convergence of adult stem cell evolution and regenerative processes

How do animals regenerate specialised tissues or their entire body after a traumatic injury, how has this ability evolved and what are the genetic and cellular components underpinning this remarkable feat? While some progress has been made in understanding mechanisms, relatively little is known about the evolution of regenerative ability. Which elements of regeneration are due to lineage specific evolutionary novelties or have deeply conserved roots within the Metazoa remains an open question. The renaissance in regeneration research, fuelled by the development of modern functional and comparative genomics, now enable us to gain a detailed understanding of both the mechanisms and evolutionary forces underpinning regeneration in diverse animal phyla. Here we review existing and emerging model systems, with the focus on invertebrates, for studying regeneration. We summarize findings across these taxa that tell us something about the evolution of adult stem cell types that fuel regeneration and the growing evidence that many highly regenerative animals harbor adult stem cells with a gene expression profile that overlaps with germline stem cells. We propose a framework in which regenerative ability broadly evolves through changes in the extent to which stem cells generated through embryogenesis are maintained into the adult life history.     

Lens regeneration in axolotl: new evidence of developmental plasticity

Background

Among vertebrates lens regeneration is most pronounced in newts, which have the ability to regenerate the entire lens throughout their lives. Regeneration occurs from the dorsal iris by transdifferentiation of the pigment epithelial cells. Interestingly, the ventral iris never contributes to regeneration. Frogs have limited lens regeneration capacity elicited from the cornea during pre-metamorphic stages. The axolotl is another salamander which, like the newt, regenerates its limbs or its tail with the spinal cord, but up until now all reports have shown that it does not regenerate the lens.

Results

Here we present a detailed analysis during different stages of axolotl development, and we show that despite previous beliefs the axolotl does regenerate the lens, however, only during a limited time after hatching. We have found that starting at stage 44 (forelimb bud stage) lens regeneration is possible for nearly two weeks. Regeneration occurs from the iris but, in contrast to the newt, regeneration can be elicited from either the dorsal or the ventral iris and, occasionally, even from both in the same eye. Similar studies in the zebra fish concluded that lens regeneration is not possible.

Conclusions

Regeneration of the lens is possible in the axolotl, but differs from both frogs and newts. Thus the axolotl iris provides a novel and more plastic strategy for lens regeneration.

     

Plant invasiveness is not linked to the capacity of regeneration from small fragments: an experimental test with 39 stoloniferous species

Fragmentation and vegetative regeneration from small fragments may contribute to population expansion, dispersal and establishment of new populations of introduced plants. However, no study has systematically tested whether a high capacity of vegetative regeneration is associated with a high degree of invasiveness. For small single-node fragments, the presence of internodes may increase regeneration capacity because internodes may store carbohydrates and proteins that can be used for regeneration. We conducted an experiment with 39 stoloniferous plant species to examine the regeneration capacity of small, single-node fragments with or without attached stolon internodes. We asked (1) whether the presence of stolon internodes increases regeneration from single-node fragments, (2) whether regeneration capacity differs between native and introduced species in China, and (3) whether regeneration capacity is positively associated with plant invasiveness at a regional scale (within China) and at a global scale. Most species could regenerate from single-node fragments, and the presence of internodes increased regeneration rate and subsequent growth and/or asexual reproduction. Regeneration capacity varied greatly among species, but showed no relationship to invasiveness, either in China or globally. High regeneration capacity from small fragments may contribute to performance of clonal plants in general, but it does not appear to explain differences in invasiveness among stoloniferous clonal species.     

Notch signaling inhibits axon regeneration

Many neurons have limited capacity to regenerate their axons after injury. Neurons in the mammalian central nervous system do not regenerate, and even neurons in the peripheral nervous system often fail to regenerate to their former targets. This failure is likely due in part to pathways that actively restrict regeneration; however, only a few factors that limit regeneration are known. Here, using single-neuron analysis of regeneration in?vivo, we show that Notch/lin-12 signaling inhibits the regeneration of mature C.?elegans neurons. Notch signaling suppresses regeneration by acting autonomously in the injured cell to prevent growth cone formation. The metalloprotease and gamma-secretase cleavage events that lead to Notch activation during development are also required for its activity in regeneration. Furthermore, blocking Notch activation immediately after injury improves regeneration. Our results define a postdevelopmental role for the Notch pathway as a repressor of axon regeneration in?vivo.     

Hair cell regeneration in the avian auditory epithelium

Regeneration of sensory hair cells in the mature avian inner ear was first described just over 20 years ago. Since then, it has been shown that many other non-mammalian species either continually produce new hair cells or regenerate them in response to trauma. However, mammals exhibit limited hair cell regeneration, particularly in the auditory epithelium. In birds and other non-mammals, regenerated hair cells arise from adjacent non-sensory (supporting) cells. Hair cell regeneration was initially described as a proliferative response whereby supporting cells re-enter the mitotic cycle, forming daughter cells that differentiate into either hair cells or supporting cells and thereby restore cytoarchitecture and function in the sensory epithelium. However, further analyses of the avian auditory epithelium (and amphibian vestibular epithelium) revealed a second regenerative mechanism, direct transdifferentiation, during which supporting cells change their gene expression and convert into hair cells without dividing. In the chicken auditory epithelium, these two distinct mechanisms show unique spatial and temporal patterns, suggesting they are differentially regulated. Current efforts are aimed at identifying signals that maintain supporting cells in a quiescent state or direct them to undergo direct transdifferentiation or cell division. Here, we review current knowledge about supporting cell properties and discuss candidate signaling molecules for regulating supporting cell behavior, in quiescence and after damage. While significant advances have been made in understanding regeneration in non-mammals over the last 20 years, we have yet to determine why the mammalian auditory epithelium lacks the ability to regenerate hair cells spontaneously and whether it is even capable of significant regeneration under additional circumstances. The continued study of mechanisms controlling regeneration in the avian auditory epithelium may lead to strategies for inducing significant and functional regeneration in mammals.     

Regeneration of vertebrate appendage: an old experimental model to study stem cells in the adult

The application of stem cell therapy to cure degenerative diseases offers immense possibilities, but the research in this field is the subject of ethical debates raised by the question of destructive research on early human embryos. Stem cells taken in the adult constitute an alternative to human embryonic stem cells, but our knowledge on totipotent or pluripotent cells is currently insufficient. Furthermore, many questions must be solved before selection and differentiation of these cells in a given cellular type can be controlled on a routine basis. What are the molecular characteristics of an adult stem cell? What are the mechanisms involved in cell reprogramming? Which signals control stem cell replication and differentiation? Basic research activities must be carried out in order to clarify all these points. In this context, the regeneration of vertebrate appendages provides a model for this type of research. The regeneration process is defined by both the morphological and functional reconstruction of a part of a living organism, which has previously been destroyed. But why are some vertebrates able to regenerate complex structures and others apparently not? Among most vertebrates, the capacity to regenerate is limited to some tissues. It is however possible to observe the regeneration of appendages (limb, tail, fin, jaw, etc.) among several amphibians and fish. This regeneration leads to re-forming of the amputated part with a complete restoration of its shape, segmentation and function. Why is the amputation of limbs not followed by regeneration in mammals and birds: absence of stem cells, absence of recruitment signals for these cells, or absence of signal receptivity? This review constitutes a report on the current understanding of the basis of on regeneration of legs in tetrapods and of fins in fish with an emphasis in the role of the nervous system in this process.     

Brassinosteroids are required for efficient root tip regeneration in Arabidopsis

Compared with other organisms, plants have an extraordinary capacity for self-repair. Even if the entire tissues, including the stem cells, are resected, most plant species are able to completely regenerate whole tissues. However, the mechanism by which plants efficiently regenerate the stem cell niche during tissue reorganization is still largely unknown. Here, we found that the signaling mediated by plant steroid hormones brassinosteroids is activated during stem cell formation after root tip excision in Arabidopsis. Treatment with brassinazole, an inhibitor of brassinosteroid biosynthesis, delayed the recovery of stem cell niche after root tip excision. Regeneration of root tip after resection was also delayed in a brassinosteroid receptor mutant. Therefore, we propose that brassinosteroids participate in efficient root tip regeneration, thereby enabling efficient tissue regeneration to ensure continuous root growth after resection.     

Regeneration: every clot has a thrombin lining

Why do some amphibian organs regenerate and others do not? And why do most mammalian organs not regenerate? New work on lens regeneration indicates that the answer lies in the need for cells to dedifferentiate first, and that what makes them dedifferentiate may be the presence of thrombin in combination with cellular clotting factors.     

The regeneration capacity of the flatworm Macrostomum lignano—on repeated regeneration, rejuvenation, and the minimal size needed for regeneration

The lion’s share of studies on regeneration in Plathelminthes (flatworms) has been so far carried out on a derived taxon of rhabditophorans, the freshwater planarians (Tricladida), and has shown this group’s outstanding regeneration capabilities in detail. Sharing a likely totipotent stem cell system, many other flatworm taxa are capable of regeneration as well. In this paper, we present the regeneration capacity of Macrostomum lignano, a representative of the Macrostomorpha, the basal-most taxon of rhabditophoran flatworms and one of the most basal extant bilaterian protostomes. Amputated or incised transversally, obliquely, and longitudinally at various cutting levels, M. lignano is able to regenerate the anterior-most body part (the rostrum) and any part posterior of the pharynx, but cannot regenerate a head. Repeated regeneration was observed for 29 successive amputations over a period of almost 12 months. Besides adults, also first-day hatchlings and older juveniles were shown to regenerate after transversal cutting. The minimum number of cells required for regeneration in adults (with a total of 25,000 cells) is 4,000, including 160 neoblasts. In hatchlings only 1,500 cells, including 50 neoblasts, are needed for regeneration. The life span of untreated M. lignano was determined to be about 10 months.An erratum to this article can be found at     

Subtractive screen of potential limb regeneration related genes from Pachytriton brevipes

Regeneration capacity varies greatly among different animal species. In vertebrate, amphibian especially the Urodela, has been used as a powerful model system to study the mechanism of tissue regeneration because of the strong ability to regenerate their damaged or lost appendages. Pachytriton brevipes, a species of newt, which is widely distributed in south of China, can completely restore their damaged limbs within several months. In this study, we use modified suppression subtractive hybridization assay and dot-blot screening to identify candidate genes involved in tissue regeneration in P. brevipes. We successfully isolated 81 ESTs from a forward regeneration subtraction library. And we further verified the differential expression of four candidate genes, Rpl11, Cirbp, Ag2 and Trimx, between regenerating blastema and non-regeneration tissues by in situ hybridization. These genes were also be further characterized by phylogenetic and bioinformatic analysis. In general, we provided a comparative experimental approach to study the mechanisms of vertebrate regeneration.     

Rigenerazione e totipotenza

Abstract

Regeneration and totipotency.—Regeneration and totipotency of cells and nuclei in plants and animals are discussed. The paper consists of the following sections: regeneration in coelenterates and planarians; limb regeneration in amphibians; regeneration of mammalian liver; Wolffian lens regeneration in urodeles; nuclear transplantation in animal eggs; regeneration of plants from single cells (in vivo; in vitro; from somatic cell hybrids); general remarks.     

Infestation of the clam Chione fluctifraga by the burrowing worm Polydora sp. nov. in laboratory conditions

Burrowing worms that belong to Polydora spp. infest marine mollusks cultured worldwide, causing problems for production and marketing. The clam Chione fluctifraga is semi-cultured in Bahía Falsa, Baja California, NW Mexico, and some clams harbor burrowing worms. The present study was carried out to determine the identity of the worm species infesting the clam, the infesting process by cohabitation of infested and non-infested clams in aquaria with a variety of substrates (fine sand, gross sand, plastic bag used for clam culture, and aquarium without substrate) and turbulence conditions, and the occurrence of architomy phenomena in connection with infestation of the clam. The burrowing worm was considered as a nova species due to its singular limbate neurosetae and notosetae in the setiger 5, hooks in the setiger 6, eyes not present, and general pigmentation, among other characteristics. Infestation was similar in all substrates and turbulence conditions, but it was more abundant on clams previously infested than on those free of worms, showing a preferential settlement of worm infesting stages on pre-infested clams. Regeneration was observed in all segments of the worm: anterior (metastomium), medium, and posterior (prostomium); the complete regeneration time occurred in 40 days. This is the first record of architomy in a species of Polydora and this phenomenon could account for the increase of infestation intensity in pre-infested clams at the end of the study period. Infestation of clams by settling polichaete in the conditions studied, and the architomy process in this worm species, shows its great infesting capacity.