The brittle star genome illuminates the genetic basis of animal appendage regeneration.

Species within nearly all extant animal lineages are capable of regenerating body parts. However, it remains unclear whether the gene expression programme controlling regeneration is evolutionarily conserved. Brittle stars are a species-rich class of echinoderms with outstanding regenerative abilities, but investigations into the genetic bases of regeneration in this group have been hindered by the limited genomic resources. Here we report a chromosome-scale genome assembly for the brittle star Amphiura filiformis. We show that the brittle star genome is the most rearranged among echinoderms sequenced so far, featuring a reorganized Hox cluster reminiscent of the rearrangements observed in sea urchins. In addition, we performed an extensive profiling of gene expression during brittle star adult arm regeneration and identified sequential waves of gene expression governing wound healing, proliferation and differentiation. We conducted comparative transcriptomic analyses with other invertebrate and vertebrate models for appendage regeneration and uncovered hundreds of genes with conserved expression dynamics, particularly during the proliferative phase of regeneration. Our findings emphasize the crucial importance of echinoderms to detect long-range expression conservation between vertebrates and classical invertebrate regeneration model systems.

Functional Analyses of Four Cryptochromes From Aquatic Organisms After Heterologous Expression in Drosophila melanogaster Circadian Clock Cells.

Cryptochromes (Crys) represent a multi-facetted class of proteins closely associated with circadian clocks. They have been shown to function as photoreceptors but also to fulfill light-independent roles as transcriptional repressors within the negative feedback loop of the circadian clock. In addition, there is evidence for Crys being involved in light-dependent magneto-sensing, and regulation of neuronal activity in insects, adding to the functional diversity of this cryptic protein class. In mammals, Crys are essential components of the circadian clock, but their role in other vertebrates is less clear. In invertebrates, Crys can function as circadian photoreceptors, or as components of the circadian clock, while in some species, both light-receptive and clock factor roles coexist. In the current study, we investigate the function of Cry proteins in zebrafish (Danio rerio), a freshwater teleost expressing 6 cry genes. Zebrafish peripheral circadian clocks are intrinsically light-sensitive, suggesting the involvement of Cry in light-resetting. Echinoderms (Strongylocentrotus purpuratus) represent the only class of deuterostomes that possess an orthologue (SpuCry) of the light-sensitive Drosophila melanogaster Cry, which is an important component of the light-resetting pathway, but also works as transcriptional repressor in peripheral clocks of fruit flies. We therefore investigated the potential of different zebrafish cry genes and SpuCry to replace the light-resetting and repressor functions of Drosophila Cry by expressing them in fruit flies lacking endogenous cry function. Using various behavioral and molecular approaches, we show that most Cry proteins analyzed are able to fulfill circadian repressor functions in flies, except for one of the zebrafish Crys, encoded by cry4a. Cry4a also shows a tendency to support light-dependent Cry functions, indicating that it might act in the light-input pathway of zebrafish.

Analysis of the P. lividus sea urchin genome highlights contrasting trends of genomic and regulatory evolution in deuterostomes.

Sea urchins are emblematic models in developmental biology and display several characteristics that set them apart from other deuterostomes. To uncover the genomic cues that may underlie these specificities, we generated a chromosome-scale genome assembly for the sea urchin Paracentrotus lividus and an extensive gene expression and epigenetic profiles of its embryonic development. We found that, unlike vertebrates, sea urchins retained ancestral chromosomal linkages but underwent very fast intrachromosomal gene order mixing. We identified a burst of gene duplication in the echinoid lineage and showed that some of these expanded genes have been recruited in novel structures (water vascular system, Aristotle's lantern, and skeletogenic micromere lineage). Finally, we identified gene-regulatory modules conserved between sea urchins and chordates. Our results suggest that gene-regulatory networks controlling development can be conserved despite extensive gene order rearrangement.

Molecular and Cellular Characterization of the TH Pathway in the Sea Urchin Strongylocentrotus purpuratus.

Thyroid Hormones (THs) are a class of signaling molecules produced by coupling iodine with tyrosine residues. In vertebrates, extensive data support their important role in a variety of processes such as metabolism, development and metamorphosis. On the other hand, in invertebrates, the synthesis and role of the THs have been, so far, poorly investigated, thus limiting our understanding of the function and evolution of this important animal signaling pathway. In sea urchins, for example, while several studies focused on the availability and function of external sources of iodotyrosines, preliminary evidence suggests that an endogenous TH pathway might be in place. Here, integrating available literature with an in silico analysis, various homologous genes of the vertebrate TH molecular toolkit have been identified in the sea urchin Strongylocentrotus purpuratus. They include genes involved in the synthesis (Sp-Pxdn), metabolism (Sp-Dios), transport (Sp-Ttrl, Sp-Mct7/8/10) and response (Sp-Thr, Sp-Rxr and Sp-Integrin αP) to thyroid hormones. To understand the cell type(s) involved in TH synthesis and/or response, we studied the spatial expression of the TH toolkit during urchin development. Exploiting single-cell transcriptomics data in conjunction with in situ hybridization and immunohistochemistry, we identified cell types that are potentially producing or responding to THs in the sea urchin. Finally, growing sea urchin embryos until the larva stage with and without a source of inorganic iodine, we provided evidence that iodine organification is important for larval skeleton growth.

Neurogenesis during Brittle Star Arm Regeneration Is Characterised by a Conserved Set of Key Developmental Genes.

Neural regeneration is very limited in humans but extremely efficient in echinoderms. The brittle star Amphiura filiformis can regenerate both components of its central nervous system as well as the peripheral system, and understanding the molecular mechanisms underlying this ability is key for evolutionary comparisons not only within the echinoderm group, but also wider within deuterostomes. Here we characterise the neural regeneration of this brittle star using a combination of immunohistochemistry, in situ hybridization and Nanostring nCounter to determine the spatial and temporal expression of evolutionary conserved neural genes. We find that key genes crucial for the embryonic development of the nervous system in sea urchins and other animals are also expressed in the regenerating nervous system of the adult brittle star in a hierarchic and spatio-temporally restricted manner.

The Development and Neuronal Complexity of Bipinnaria Larvae of the Sea Star Asterias rubens.

Free-swimming planktonic larvae are a key stage in the development of many marine phyla, and studies of these organisms have contributed to our understanding of major genetic and evolutionary processes. Although transitory, these larvae often attain a remarkable degree of tissue complexity, with well-defined musculature and nervous systems. Among the best studied are larvae belonging to the phylum Echinodermata, but with work largely focused on the pluteus larvae of sea urchins (class Echinoidea). The greatest diversity of larval strategies among echinoderms is found in the class Asteroidea (sea stars), organisms that are rapidly emerging as experimental systems for genetic and developmental studies. However, the bipinnaria larvae of sea stars have only been studied in detail in a small number of species and although they have been relatively well described neuro-anatomically, they are poorly understood neurochemically. Here, we have analyzed embryonic development and bipinnaria larval anatomy in the common North Atlantic sea star Asterias rubens, using a variety of staining methods in combination with confocal microscopy. Importantly, the chemical complexity of the nervous system of bipinnaria larvae was revealed through use of a diverse set of antibodies, with identification of at least three centers of differing neurochemical signature within the previously described nervous system: the anterior apical organ, oral region, and ciliary bands. Furthermore, the anatomy of the musculature and sites of cell division in bipinnaria larvae was analyzed. Comparisons of developmental progression and molecular anatomy across the Echinodermata provided a basis for hypotheses on the shared evolutionary and developmental processes that have shaped this group of animals. We conclude that bipinnaria larvae appear to be remarkably conserved across ∼200 million years of evolutionary time and may represent a strong evolutionary and/or developmental constraint on species utilizing this larval strategy.

FGF signalling plays similar roles in development and regeneration of the skeleton in the brittle star Amphiura filiformis.

Regeneration as an adult developmental process is in many aspects similar to embryonic development. Although many studies point out similarities and differences, no large-scale, direct and functional comparative analyses between development and regeneration of a specific cell type or structure in one animal exist. Here, we use the brittle star Amphiura filiformis to characterise the role of the FGF signalling pathway during skeletal development in embryos and arm regeneration. In both processes, we find ligands expressed in ectodermal cells that flank underlying skeletal mesenchymal cells, which express the receptors. Perturbation of FGF signalling showed inhibited skeleton formation in both embryogenesis and regeneration, without affecting other key developmental processes. Differential transcriptome analysis finds mostly differentiation genes rather than transcription factors to be downregulated in both contexts. Moreover, comparative gene analysis allowed us to discover brittle star-specific differentiation genes. In conclusion, our results show that the FGF pathway is crucial for skeletogenesis in the brittle star, as in other deuterostomes, and provide evidence for the re-deployment of a developmental gene regulatory module during regeneration.

Post-metamorphic skeletal growth in the sea urchin Paracentrotus lividus and implications for body plan evolution.

BACKGROUND: Understanding the molecular and cellular processes that underpin animal development are crucial for understanding the diversity of body plans found on the planet today. Because of their abundance in the fossil record, and tractability as a model system in the lab, skeletons provide an ideal experimental model to understand the origins of animal diversity. We herein use molecular and cellular markers to understand the growth and development of the juvenile sea urchin (echinoid) skeleton. RESULTS: We developed a detailed staging scheme based off of the first ~ 4 weeks of post-metamorphic life of the regular echinoid Paracentrotus lividus. We paired this scheme with immunohistochemical staining for neuronal, muscular, and skeletal tissues, and fluorescent assays of skeletal growth and cell proliferation to understand the molecular and cellular mechanisms underlying skeletal growth and development of the sea urchin body plan. CONCLUSIONS: Our experiments highlight the role of skeletogenic proteins in accretionary skeletal growth and cell proliferation in the addition of new metameric tissues. Furthermore, this work provides a framework for understanding the developmental evolution of sea urchin body plans on macroevolutionary timescales.

Ultrastructural and molecular analysis of the origin and differentiation of cells mediating brittle star skeletal regeneration.

BACKGROUND: Regeneration is the ability to re-grow body parts or tissues after trauma, and it is widespread across metazoans. Cells involved in regeneration can arise from a pool of undifferentiated proliferative cells or be recruited from pre-existing differentiated tissues. Both mechanisms have been described in different phyla; however, the cellular and molecular mechanisms employed by different animals to restore lost tissues as well as the source of cells involved in regeneration remain largely unknown. Echinoderms are a clade of deuterostome invertebrates that show striking larval and adult regenerative abilities in all extant classes. Here, we use the brittle star Amphiura filiformis to investigate the origin and differentiation of cells involved in skeletal regeneration using a combination of microscopy techniques and molecular markers. RESULTS: Our ultrastructural analyses at different regenerative stages identify a population of morphologically undifferentiated cells which appear in close contact with the proliferating epithelium of the regenerating aboral coelomic cavity. These cells express skeletogenic marker genes, such as the transcription factor alx1 and the differentiation genes c-lectin and msp130L, and display a gradient of morphological differentiation from the aboral coelomic cavity towards the epidermis. Cells closer to the epidermis, which are in contact with developing spicules, have the morphology of mature skeletal cells (sclerocytes), and express several skeletogenic transcription factors and differentiation genes. Moreover, as regeneration progresses, sclerocytes show a different combinatorial expression of genes in various skeletal elements. CONCLUSIONS: We hypothesize that sclerocyte precursors originate from the epithelium of the proliferating aboral coelomic cavity. As these cells migrate towards the epidermis, they differentiate and start secreting spicules. Moreover, our study shows that molecular and cellular processes involved in skeletal regeneration resemble those used during skeletal development, hinting at a possible conservation of developmental programmes during adult regeneration. Finally, we highlight that many genes involved in echinoderm skeletogenesis also play a role in vertebrate skeleton formation, suggesting a possible common origin of the deuterostome endoskeleton pathway.

Unravelling the evolutionary history of kisspeptin.

Experiments in sea cucumbers reveal how the physiological responses regulated by a neuropeptide called kisspeptin have evolved.

Extracellular matrix gene expression during arm regeneration in Amphiura filiformis.

Extracellular matrix (ECM) plays a dynamic role during tissue development and re-growth. Body part regeneration efficiency relies also on effective ECM remodelling and deposition. Among invertebrates, echinoderms are well known for their striking regenerative abilities since they can rapidly regenerate functioning complex structures. To gather insights on the involvement of ECM during arm regeneration, the brittle star Amphiura filiformis was chosen as experimental model. Eight ECM genes were identified and cloned, and their spatio-temporal and quantitative expression patterns were analysed by means of whole mount in situ hybridisation and quantitative PCR on early and advanced regenerative stages. Our results show that almost none of the selected ECM genes are expressed at early stages of regeneration, suggesting a delay in their activation that may be responsible for the high regeneration efficiency of these animals, as described for other echinoderms and in contrast to most vertebrates. Moreover, at advanced stages, these genes are spatially and temporally differentially expressed, suggesting that the molecular regulation of ECM deposition/remodelling varies throughout the regenerative process. Phylogenetic analyses of the identified collagen-like genes reveal complex evolutionary dynamics with many rounds of duplications and losses and pinpointed their homologues in selected vertebrates. The study of other ECM genes will allow a better understanding of ECM contribution to brittle star arm regeneration.

A conceptual history of the "regulatory genome": From Theodor Boveri to Eric Davidson.

The formalization of the idea of "Regulatory Genome" is a recent one. However, it stems from a long tradition in the study of how the genetic information is transferred between generations. Theodore Boveri suggested for the first time that the whole genome participates in the shaping of individuals. Through a long lineage of researchers, we have learned how this whole-genome activity is regulated, in space and time. It is, however, due to the insights and experimental approaches taken by different researchers, among them Eric Davidson and associates, that we understand the mechanistic basis of this regulation. Whole batteries of regulatory genes interact through their cis-regulatory modules, generating a precise pattern of cross-controlled gene activity (Gene Regulatory Networks). How these genes are deployed in development and evolution has become an area of vibrant research. Here we revisit the history of this intellectual endeavour, taking as key defining points along this historical trajectory the contributions of Theodor Boveri and Eric Davidson.

Evolutionary transition between invertebrates and vertebrates via methylation reprogramming in embryogenesis.

Major evolutionary transitions are enigmas, and the most notable enigma is between invertebrates and vertebrates, with numerous spectacular innovations. To search for the molecular connections involved, we asked whether global epigenetic changes may offer a clue by surveying the inheritance and reprogramming of parental DNA methylation across metazoans. We focused on gametes and early embryos, where the methylomes are known to evolve divergently between fish and mammals. Here, we find that methylome reprogramming during embryogenesis occurs neither in pre-bilaterians such as cnidarians nor in protostomes such as insects, but clearly presents in deuterostomes such as echinoderms and invertebrate chordates, and then becomes more evident in vertebrates. Functional association analysis suggests that DNA methylation reprogramming is associated with development, reproduction and adaptive immunity for vertebrates, but not for invertebrates. Interestingly, the single HOX cluster of invertebrates maintains unmethylated status in all stages examined. In contrast, the multiple HOX clusters show dramatic dynamics of DNA methylation during vertebrate embryogenesis. Notably, the methylation dynamics of HOX clusters are associated with their spatiotemporal expression in mammals. Our study reveals that DNA methylation reprogramming has evolved dramatically during animal evolution, especially after the evolutionary transitions from invertebrates to vertebrates, and then to mammals.

New Neuronal Subtypes With a "Pre-Pancreatic" Signature in the Sea Urchin Stongylocentrotus purpuratus.

Neurons and pancreatic endocrine cells have a common physiology and express a similar toolkit of transcription factors during development. To explain these common features, it has been hypothesized that pancreatic cells most likely co-opted a pre-existing gene regulatory program from ancestral neurons. To test this idea, we looked for neurons with a "pre-pancreatic" program in an early-branched deuterostome, the sea urchin. Only vertebrates have a proper pancreas, however, our lab previously found that cells with a pancreatic-like signature are localized within the sea urchin embryonic gut. We also found that the pancreatic transcription factors Xlox/Pdx1 and Brn1/2/4 co-localize in a sub-population of ectodermal cells. Here, we find that the ectodermal SpLox+ SpBrn1/2/4 cells are specified as SpSoxC and SpPtf1a neuronal precursors that become the lateral ganglion and the apical organ neurons. Two of the SpLox+ SpBrn1/2/4 cells also express another pancreatic transcription factor, the LIM-homeodomain gene islet-1. Moreover, we find that SpLox neurons produce the neuropeptide SpANP2, and that SpLox regulates SpANP2 expression. Taken together, our data reveal that there is a subset of sea urchin larval neurons with a gene program that predated pancreatic cells. These findings suggest that pancreatic endocrine cells co-opted a regulatory signature from an ancestral neuron that was already present in an early-branched deuterostome.

Neuropeptidergic Systems in Pluteus Larvae of the Sea Urchin Strongylocentrotus purpuratus: Neurochemical Complexity in a "Simple" Nervous System.

The nervous system of the free-living planktonic larvae of sea urchins is relatively "simple," but sufficiently complex to enable sensing of the environment and control of swimming and feeding behaviors. At the pluteus stage of development, the nervous system comprises a central ganglion of serotonergic neurons located in the apical organ and sensory and motor neurons associated with the ciliary band and the gut. Neuropeptides are key mediators of neuronal signaling in nervous systems but currently little is known about neuropeptidergic systems in sea urchin larvae. Analysis of the genome sequence of the sea urchin Strongylocentrotus purpuratus has enabled the identification of 38 genes encoding neuropeptide precursors (NP) in this species. Here we characterize for the first time the expression of nine of these NP genes in S. purpuratus larvae, providing a basis for a functional understanding of the neurochemical organization of the larval nervous system. In order to accomplish this we used single and double in situ hybridization, coupled with immunohistochemistry, to investigate NP gene expression in comparison with known markers (e.g., the neurotransmitter serotonin). Several sub-populations of cells that express one or more NP genes were identified, which are located in the apica organ, at the base of the arms, around the mouth, in the ciliary band and in the mid- and fore-gut. Furthermore, high levels of cell proliferation were observed in neurogenic territories, consistent with an increase in the number of neuropeptidergic cells at late larval stages. This study has revealed that the sea urchin larval nervous system is far more complex at a neurochemical level than was previously known. Our NP gene expression map provides the basis for future work, aimed at understanding the role of diverse neuropeptides in control of various aspects of embryonic and larval behavior.

Regeneration in Stellate Echinoderms: Crinoidea, Asteroidea and Ophiuroidea.

Reparative regeneration is defined as the replacement of lost adult body parts and is a phenomenon widespread yet highly variable among animals. This raises the question of which key cellular and molecular mechanisms have to be implemented in order to efficiently and correctly replace entire body parts in any animal. To address this question, different studies using an integrated cellular and functional genomic approach to study regeneration in stellate echinoderms (crinoids, asteroids and ophiuroids) had been carried out over the last few years. The phylum Echinodermata is recognized for the striking regeneration potential shown by the members of its different clades. Indeed, stellate echinoderms are considered among the most useful and tractable experimental models for carrying comprehensive studies focused on ecological, developmental and evolutionary aspects. Moreover, most of them are tractable in the laboratory and, thus, should allow us to understand the underlying mechanisms, cellular and molecular, which are involved. Here, a comprehensive analysis of the cellular/histological components of the regenerative process in crinoids, asteroids and ophiuroids is described and compared. However, though this knowledge provided us with some clear insights into the global distribution of cell types at different times, it did not explain us how the recruited cells are specified (and from which precursors) over time and where are they located in the animal. The precise answer to these queries needs the incorporation of molecular approaches, both descriptive and functional. Yet, the molecular studies in stellate echinoderms are still limited to characterization of some gene families and protein factors involved in arm regeneration but, at present, have not shed light on most of the basic mechanisms. In this context, further studies are needed specifically to understand the role of regulatory factors and their spatio-temporal deployment in the growing arms. A focus on developing functional tools over the next few years should be of fundamental importance.

Developmental transcriptomics of the brittle star Amphiura filiformis reveals gene regulatory network rewiring in echinoderm larval skeleton evolution.

BACKGROUND: Amongst the echinoderms the class Ophiuroidea is of particular interest for its phylogenetic position, ecological importance and developmental and regenerative biology. However, compared to other echinoderms, notably echinoids (sea urchins), relatively little is known about developmental changes in gene expression in ophiuroids. To address this issue, we have generated and assembled a large RNAseq data set of four key stages of development in the brittle star Amphiura filiformis and a de novo reference transcriptome of comparable quality to that of a model echinoderm-the sea urchin Strongylocentrotus purpuratus. Furthermore, we provide access to the new data via a web interface: http://www.echinonet.eu/shiny/Amphiura_filiformis/ . RESULTS: We have identified highly conserved genes associated with the development of a biomineralised skeleton. We also identify important class-specific characters, including the independent duplication of the msp130 class of genes in different echinoderm classes and the unique occurrence of spicule matrix (sm) genes in echinoids. Using a new quantification pipeline for our de novo transcriptome, validated with other methodologies, we find major differences between brittle stars and sea urchins in the temporal expression of many transcription factor genes. This divergence in developmental regulatory states is more evident in early stages of development when cell specification begins, rather than when cells initiate differentiation. CONCLUSIONS: Our findings indicate that there has been a high degree of gene regulatory network rewiring and clade-specific gene duplication, supporting the hypothesis of a convergent evolution of larval skeleton development in echinoderms.

Fundamental aspects of arm repair phase in two echinoderm models.

Regeneration is a post-embryonic developmental process that ensures complete morphological and functional restoration of lost body parts. The repair phase is a key step for the effectiveness of the subsequent regenerative process: in vertebrates, efficient re-epithelialisation, rapid inflammatory/immune response and post-injury tissue remodelling are fundamental aspects for the success of this phase, their impairment leading to an inhibition or total prevention of regeneration. Among deuterostomes, echinoderms display a unique combination of striking regenerative abilities and diversity of useful experimental models, although still largely unexplored. Therefore, the brittle star Amphiura filiformis and the starfish Echinaster sepositus were here used to comparatively investigate the main repair phase events after injury as well as the presence and expression of immune system and extracellular matrix (i.e. collagen) molecules using both microscopy and molecular tools. Our results showed that emergency reaction and re-epithelialisation are similar in both echinoderm models, being faster and more effective than in mammals. Moreover, in comparison to the latter, both echinoderms showed delayed and less abundant collagen deposition at the wound site (absence of fibrosis). The gene expression patterns of molecules related to the immune response, such as Ese-fib-like (starfishes) and Afi-ficolin (brittle stars), were described for the first time during echinoderm regeneration providing promising starting points to investigate the immune system role in these regeneration models. Overall, the similarities in repair events and timing within the echinoderms and the differences with what has been reported in mammals suggest that effective repair processes in echinoderms play an important role for their subsequent ability to regenerate. Targeted molecular and functional analyses will shed light on the evolution of these abilities in the deuterostomian lineage.

Sex and Pubertal Differences in the Type 1 Interferon Pathway Associate With Both X Chromosome Number and Serum Sex Hormone Concentration.

Type 1 interferons (IFN) are an antiviral cytokine family, important in juvenile onset systemic lupus erythematosus (jSLE) which is more common in females, around puberty. We report that plasmacytoid dendritic cells (pDC) from healthy females produced more type 1 IFN after toll like receptor (TLR) 7 signaling than males, even before puberty, but that puberty itself associated with increased production of type 1 IFN. A unique human model allows us to show that this was related to X chromosome number, and serum testosterone concentration, in a manner which differed depending on the number of X chromosomes present. In addition, we have showed that pDC were more activated in females overall, and immune cell TLR7 gene expression was higher in females after puberty. Therefore, sex hormones and X chromosome number were associated individually and interactively with the type 1 IFN response, which contributes to our understanding of why females are more likely to develop an IFN mediated disease like jSLE after puberty.

Skeletal regeneration in the brittle star Amphiura filiformis.

BACKGROUND: Brittle stars regenerate their whole arms post-amputation. Amphiura filiformis can now be used for molecular characterization of arm regeneration due to the availability of transcriptomic data. Previous work showed that specific developmental transcription factors known to take part in echinoderm skeletogenesis are expressed during adult arm regeneration in A. filiformis; however, the process of skeleton formation remained poorly understood. Here, we present the results of an in-depth microscopic analysis of skeletal morphogenesis during regeneration, using calcein staining, EdU labeling and in situ hybridization. RESULTS: To better compare different samples, we propose a staging system for the early A. filiformis arm regeneration stages based on morphological landmarks identifiable in living animals and supported by histological analysis. We show that the calcified spicules forming the endoskeleton first appear very early during regeneration in the dermal layer of regenerates. These spicules then mature into complex skeletal elements of the differentiated arm during late regeneration. The mesenchymal cells in the dermal area express the skeletal marker genes Afi-c-lectin, Afi-p58b and Afi-p19; however, EdU labeling shows that these dermal cells do not proliferate. CONCLUSIONS: A. filiformis arms regenerate through a consistent set of developmental stages using a distalization-intercalation mode, despite variability in regeneration rate. Skeletal elements form in a mesenchymal cell layer that does not proliferate and thus must be supplied from a different source. Our work provides the basis for future cellular and molecular studies of skeleton regeneration in brittle stars.