Distinct WNT pathways regulating AER formation and dorsoventral polarity in the chick limb bud

Evidence for Fgf and Wnt regulation of Lhx2 during limb development via two limb-specific Lhx2-associated cis-regulatory modules

Introduction

In vertebrate limb morphogenesis, wingless-related integration site (Wnt) proteins and fibroblast growth factors (Fgfs) secreted from the apical ectodermal ridge (AER) coordinate proximodistal outgrowth. Fgfs also sustain sonic hedgehog (Shh) in the zone of polarizing activity (ZPA). Shh directs anteroposterior patterning and expansion and regulates AER-Fgfs, establishing a positive regulatory feedback loop that is vital in sustaining limb outgrowth. The transcription factor LIM homeodomain 2 (Lhx2) is expressed in the distal mesoderm and coordinates AER and ZPA signals that control cellular proliferation, differentiation, and shaping of the developing limb. Yet how Lhx2 is transcriptionally regulated to support such functions has only been partially characterized.

Methods/Results

We have identified two limb-specific cis-regulatory modules (CRMs) active within the Lhx2 expression domain in the limb. Chromatin conformation analysis of the Lhx2 locus in mouse embryonic limb bud cells predicted CRMs-Lhx2 promoter interactions. Single-cell RNA-sequencing analysis of limb bud cells revealed co-expression of several Fgf-related Ets and Wnt-related Tcf/Lef transcripts in Lhx2-expressing cells. Additionally, disruption of Ets and Tcf/Lef binding sites resulted in loss of reporter-driven CRM activity. Finally, binding of β-catenin to both Lhx2-associated CRMs supports the associated binding of Tcf/Lef transcription factors.

Discussion

These results suggest a role for Ets and Tcf/Lef transcription factors in the regulation of Lhx2 expression through these limb-specific Lhx2-associated CRMs. Moreover, these CRMs provide a mechanism for Fgf and Wnt signaling to localize and maintain distal Lhx2 expression during vertebrate limb development.

Canonical Wnt signalling from the area opaca induces and maintains the marginal zone in pre-primitive-streak stage chick embryos

In chick embryos before primitive streak formation, the outermost extra-embryonic region, known as the area opaca (AO), was generally thought to act only by providing nutrients and mechanical support to the embryo. Immediately internal to the AO is a ring of epiblast called the marginal zone (MZ), separating the former from the inner area pellucida (AP) epiblast. The MZ does not contribute cells to any part of the embryo but is involved in determining the position of primitive streak formation from the adjacent AP epiblast. Recently, it was discovered that the AO can induce an MZ from AP epiblast. Here, we explore the nature of this inductive signal. We find that WNT8C is highly expressed in the AO, whereas canonical Wnt pathway targets are enriched in the MZ, along with strong nuclear β-catenin localization. Using isolation and recombination experiments combined with gain- and loss-of-function by exogenous chemical modulators of the pathway, we reveal that Wnt signalling is essential for induction and maintenance of the MZ, as well as sufficient to induce MZ properties in AP epiblast. We propose that canonical Wnt signalling is responsible for induction of the MZ by the area opaca.

The pleomorphic cholesterol sensing motifs of transmembrane proteins

Millions of years of phylogenetic evolution have shaped the crosstalk between sterols and membrane-embedded proteins. This lengthy process, which began before the appearance of eukaryotic cells, has sculpted the two types of molecules to cover a wide spectrum of structural interconnectedness, ranging from rapid touch-and-go hits of low-affinity between surfaces to stronger lock-and-key type structural contacts. The former usually involve relatively loose contacts between linear amino acid sequences on the membrane-exposed transmembrane domains of the protein, readily accessible to the sterols as they briefly visit clefts between adjacent transmembrane segments while in rapid exchange with the bulk lipid bilayer. This operational mode is probably the most ancestral one, since it was already present in primitive bacteria interacting with hopanoid lipids. At the other end of this spectrum are more complex cholesterol binding sites that have required the acquisition of complex 3D non-sequential segments of the membrane protein to establish stereochemically elaborate 3D designs complementary to the rough and smooth surfaces of the eukaryotic neutral lipid, cholesterol. This short review explores cholesterol-membrane protein interactions using membrane protein paradigms having in common their participation in intercellular communications neurotransmission, hormone signalling, amino acid/neurotransmitter transport- and in cancer.

The limb dorsoventral axis: Lmx1b's role in development, pathology, evolution, and regeneration

The limb anatomy displays well-defined dorsal and ventral compartments, housing extensor, and flexor muscles, which play a crucial role in facilitating limb locomotion and manipulation. Despite its importance, the study of limb dorsoventral patterning has been relatively neglected compared to the other two axes leaving many crucial questions about the genes and developmental processes implicated unanswered. This review offers a thorough overview of the current understanding of limb dorsoventral patterning, synthesizing classical literature with recent research. It covers the specification of dorsal fate in the limb mesoderm and its subsequent translation into dorsal morphologies-a process directed by the transcription factor Lmx1b. We also discuss the potential role of dorsoventral patterning in the evolution of paired appendages and delve into the involvement of LMX1B in Nail-Patella syndrome, discussing the molecular and genetic aspects underlying this condition. Finally, the potential role of dorsoventral polarity in digit tip regeneration, a prominent instance of multi-tissue regeneration in mammals is also considered. We anticipate that this review will renew interest in a process that is critical to limb function and evolutionary adaptations but has nonetheless been overlooked.

Protein signaling and morphological development of the tail fluke in the embryonic beluga whale (Delphinapterus leucas)

BACKGROUND

During the land-to-sea transition of cetaceans (whales, dolphins, and porpoises), the hindlimbs were lost and replaced by an elaborate tail fluke that evolved 32 Ma. All modern cetaceans utilize flukes for lift-based propulsion, and nothing is known of this organ's molecular origins during embryonic development. This study utilizes immunohistochemistry to identify the spatiotemporal location of protein signals known to drive appendage outgrowth in other vertebrates (e.g., Sonic Hedgehog [SHH], GREMLIN [GREM], wingless-type family member 7a [WNT], and fibroblast growth factors [FGFs]) and to test the hypothesis that signals associated with outgrowth and patterning of the tail fluke are similar to a tetrapod limb. Specifically, this study utilizes an embryo of a beluga whale (Delphinapterus leucas) as a case study.

RESULTS

Results showed epidermal signals of WNT and FGFs, and mesenchymal/epidermal signals of SHH and GREM. These patterns are most consistent with vertebrate limb development. Overall, these data are most consistent with the hypothesis that outgrowth of tail flukes in cetaceans employs a signaling pattern that suggests genes essential for limb outgrowth and patterning shape this evolutionarily novel appendage.

CONCLUSIONS

While these data add insights into the molecular signals potentially driving the evolution and development of tail flukes in cetaceans, further exploration of the molecular drivers of fluke development is required.

Conserved regulatory switches for the transition from natal down to juvenile feather in birds

The transition from natal downs for heat conservation to juvenile feathers for simple flight is a remarkable environmental adaptation process in avian evolution. However, the underlying epigenetic mechanism for this primary feather transition is mostly unknown. Here we conducted time-ordered gene co-expression network construction, epigenetic analysis, and functional perturbations in developing feather follicles to elucidate four downy-juvenile feather transition events. We report that extracellular matrix reorganization leads to peripheral pulp formation, which mediates epithelial-mesenchymal interactions for branching morphogenesis. α-SMA (ACTA2) compartmentalizes dermal papilla stem cells for feather renewal cycling. LEF1 works as a key hub of Wnt signaling to build rachis and converts radial downy to bilateral symmetry. Novel usage of scale keratins strengthens feather sheath with SOX14 as the epigenetic regulator. We show that this primary feather transition is largely conserved in chicken (precocial) and zebra finch (altricial) and discuss the possibility that this evolutionary adaptation process started in feathered dinosaurs.

Transition from natal downs to juvenile feathers: conserved regulatory switches in Neoaves

The transition from natal downs for heat conservation to juvenile feathers for simple flight is a remarkable environmental adaptation process in avian evolution. However, the underlying epigenetic mechanism for this primary feather transition is mostly unknown. Here we conducted time-ordered gene co-expression network construction, epigenetic analysis, and functional perturbations in developing feather follicles to elucidate four downy-juvenile feather transition events. We discovered that LEF1 works as a key hub of Wnt signaling to build rachis and converts radial downy to bilateral symmetry. Extracellular matrix reorganization leads to peripheral pulp formation, which mediates epithelial -mesenchymal interactions for branching morphogenesis. ACTA2 compartments dermal papilla stem cells for feather cycling. Novel usage of scale keratins strengthens feather sheath with SOX14 as the epigenetic regulator. We found this primary feather transition largely conserved in chicken (precocious) and zebra finch (altricial) and discussed the possibility that this evolutionary adaptation process started in feathered dinosaurs.

Primary cilia in skeletal development and disease

Primary cilia are non-motile, microtubule-based sensory organelle present in most vertebrate cells with a fundamental role in the modulation of organismal development, morphogenesis, and repair. Here we focus on the role of primary cilia in embryonic and postnatal skeletal development. We examine evidence supporting its involvement in physiochemical and developmental signaling that regulates proliferation, patterning, differentiation and homeostasis of osteoblasts, chondrocytes, and their progenitor cells in the skeleton. We discuss how signaling effectors in mechanotransduction and bone development, such as Hedgehog, Wnt, Fibroblast growth factor and second messenger pathways operate at least in part at the primary cilium. The relevance of primary cilia in bone formation and maintenance is underscored by a growing list of rare genetic skeletal ciliopathies. We collate these findings and summarize the current understanding of molecular factors and mechanisms governing primary ciliogenesis and ciliary function in skeletal development and disease.

Ectodermal Wnt signaling, cell fate determination, and polarity of the skate gill arch skeleton

The gill skeleton of cartilaginous fishes (sharks, skates, rays, and holocephalans) exhibits a striking anterior-posterior polarity, with a series of fine appendages called branchial rays projecting from the posterior margin of the gill arch cartilages. We previously demonstrated in the skate (Leucoraja erinacea) that branchial rays derive from a posterior domain of pharyngeal arch mesenchyme that is responsive to Sonic hedgehog (Shh) signaling from a distal gill arch epithelial ridge (GAER) signaling centre. However, how branchial ray progenitors are specified exclusively within posterior gill arch mesenchyme is not known. Here, we show that genes encoding several Wnt ligands are expressed in the ectoderm immediately adjacent to the skate GAER, and that these Wnt signals are transduced largely in the anterior arch environment. Using pharmacological manipulation, we show that inhibition of Wnt signalling results in an anterior expansion of Shh signal transduction in developing skate gill arches, and in the formation of ectopic anterior branchial ray cartilages. Our findings demonstrate that ectodermal Wnt signalling contributes to gill arch skeletal polarity in skate by restricting Shh signal transduction and chondrogenesis to the posterior arch environment and highlights the importance of signalling interactions at embryonic tissue boundaries for cell fate determination in vertebrate pharyngeal arches.

Gene expression changes during the evolution of the tetrapod limb

Major changes in the vertebrate anatomy have preceded the conquest of land by the members of this taxon, and continuous changes in limb shape and use have occurred during the later radiation of tetrapods. While the main, conserved mechanisms of limb development have been discerned over the past century using a combination of classical embryological and molecular methods, only recent advances made it possible to identify and study the regulatory changes that have contributed to the evolution of the tetrapod appendage. These advances include the expansion of the model repertoire from traditional genetic model species to non-conventional ones, a proliferation of predictive mathematical models that describe gene interactions, an explosion in genomic data and the development of high-throughput methodologies. These revolutionary innovations make it possible to identify specific mutations that are behind specific transitions in limb evolution. Also, as we continue to apply them to more and more extant species, we can expect to gain a fine-grained view of this evolutionary transition that has been so consequential for our species as well.

Cellular responses in the FGF10-mediated improvement of hindlimb regenerative capacity in Xenopus laevis revealed by single-cell transcriptomics

Xenopus laevis tadpoles possess regenerative capacity in their hindlimb buds at early developmental stages (stages ~52-54); they can regenerate complete hindlimbs with digits after limb bud amputation. However, they gradually lose their regenerative capacity as metamorphosis proceeds. Tadpoles in late developmental stages regenerate fewer digits (stage ~56), or only form cartilaginous spike without digits or joints (stage ~58 or later) after amputation. Previous studies have shown that administration of fibroblast growth factor 10 (FGF10) in late-stage (stage 56) tadpole hindlimb buds after amputation can improve their regenerative capacity, which means that the cells responding to FGF10 signaling play an important role in limb bud regeneration. In this study, we performed single-cell RNA sequencing (scRNA-seq) of hindlimb buds that were amputated and administered FGF10 by implanting FGF10-soaked beads at a late stage (stage 56), and explored cell clusters exhibiting a differential gene expression pattern compared with that in controls treated with phosphate-buffered saline. The scRNA-seq data showed expansion of fgf8-expressing cells in the cluster of the apical epidermal cap of FGF10-treated hindlimb buds, which was reported previously, indicating that the administration of FGF10 was successful. On analysis, in addition to the epidermal cluster, a subset of myeloid cells and a newly identified cluster of steap4-expressing cells showed remarkable differences in their gene expression profiles between the FGF10- or phosphate-buffered saline-treatment conditions, suggesting a possible role of these clusters in improving the regenerative capacity of hindlimbs via FGF10 administration.

Transgenic quails reveal dynamic TCF/β-catenin signaling during avian embryonic development

The Wnt/β-catenin signaling pathway is highly conserved throughout evolution, playing crucial roles in several developmental and pathological processes. Wnt ligands can act at a considerable distance from their sources and it is therefore necessary to examine not only the Wnt-producing but also the Wnt-receiving cells and tissues to fully appreciate the many functions of this pathway. To monitor Wnt activity, multiple tools have been designed which consist of multimerized Wnt signaling response elements (TCF/LEF binding sites) driving the expression of fluorescent reporter proteins (e.g. GFP, RFP) or of LacZ. The high stability of those reporters leads to a considerable accumulation in cells activating the pathway, thereby making them easily detectable. However, this makes them unsuitable to follow temporal changes of the pathway’s activity during dynamic biological events. Even though fluorescent transcriptional reporters can be destabilized to shorten their half-lives, this dramatically reduces signal intensities, particularly when applied in vivo. To alleviate these issues, we developed two transgenic quail lines in which high copy number (12× or 16×) of the TCF/LEF binding sites drive the expression of destabilized GFP variants. Translational enhancer sequences derived from viral mRNAs were used to increase signal intensity and specificity. This resulted in transgenic lines efficient for the characterization of TCF/β-catenin transcriptional dynamic activities during embryogenesis, including using in vivo imaging. Our analyses demonstrate the use of this transcriptional reporter to unveil novel aspects of Wnt signaling, thus opening new routes of investigation into the role of this pathway during amniote embryonic development.

Changes in the Expression of Smooth Muscle Cell–Related Genes in Human Dermal Sheath Cup Cells Associated with the Treatment Outcome of Autologous Cell–Based Therapy for Male and Female Pattern Hair Loss

In a clinical study of autologous cell–based therapy using dermal sheath cup (DSC) cells, the treatment of hair loss showed improvements. However, the outcomes were variable. Here, correlations between marker gene expression in DSC cells and treatment outcomes were assessed to predict therapeutic efficacy. Overall, 32 DSC cell lines were used to evaluate correlations between marker gene expression and treatment outcomes. Correlations between vascular pericyte and preadipocyte marker expression and treatment outcomes were inconsistent. As smooth muscle cell markers, MYOCD correlated negatively with treatment outcomes and SRF consistently demonstrated an inverse correlation. Additionally, CALD1 correlated negatively and ACTA2 correlated inversely with treatment outcomes. DSC cell lines were divided into good and moderate/poor responders to further investigate the correlations. SRF and CALD1 were lower in a good responder compared with a moderate responder. Next, DSC cells were differentiated toward dermal papilla cells. Dermal papilla markers SOX2 and LEF1 before differentiation had moderate positive and inverse correlations with the treatment outcome, respectively. SOX2 after differentiation more consistently demonstrated a positive correlation. Significant downregulation of smooth muscle–related genes was also observed after differentiation. These findings revealed putative markers for preclinical evaluation of DSC cells to improve hair loss.

Canonical Wnt signaling and the regulation of divergent mesenchymal Fgf8 expression in axolotl limb development and regeneration

The expression of fibroblast growth factors (Fgf) ligands in a specialized epithelial compartment, the Apical Ectodermal Ridge (AER), is a conserved feature of limb development across vertebrate species. In vertebrates, Fgf 4, 8, 9, and 17 are all expressed in the AER. An exception to this paradigm is the salamander (axolotl) developing and regenerating limb, where key Fgf ligands are expressed in the mesenchyme. The mesenchymal expression of Amex.Fgf8 in axolotl has been suggested to be critical for regeneration. To date, there is little knowledge regarding what controls Amex.Fgf8 expression in the axolotl limb mesenchyme. A large body of mouse and chick studies have defined a set of transcription factors and canonical Wnt signaling as the main regulators of epidermal Fgf8 expression in these organisms. In this study, we address the hypothesis that alterations to one or more of these components during evolution has resulted in mesenchymal Amex.Fgf8 expression in the axolotl. To sensitively quantify gene expression with spatial precision, we combined optical clearing of whole-mount axolotl limb tissue with single molecule fluorescent in situ hybridization and a semiautomated quantification pipeline. Several candidate upstream components were found expressed in the axolotl ectoderm, indicating that they are not direct regulators of Amex.Fgf8 expression. We found that Amex.Wnt3a is expressed in axolotl limb epidermis, similar to chicken and mouse. However, unlike in amniotes, Wnt target genes are activated preferentially in limb mesenchyme rather than in epidermis. Inhibition and activation of Wnt signaling results in downregulation and upregulation of mesenchymal Amex.Fgf8 expression, respectively. These results implicate a shift in tissue responsiveness to canonical Wnt signaling from epidermis to mesenchyme as one step contributing to the unique mesenchymal Amex.Fgf8 expression seen in the axolotl.

The WNT7A/WNT7B/GPR124/RECK signaling module plays an essential role in mammalian limb development

In central nervous system vascular endothelial cells, signaling via the partially redundant ligands WNT7A and WNT7B requires two co-activator proteins, GPR124 and RECK. WNT7A and RECK have been shown previously to play a role in limb development, but the mechanism of RECK action in this context is unknown. The roles of WNT7B and GPR124 in limb development have not been investigated. Using combinations of conventional and/or conditional loss-of-function alleles for mouse Wnt7a, Wnt7b, Gpr124 and Reck, including a Reck allele that codes for a protein that is specifically defective in WNT7A/WNT7B signaling, we show that reductions in ligand and/or co-activator function synergize to cause reduced and dysmorphic limb bone growth. Two additional limb phenotypes – loss of distal Lmx1b expression and ectopic growth of nail-like structures – occur with reduced Wnt7a/Wnt7b gene copy number and, respectively, with Reck mutations and with combined Reck and Gpr124 mutations. A third limb phenotype – bleeding into a digit – occurs with the most severe combinations of Wnt7a/Wnt7b, Reck and Gpr124 mutations. These data imply that the WNT7A/WNT7B-FRIZZLED-LRP5/LRP6-GPR124-RECK signaling system functions as an integral unit in limb development.

Summary: Genetic analyses in mice show that the WNT7A/WNT7B-FRIZZLED-LRP5/LRP6-GPR124-RECK signaling system, first defined in the context of CNS angiogenesis and barrier development, also functions as an integral unit in limb development.

Wnt Signaling Coordinates the Expression of Limb Patterning Genes During Axolotl Forelimb Development and Regeneration

After amputation, axolotl salamanders can regenerate their limbs, but the degree to which limb regeneration recapitulates limb development remains unclear. One limitation in answering this question is our lack of knowledge about salamander limb development. Here, we address this question by studying expression patterns of genes important for limb patterning during axolotl salamander limb development and regeneration. We focus on the Wnt signaling pathway because it regulates multiple functions during tetrapod limb development, including limb bud initiation, outgrowth, patterning, and skeletal differentiation. We use fluorescence in situ hybridization to show the expression of Wnt ligands, Wnt receptors, and limb patterning genes in developing and regenerating limbs. Inhibition of Wnt ligand secretion permanently blocks limb bud outgrowth when treated early in limb development. Inhibiting Wnt signaling during limb outgrowth decreases the expression of critical signaling genes, including Fgf10, Fgf8, and Shh, leading to the reduced outgrowth of the limb. Patterns of gene expression are similar between developing and regenerating limbs. Inhibition of Wnt signaling during regeneration impacted patterning gene expression similarly. Overall, our findings suggest that limb development and regeneration utilize Wnt signaling similarly. It also provides new insights into the interaction of Wnt signaling with other signaling pathways during salamander limb development and regeneration.

Aberrant Expression of Lymphoid Enhancer-Binding Factor 1 (LEF1) in Hodgkin Lymphoma

Lymphoid enhancer-binding factor 1 (LEF1) is a transcription factor involved in T-cell maturation and is usually absent in mature B-cells. Previous studies have shown aberrant LEF1 expression as a sensitive and specific marker in chronic lymphocytic leukemia/small lymphocytic lymphoma. Our primary aims were i) to analyze LEF1 expression in classic Hodgkin lymphomas (CHL) including de novo and Richter syndrome (RS), and to assess if LEF1 can be a surrogate marker to assess clonal relationship in RS, and ii) to compare LEF1 expression in CHL and Nodular lymphocyte predominant Hodgkin lymphoma (NLPHL). We included 117 patients: 24 CHL-RS, 66 CHL-de novo and 27 NLPHL. There was no significant difference in LEF1 expression between CHL-RS and CHL-de novo (79.2% vs 87.9%, P = 0.299), or in type I and type II CHL-RS (75% vs 81.3%, P = 1.000). However, CHL showed a significantly higher LEF1 expression compared to NLPHL (85.6% vs 44.4%, P<0.0001). As the Wnt/β-catenin pathway directly regulates LEF1 expression in a β-catenin-dependent way, β-catenin expression was assessed in 76 cases and all were negative. Additionally, no association between EBV-positivity and LEF1-expression was detected. Overall, our findings show high LEF1 expression in CHL, regardless of RS or de novo, indicating LEF1 cannot be utilized as a surrogate marker to suggest clonal relationship in RS. Compared with CHL, LEF1 expression is significantly less common in NLPHL, further attesting that they are biologically distinct entities. The absent β-catenin expression suggests LEF1 expression is independent of Wnt/β-catenin signaling pathway in Hodgkin lymphomas.

Limb positioning and initiation: An evolutionary context of pattern and formation

Before limbs or fins, can be patterned and grow they must be initiated. Initiation of the limb first involves designating a portion of lateral plate mesoderm along the flank as the site of the future limb. Following specification, a myriad of cellular and molecular events interact to generate a bud that will grow and form the limb. The past three decades has provided a wealth of understanding on how those events generate the limb bud and how variations in them result in different limb forms. Comparatively, much less attention has been given to the earliest steps of limb formation and what impacts altering the position and initiation of the limb have had on evolution. Here, we first review the processes and pathways involved in these two phases of limb initiation, as determined from amniote model systems. We then broaden our scope to examine how variation in the limb initiation module has contributed to biological diversity in amniotes. Finally, we review what is known about limb initiation in fish and amphibians, and consider what mechanisms are conserved across vertebrates.

In ovo electroporation of chicken limb bud ectoderm: Electroporation to chick limb ectoderm

BACKGROUND

Deciphering how ectodermal tissues form, and how they maintain their integrity, is crucial for understanding epidermal development and pathogenesis. However, lack of simple and rapid gene manipulation techniques limits genetic studies to elucidate mechanisms underlying these events.

RESULTS

Here we describe an easy method for electroporation of chick limb bud ectoderm enabling gene manipulation during ectoderm development and wound healing. Taking advantage of a small parafilm well that constrains DNA plasmids locally and the fact that the limb ectoderm arises from a defined site, we target the limb ectoderm forming region by in ovo electroporation. This approach results in focal and efficient transgenesis of the limb ectodermal cells. Further, using a previously described Msx2 promoter, gene manipulation can be specifically targeted to the apical ectodermal ridge (AER), a signaling center regulating limb development. Using the electroporation technique to deliver a fluorescent marker into the embryonic limb ectoderm, we show its utility in performing time-lapse imaging during wound healing. This analysis revealed previously unrecognized dynamic remodeling of the actin cytoskeleton and lamellipodia formation at the edges of the wound. We find that the lamellipodia formation requires activity of Rac1 GTPase, suggesting its necessity for wound closure.

CONCLUSION

Our method is simple and easy. Thus, it would permit high throughput tests for gene function during limb ectodermal development and wound healing.

Wnt Signaling Drives Correlated Changes in Facial Morphology and Brain Shape

Canonical Wnt signaling plays multiple roles critical to normal craniofacial development while its dysregulation is known to be involved in structural birth defects of the face. However, when and how Wnt signaling influences phenotypic variation, including those associated with disease, remains unclear. One potential mechanism is via Wnt signaling’s role in the patterning of an early facial signaling center, the frontonasal ectodermal zone (FEZ), and its subsequent regulation of early facial morphogenesis. For example, Wnt signaling may directly alter the shape and/or magnitude of expression of the sonic hedgehog (SHH) domain in the FEZ. To test this idea, we used a replication-competent avian sarcoma retrovirus (RCAS) encoding Wnt3a to modulate its expression in the facial mesenchyme. We then quantified and compared ontogenetic changes in treated to untreated embryos in the three-dimensional (3D) shape of both the SHH expression domain of the FEZ, and the morphology of the facial primordia and brain using iodine-contrast microcomputed tomography imaging and 3D geometric morphometrics (3DGM). We found that increased Wnt3a expression in early stages of head development produces correlated variation in shape between both structural and signaling levels of analysis. In addition, altered Wnt3a activation disrupted the integration between the forebrain and other neural tube derivatives. These results show that activation of Wnt signaling influences facial shape through its impact on the forebrain and SHH expression in the FEZ, and highlights the close relationship between morphogenesis of the forebrain and midface.

The Progress of Stem Cell Technology for Skeletal Regeneration

Skeletal disorders, such as osteoarthritis and bone fractures, are among the major conditions that can compromise the quality of daily life of elderly individuals. To treat them, regenerative therapies using skeletal cells have been an attractive choice for patients with unmet clinical needs. Currently, there are two major strategies to prepare the cell sources. The first is to use induced pluripotent stem cells (iPSCs) or embryonic stem cells (ESCs), which can recapitulate the skeletal developmental process and differentiate into various skeletal cells. Skeletal tissues are derived from three distinct origins: the neural crest, paraxial mesoderm, and lateral plate mesoderm. Thus, various protocols have been proposed to recapitulate the sequential process of skeletal development. The second strategy is to extract stem cells from skeletal tissues. In addition to mesenchymal stem/stromal cells (MSCs), multiple cell types have been identified as alternative cell sources. These cells have distinct multipotent properties allowing them to differentiate into skeletal cells and various potential applications for skeletal regeneration. In this review, we summarize state-of-the-art research in stem cell differentiation based on the understanding of embryogenic skeletal development and stem cells existing in skeletal tissues. We then discuss the potential applications of these cell types for regenerative medicine.

Lessons from joint development for cartilage repair in the clinic

More than 250 years ago, William Hunter stated that when cartilage is destroyed it never recovers. In the last 20 years, the understanding of the mechanisms that lead to joint formation and the knowledge that some of these mechanisms are reactivated in the homeostatic responses of cartilage to injury has offered an unprecedented therapeutic opportunity to achieve cartilage regeneration. Very large investments in ambitious clinical trials are finally revealing that, although we do not have perfect medicines yet, disease modification is a feasible possibility for human osteoarthritis.

Developmental constraints on fin diversity

The fish fin is a breathtaking repository full of evolutionary diversity, novelty, and convergence. Over 500 million years, the adaptation to novel habitats has provided landscapes of fin diversity. Although comparative anatomy of evolutionarily divergent patterns over centuries has highlighted the fundamental architectures and evolutionary trends of fins, including convergent evolution, the developmental constraints on fin evolution, which bias the evolutionary trajectories of fin morphology, largely remain elusive. Here, we review the evolutionary history, developmental mechanisms, and evolutionary underpinnings of paired fins, illuminating possible developmental constraints on fin evolution. Our compilation of anatomical and genetic knowledge of fin development sheds light on the canalized and the unpredictable aspects of fin shape in evolution. Leveraged by an arsenal of genomic and genetic tools within the working arena of spectacular fin diversity, evolutionary developmental biology embarks on the establishment of conceptual framework for developmental constraints, previously enigmatic properties of evolution.

Fin ray patterns at the fin-to-limb transition

The fin-to-limb transition was marked by the origin of digits and the loss of dermal fin rays. Paleontological research into this transformation has focused on the evolution of the endoskeleton, with little attention paid to fin ray structure and function. To address this knowledge gap, we study the dermal rays of the pectoral fins of 3 key tetrapodomorph taxa-Sauripterus taylori (Rhizodontida), Eusthenopteron foordi (Tristichopteridae), and Tiktaalik roseae (Elpistostegalia)-using computed tomography. These data show several trends in the lineage leading to digited forms, including the consolidation of fin rays (e.g., reduced segmentation and branching), reduction of the fin web, and unexpectedly, the evolution of asymmetry between dorsal and ventral hemitrichia. In Eusthenopteron, dorsal rays cover the preaxial endoskeleton slightly more than ventral rays. In Tiktaalik, dorsal rays fully cover the third and fourth mesomeres, while ventral rays are restricted distal to these elements, suggesting the presence of ventralized musculature at the fin tip analogous to a fleshy "palm." Asymmetry is also observed in cross-sectional areas of dorsal and ventral rays. Eusthenopteron dorsal rays are slightly larger than ventral rays; by contrast, Tiktaalik dorsal rays can be several times larger than ventral rays, and degree of asymmetry appears to be greater at larger sizes. Analysis of extant osteichthyans suggests that cross-sectional asymmetry in the dermal rays of paired fins is plesiomorphic to crown group osteichthyans. The evolution of dermal rays in crownward stem tetrapods reflects adaptation for a fin-supported elevated posture and resistance to substrate-based loading prior to the origin of digits.

Signaling regulation during gastrulation: Insights from mouse embryos and in vitro systems

Gastrulation is the process whereby cells exit pluripotency and concomitantly acquire and pattern distinct cell fates. This is driven by the convergence of WNT, BMP, Nodal and FGF signals, which are tightly spatially and temporally controlled, resulting in regional and stage-specific signaling environments. The combination, level and duration of signals that a cell is exposed to, according its position within the embryo and the developmental time window, dictates the fate it will adopt. The key pathways driving gastrulation exhibit complex interactions, which are difficult to disentangle in vivo due to the complexity of manipulating multiple signals in parallel with high spatiotemporal resolution. Thus, our current understanding of the signaling dynamics regulating gastrulation is limited. In vitro stem cell models have been established, which undergo organized cellular differentiation and patterning. These provide amenable, simplified, deconstructed and scalable models of gastrulation. While the foundation of our understanding of gastrulation stems from experiments in embryos, in vitro systems are now beginning to reveal the intricate details of signaling regulation. Here we discuss the current state of knowledge of the role, regulation and dynamic interaction of signaling pathways that drive mouse gastrulation.

Understanding the Cellular and Molecular Mechanisms That Control Early Cell Fate Decisions During Appendicular Skeletogenesis

The formation of the vertebrate skeleton is orchestrated in time and space by a number of gene regulatory networks that specify and position all skeletal tissues. During embryonic development, bones have two distinct origins: bone tissue differentiates directly from mesenchymal progenitors, whereas most long bones arise from cartilaginous templates through a process known as endochondral ossification. Before endochondral bone development takes place, chondrocytes form a cartilage analgen that will be sequentially segmented to form joints; thus, in the cartilage template, either the cartilage maturation programme or the joint formation programme is activated. Once the cartilage differentiation programme starts, the growth plate begins to form. In contrast, when the joint formation programme is activated, a capsule begins to form that contains special articular cartilage and synovium to generate a functional joint. In this review, we will discuss the mechanisms controlling the earliest molecular events that regulate cell fate during skeletogenesis in long bones. We will explore the initial processes that lead to the recruitment of mesenchymal stem/progenitor cells, the commitment of chondrocyte lineages, and the formation of skeletal elements during morphogenesis. Thereafter, we will review the process of joint specification and joint morphogenesis. We will discuss the links between transcription factor activity, cell–cell interactions, cell–extracellular matrix interactions, growth factor signalling, and other molecular interactions that control mesenchymal stem/progenitor cell fate during embryonic skeletogenesis.

Localization of β-Catenin and Islet in the Pelvic Fin Field in Zebrafish

In zebrafish, pelvic fin buds appear at 3 weeks post fertilization (wpf) during the larval to juvenile transition (metamorphosis), but their fate is already determined during embryogenesis. Thus, presumptive pelvic fin cells appear to memorize their positional information for three weeks, but no factors expressed in the pelvic fin field from the embryonic to the metamorphic stages have been identified. In mice, Islet1 is proposed to promote nuclear accumulation of β-catenin in the hindlimb field, which leads to the initiation of hindlimb bud outgrowth through activation of the Wnt/βcatenin pathway. Here, we examined the distribution of β-catenin and islet proteins in the pelvic fin field of zebrafish from the embryonic to the metamorphic stages. We found that transcripts of islet2a, but not islet1, are detected in the posterior lateral plate mesoderm, including the presumptive pelvic fin field, at the embryonic stage as well as in the pelvic fin bud at the metamorphic stage. Immunolocalization revealed that β-catenin and islet proteins, which are synthesized during the embryonic stage, remain in the cytoplasm of the presumptive pelvic fin cells during the larval stage, and are then translocated into the nuclei of the pelvic fin bud at the metamorphic stage. We propose that cytoplasmic localization of these proteins in the presumptive pelvic fin cells that remained during the larval stage may underlie the mechanism by which pelvic fin cells memorize their positional information from the embryonic stage to the metamorphic stage.

Fibroblast Growth Factor Receptors Function Redundantly During Zebrafish Embryonic Development

Fibroblast growth factor (Fgf) signaling regulates many processes during development. In most cases, one tissue layer secretes an Fgf ligand that binds and activates an Fgf receptor (Fgfr) expressed by a neighboring tissue. Although studies have identified the roles of specific Fgf ligands during development, less is known about the requirements for the receptors. We have generated null mutations in each of the five fgfr genes in zebrafish. Considering the diverse requirements for Fgf signaling throughout development, and that null mutations in the mouse Fgfr1 and Fgfr2 genes are embryonic lethal, it was surprising that all zebrafish homozygous mutants are viable and fertile, with no discernable embryonic defect. Instead, we find that multiple receptors are involved in coordinating most Fgf-dependent developmental processes. For example, mutations in the ligand fgf8a cause loss of the midbrain-hindbrain boundary, whereas, in the fgfr mutants, this phenotype is seen only in embryos that are triple mutant for fgfr1a;fgfr1b;fgfr2, but not in any single or double mutant combinations. We show that this apparent fgfr redundancy is also seen during the development of several other tissues, including posterior mesoderm, pectoral fins, viscerocranium, and neurocranium. These data are an essential step toward defining the specific Fgfrs that function with particular Fgf ligands to regulate important developmental processes in zebrafish.

Roles of Wnt7a in embryo development, tissue homeostasis, and human diseases

Human Wnt family comprises 19 proteins which are critical to embryo development and tissue homeostasis. Binding to different frizzled (FZD) receptor, Wnt7a initiates both β-catenin dependent pathway, and β-catenin independent pathways such as PI3K/Akt, RAC/JNK, and extracellular signal-regulated kinase 5/peroxisome proliferator-activated receptor-γ. In the embryo, Wnt7a plays a crucial role in cerebral cortex development, synapse formation, and central nervous system vasculature formation and maintenance. Wnt7a is also involved in the development of limb and female reproductive system. Wnt7a mutation leads to human limb malformations and animal female reproductive system defects. Wnt7a is implicated in homeostasis maintenance of skeletal muscle, cartilage, cornea and hair follicle, and Wnt7a treatment may be potentially applied in skeletal muscle dystrophy, corneal damage, wound repair, and hair follicle regeneration. Wnt7a plays dual roles in human tumors. Wnt7a is downregulated in lung cancers, functioning as a tumor suppressor, however, it is upregulated in several other malignancies such as ovarian cancer, breast cancer, and glioma, acting as a tumor promoter. Moreover, Wnt7a overexpression is associated with inflammation and fibrosis, but its roles need to be further investigated.

Primary myogenesis in the sand lizard (Lacerta agilis) limb bud

Our studies conducted on reptilian limb muscle development revealed, for the first time, early forelimb muscle differentiation at the morphological and molecular level. Sand lizard skeletal muscle differentiation in the early forelimb bud was investigated by light, confocal, and transmission electron microscopy as well as western blot. The early forelimb bud, filled with mesenchymal cells, is surrounded by monolayer epithelium cells. The immunocytochemical analysis revealed the presence of Pax3- and Lbx-positive cells in the vicinity of the ventro-lateral lip (VLL) of the dermomyotome, suggesting that VLL is the source of limb muscle progenitor cells. Furthermore, Pax3- and Lbx-positive cells were observed in the dorsal and ventral myogenic pools of the forelimb bud. Skeletal muscle development in the early limb bud is asynchronous, which is manifested by the presence of myogenic cells in different stages of differentiation: multinucleated myotubes with well-developed contractile apparatus, myoblasts, and mitotically active premyoblasts. The western blot analysis revealed the presence of MyoD and Myf5 proteins in all investigated developmental stages. The MyoD western blot analysis showed two bands corresponding to monomeric (mMyoD) and dimeric (dMyoD) fractions. Two separate bands were also detected in the case of Myf5. The observed bands were related to non-phosphorylated (Myf5) and phosphorylated (pMyf5) fractions of Myf5. Our investigations on sand lizard forelimb myogenesis showed that the pattern of muscle differentiation in the early forelimb bud shares many features with rodents and chicks.

Unique Gene Expression Signatures in the Intestinal Mucosa and Organoids Derived from Germ-Free and Monoassociated Mice

Commensal microbiota contribute to gut homeostasis by inducing transcription of mucosal genes. Analysis of the impact of various microbiota on intestinal tissue provides an important insight into the function of this organ. We used cDNA microarrays to determine the gene expression signature of mucosa isolated from the small intestine and colon of germ-free (GF) mice and animals monoassociated with two E. coli strains. The results were compared to the expression data obtained in conventionally reared (CR) mice. In addition, we analyzed gene expression in colon organoids derived from CR, GF, and monoassociated animals. The analysis revealed that the complete absence of intestinal microbiota mainly affected the mucosal immune system, which was not restored upon monoassociation. The most important expression changes observed in the colon mucosa indicated alterations in adipose tissue and lipid metabolism. In the comparison of differentially expressed genes in the mucosa or organoids obtained from GF and CR mice, only six genes were common for both types of samples. The results show that the increased expression of the angiopoietin-like 4 (Angptl4) gene encoding a secreted regulator of lipid metabolism indicates the GF status.

Fibroblast Growth Factor 10 and Vertebrate Limb Development

Early limb development requires fibroblast growth factor (Fgf)-mediated coordination between growth and patterning to ensure the proper formation of a functional organ. The apical ectodermal ridge (AER) is a domain of thickened epithelium located at the distal edge of the limb bud that coordinates outgrowth along the proximodistal axis. Considerable amount of work has been done to elucidate the cellular and molecular mechanisms underlying induction, maintenance and regression of the AER. Fgf10, a paracrine Fgf that elicits its biological responses by activating the fibroblast growth factor receptor 2b (Fgfr2b), is crucial for governing proximal distal outgrowth as well as patterning and acts upstream of the known AER marker Fgf8. A transgenic mouse line allowing doxycycline-based inducible and ubiquitous expression of a soluble form of Fgfr2b has been extensively used to identify the role of Fgfr2b ligands at different time points during development. Overexpression of soluble Fgfr2b (sFgfr2b) post-AER induction leads to irreversible loss of cellular β-catenin organization and decreased Fgf8 expression in the AER. A similar approach has been carried out pre-AER induction. The observed limb phenotype is similar to the severe proximal truncations observed in human babies exposed to thalidomide, which has been proposed to block the Fgf10-AER-Fgf8 feedback loop. Novel insights on the role of Fgf10 signaling in limb formation pre- and post-AER induction are summarized in this review and will be integrated with possible future investigations on the role of Fgf10 throughout limb development.

Characterization of cis-regulatory elements for Fgf10 expression in the chick embryo

BACKGROUND

Fgf10 is expressed in various tissues and organs, such as the limb bud, heart, inner ear, and head mesenchyme. Previous studies identified Fgf10 enhancers for the inner ear and heart. However, Fgf10 enhancers for other tissues have not been identified.

RESULTS

By using primary culture chick embryo lateral plate mesoderm cells, we compared activities of deletion constructs of the Fgf10 promoter region, cloned into a promoter-less luciferase reporter vector. We identified a 0.34-kb proximal promoter that can activate luciferase expression. Then, we cloned 11 evolutionarily conserved sequences located within or outside of the Fgf10 gene into the 0.34-kb promoter-luciferase vector, and tested their activities in vitro using primary cultured cells. Two sequences showed the highest activities. By using the Tol2 system and electroporation into chick embryos, activities of the 0.34-kb promoter with and without the two sequences were tested in vivo. No activities were detected in limb buds. However, the 0.34-kb promoter exhibited activities in the dorsal midline of the brain, while Fgf10 is detected in broader region in the brain. The two noncoding sequences negatively acted on the 0.34-kb promoter in the brain.

CONCLUSIONS

The proximal 0.34-kb promoter has activities to drive expression in restricted areas of the brain. Developmental Dynamics 247:1253-1263, 2018. © 2018 Wiley Periodicals, Inc.

Identity and novelty in the avian syrinx

In its most basic conception, a novelty is simply something new. However, when many previously proposed evolutionary novelties have been illuminated by genetic, developmental, and fossil data, they have refined and narrowed our concept of biological "newness." For example, they show that these novelties can occur at one or multiple levels of biological organization. Here, we review the identity of structures in the avian vocal organ, the syrinx, and bring together developmental data on airway patterning, structural data from across tetrapods, and mathematical modeling to assess what is novel. In contrast with laryngeal cartilages that support vocal folds in other vertebrates, we find no evidence that individual cartilage rings anchoring vocal folds in the syrinx have homology with any specific elements in outgroups. Further, unlike all other vertebrate vocal organs, the syrinx is not derived from a known valve precursor, and its origin involves a transition from an evolutionary "spandrel" in the respiratory tract, the site where the trachea meets the bronchi, to a target for novel selective regimes. We find that the syrinx falls into an unusual category of novel structures: those having significant functional overlap with the structures they replace. The syrinx, along with other evolutionary novelties in sensory and signaling modalities, may more commonly involve structural changes that contribute to or modify an existing function rather than those that enable new functions.

Genetic regulatory pathways of split-hand/foot malformation

Split-hand/foot malformation (SHFM) is caused by mutations in TP63, DLX5, DLX6, FGF8, FGFR1, WNT10B, and BHLHA9. The clinical features of SHFM caused by mutations of these genes are not distinguishable. This implies that in normal situations these SHFM-associated genes share an underlying regulatory pathway that is involved in the development of the central parts of the hands and feet. The mutations in SHFM-related genes lead to dysregulation of Fgf8 in the central portion of the apical ectodermal ridge (AER) and subsequently lead to misexpression of a number of downstream target genes, failure of stratification of the AER, and thus SHFM. Syndactyly of the remaining digits is most likely the effects of dysregulation of Fgf-Bmp-Msx signaling on apoptotic cell death. Loss of digit identity in SHFM is hypothesized to be the effects of misexpression of HOX genes, abnormal SHH gradient, or the loss of balance between GLI3A and GLI3R. Disruption of canonical and non-canonical Wnt signaling is involved in the pathogenesis of SHFM. Whatever the causative genes of SHFM are, the mutations seem to lead to dysregulation of Fgf8 in AER cells of the central parts of the hands and feet and disruption of Wnt-Bmp-Fgf signaling pathways in AER.

FACS-Seq analysis of Pax3-derived cells identifies non-myogenic lineages in the embryonic forelimb

Skeletal muscle in the forelimb develops during embryonic and fetal development and perinatally. While much is known regarding the molecules involved in forelimb myogenesis, little is known about the specific mechanisms and interactions. Migrating skeletal muscle precursor cells express Pax3 as they migrate into the forelimb from the dermomyotome. To compare gene expression profiles of the same cell population over time, we isolated lineage-traced Pax3⁺ cells (Pax3^EGFP) from forelimbs at different embryonic days. We performed whole transcriptome profiling via RNA-Seq of Pax3⁺ cells to construct gene networks involved in different stages of embryonic and fetal development. With this, we identified genes involved in the skeletal, muscular, vascular, nervous and immune systems. Expression of genes related to the immune, skeletal and vascular systems showed prominent increases over time, suggesting a non-skeletal myogenic context of Pax3-derived cells. Using co-expression analysis, we observed an immune-related gene subnetwork active during fetal myogenesis, further implying that Pax3-derived cells are not a strictly myogenic lineage, and are involved in patterning and three-dimensional formation of the forelimb through multiple systems.

The function of tcf3 in medaka embryos: efficient knockdown with pePNAs

Background

The application of antisense molecules, such as morpholino oligonucleotides, is an efficient method of gene inactivation in vivo. We recently introduced phosphonic ester modified peptide nucleic acids (PNA) for in vivo loss-of-function experiments in medaka embryos. Here we tested novel modifications of the PNA backbone to knockdown the medaka tcf3 gene.

Results

A single tcf3 gene exists in the medaka genome and its inactivation strongly affected eye development of the embryos, leading to size reduction and anophthalmia in severe cases. The function of Tcf3 strongly depends on co-repressor interactions. We found interactions with Groucho/Tle proteins to be most important for eye development. Using a dominant negative approach for combined inactivation of all groucho/tle genes also resulted in eye phenotypes, as did interference with three individual tle genes.

Conclusions

Our results show that side chain modified PNAs come close to the knockdown efficiency of morpholino oligonucleotides in vivo. A single medaka tcf3 gene combines the function of the two zebrafish paralogs hdl and tcf3b. In combination with Groucho/Tle corepressor proteins Tcf3 acts in anterior development and is critical for eye formation.

Electronic supplementary material

The online version of this article (10.1186/s12896-017-0411-0) contains supplementary material, which is available to authorized users.

Postaxial limb hypoplasia (PALH): the classification, clinical features, and related developmental biology

Postaxial limb hypoplasia (PALH) is a group of nonhereditary diseases with congenital lower limb deficiency affecting the fibular ray, including fibular hemimelia, proximal femoral focal deficiency, and tarsal coalition. The etiology and the developmental biology of the anomaly are still not fully understood. Here, we review the previous classification systems, present the clinical features, and discuss the developmental biology of PALH.

Spatio-temporal expression patterns of Wnt signaling pathway during the development of temporomandibular condylar cartilage

There is a growing body of evidence supporting the involvement of the Wnt signaling pathway in various aspects of skeletal and joint development; however, it is unclear whether it is involved in the process of temporomandibular joint development. In order to clarify this issue, we examined the spatio-temporal distribution of mRNAs and proteins of the Wnt family during the formation of the mandibular condylar cartilage at the prenatal and postnatal stages. An in situ hybridization test revealed no mRNAs of β-catenin and Axin2 during early mesenchymal condensation; the ligands surveyed in this study (including Wnt-4, 5a, and 9a) were clearly detected at various ranges of expression, mainly in the condylar blastema and later distinct cartilaginous layers. Apart from β-catenin and Axin2, the Wnt family members surveyed in this study, including Lef-1, were found to be immunopositive during early chondrogenesis in the condylar cartilage at E14.5. After distinct chondrocyte layers were identified within the cartilage at E16.5, the expression of the Wnt signaling members was different and mainly restricted to proliferating cells and mineralized hypertrophic chondrocytes. In the adult mandibular condylar cartilage, the Wnt-4 mRNA, as well as the Wnt-4 and Wnt-9a proteins, was not observed. Our findings demonstrated that the Wnt signaling pathway was associated with the development of mandibular condylar cartilage.

Reduced β-catenin expression affects patterning of bone primordia, but not bone maturation

Wnt/β-catenin signaling is involved in patterning of bone primordia, but also plays an important role in the differentiation of chondrocytes and osteoblasts. During these processes the level of β-catenin must be tightly regulated. Excess β-catenin leads to conditions with increased bone mass, whereas loss of β-catenin is associated with osteoporosis or, in extreme cases, the absence of limbs. In this study, we examined skeletogenesis in mice, which retain only 25% of β-catenin. These embryos showed severe morphological abnormalities of which the lack of hindlimbs and misshaped front paws were the most striking. Surprisingly however, calcification of bone primordia occurred normally. Moreover, the Wnt-dependent regulatory network of transcription factors driving the differentiation of cartilage and bone, as well as the expression of extracellular matrix components, were preserved. These findings show that 25% β-catenin is insufficient for the correct patterning of bone primordia, but sufficient for their mineralization. Our approach helps to identify bone morphogenetic processes that can proceed normally even at low β-catenin levels, in contrast to those that require high β-catenin dosages. This information could be exploited to improve the treatment of bone diseases by fine-tuning the individual β-catenin dosage requirements.

Summary: The correct allocation and patterning of skeletal elements during development requires a higher dosage of β-catenin than the maturation and mineralization of these primordia.

Limb development: a paradigm of gene regulation

The limb is a commonly used model system for developmental biology. Given the need for precise control of complex signalling pathways to achieve proper patterning, the limb is also becoming a model system for gene regulation studies. Recent developments in genomic technologies have enabled the genome-wide identification of regulatory elements that control limb development, yielding insights into the determination of limb morphology and forelimb versus hindlimb identity. The modulation of regulatory interactions - for example, through the modification of regulatory sequences or chromatin architecture - can lead to morphological evolution, acquired regeneration capacity or limb malformations in diverse species, including humans.

Expression Pattern of Axin2 During Chicken Development

Canonical Wnt-signalling is well understood and has been extensively described in many developmental processes. The regulation of this signalling pathway is of outstanding relevance for proper development of the vertebrate and invertebrate embryo. Axin2 provides a negative-feedback-loop in the canonical Wnt-pathway, being a target gene and a negative regulator. Here we provide a detailed analysis of the expression pattern in the development of the chicken embryo. By performing in-situ hybridization on chicken embryos from stage HH 04+ to HH 32 we detected a temporally and spatially restricted dynamic expression of Axin2. In particular, data about the expression of Axin2 mRNA in early embryogenesis, somites, neural tube, limbs, kidney and eyes was obtained.

Molecular mechanisms underlying the exceptional adaptations of batoid fins

Extreme novelties in the shape and size of paired fins are exemplified by extinct and extant cartilaginous and bony fishes. Pectoral fins of skates and rays, such as the little skate (Batoid, Leucoraja erinacea), show a strikingly unique morphology where the pectoral fin extends anteriorly to ultimately fuse with the head. This results in a morphology that essentially surrounds the body and is associated with the evolution of novel swimming mechanisms in the group. In an approach that extends from RNA sequencing to in situ hybridization to functional assays, we show that anterior and posterior portions of the pectoral fin have different genetic underpinnings: canonical genes of appendage development control posterior fin development via an apical ectodermal ridge (AER), whereas an alternative Homeobox (Hox)-Fibroblast growth factor (Fgf)-Wingless type MMTV integration site family (Wnt) genetic module in the anterior region creates an AER-like structure that drives anterior fin expansion. Finally, we show that GLI family zinc finger 3 (Gli3), which is an anterior repressor of tetrapod digits, is expressed in the posterior half of the pectoral fin of skate, shark, and zebrafish but in the anterior side of the pelvic fin. Taken together, these data point to both highly derived and deeply ancestral patterns of gene expression in skate pectoral fins, shedding light on the molecular mechanisms behind the evolution of novel fin morphologies.

How the embryo makes a limb: determination, polarity and identity

The vertebrate limb with its complex anatomy develops from a small bud of undifferentiated mesoderm cells encased in ectoderm. The bud has its own intrinsic polarity and can develop autonomously into a limb without reference to the rest of the embryo. In this review, recent advances are integrated with classical embryology, carried out mainly in chick embryos, to present an overview of how the embryo makes a limb bud. We will focus on how mesoderm cells in precise locations in the embryo become determined to form a limb and express the key transcription factors Tbx4 (leg/hindlimb) or Tbx5 (wing/forelimb). These Tbx transcription factors have equivalent functions in the control of bud formation by initiating a signalling cascade involving Wnts and fibroblast growth factors (FGFs) and by regulating recruitment of mesenchymal cells from the coelomic epithelium into the bud. The mesoderm that will form limb buds and the polarity of the buds is determined with respect to both antero-posterior and dorso-ventral axes of the body. The position in which a bud develops along the antero-posterior axis of the body will also determine its identity - wing/forelimb or leg/hindlimb. Hox gene activity, under the influence of retinoic acid signalling, is directly linked with the initiation of Tbx5 gene expression in the region along the antero-posterior axis of the body that will form wings/forelimbs and determines antero-posterior polarity of the buds. In contrast, Tbx4 expression in the regions that will form legs/hindlimbs is regulated by the homeoprotein Pitx1 and there is no evidence that Hox genes determine antero-posterior polarity of the buds. Bone morphogenetic protein (BMP) signalling determines the region along the dorso-ventral axis of the body in which both wings/forelimbs and legs/hindlimbs develop and dorso-ventral polarity of the buds. The polarity of the buds leads to the establishment of signalling regions - the dorsal and ventral ectoderm, producing Wnts and BMPs, respectively, the apical ectodermal ridge producing fibroblast growth factors and the polarizing region, Sonic hedgehog (Shh). These signals are the same in both wings/forelimbs and legs/hindlimbs and control growth and pattern formation by providing the mesoderm cells of the limb bud as it develops with positional information. The precise anatomy of the limb depends on the mesoderm cells in the developing bud interpreting positional information according to their identity - determined by Pitx1 in hindlimbs - and genotype. The competence to form a limb extends along the entire antero-posterior axis of the trunk - with Hox gene activity inhibiting the formation of forelimbs in the interlimb region - and also along the dorso-ventral axis.