The molecular regulation of vertebrate limb patterning

Regulation of vertebrate forelimb development and wing reduction in the flightless emu

The vertebrate limb is a dynamic structure which has evolved into many diverse forms to facilitate complex behavioral adaptations. The principle molecular and cellular processes that underlie development of the vertebrate limb are well characterized. However, how these processes are altered to drive differential limb development between vertebrates is less well understood. Several vertebrate models are being utilized to determine the developmental basis of differential limb morphogenesis, though these typically focus on later patterning of the established limb bud and may not represent the complete developmental trajectory. Particularly, heterochronic limb development can occur prior to limb outgrowth and patterning but receives little attention. This review summarizes the genetic regulation of vertebrate forelimb diversity, with particular focus on wing reduction in the flightless emu as a model for examining limb heterochrony. These studies highlight that wing reduction is complex, with heterochronic cellular and genetic events influencing the major stages of limb development. Together, these studies provide a broader picture of how different limb morphologies may be established during development.

Tissue cross talks governing limb muscle development and regeneration

For decades, limb development has been a paradigm of three-dimensional patterning. Moreover, as the limb muscles and the other tissues of the limb's musculoskeletal system arise from distinct developmental sources, it has been a prime example of integrative morphogenesis and cross-tissue communication. As the limbs grow, all components of the musculoskeletal system (muscles, tendons, connective tissue, nerves) coordinate their growth and differentiation, ultimately giving rise to a functional unit capable of executing elaborate movement. While the molecular mechanisms governing global three-dimensional patterning and formation of the skeletal structures of the limbs has been a matter of intense research, patterning of the soft tissues is less understood. Here, we review the development of limb muscles with an emphasis on their interaction with other tissue types and the instructive roles these tissues play. Furthermore, we discuss the role of adult correlates of these embryonic accessory tissues in muscle regeneration.

Whole embryo culture, transcriptomics and RNA interference identify TBX1 and FGF11 as novel regulators of limb development in the mouse

Identifying genes involved in vertebrate developmental processes and characterizing this involvement are daunting tasks, especially in the mouse where viviparity complicates investigations. Attempting to devise a streamlined approach for this type of study we focused on limb development. We cultured E10.5 and E12.5 embryos and performed transcriptional profiling to track molecular changes in the forelimb bud over a 6-hour time-window. The expression of certain genes was found to diverge rapidly from its normal path, possibly reflecting the activation of a stress-induced response. Others, however, maintained for up to 3 hours dynamic expression profiles similar to those seen in utero. Some of these resilient genes were known regulators of limb development. The implication of the others in this process was either unsuspected or unsubstantiated. The localized knockdown of two such genes, Fgf11 and Tbx1, hampered forelimb bud development, providing evidence of their implication. These results show that combining embryo culture, transcriptome analysis and RNA interference could speed up the identification of genes involved in a variety of developmental processes, and the validation of their implication.

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.

Pitx1 determines characteristic hindlimb morphologies in cartilage micromass culture

The shapes of homologous skeletal elements in the vertebrate forelimb and hindlimb are distinct, with each element exquisitely adapted to their divergent functions. Many of the signals and signalling pathways responsible for patterning the developing limb bud are common to both forelimb and hindlimb. How disparate morphologies are generated from common signalling inputs during limb development remains poorly understood. We show that, similar to what has been shown in the chick, characteristic differences in mouse forelimb and hindlimb cartilage morphology are maintained when chondrogenesis proceeds in vitro away from the endogenous limb bud environment. Chondrogenic nodules that form in high-density micromass cultures derived from forelimb and hindlimb buds are consistently different in size and shape. We described analytical tools we have developed to quantify these differences in nodule morphology and demonstrate that characteristic hindlimb nodule morphology is lost in the absence of the hindlimb-restricted limb modifier gene Pitx1. Furthermore, we show that ectopic expression of Pitx1 in the forelimb is sufficient to generate nodule patterns characteristic of the hindlimb. We also demonstrate that hindlimb cells are less adhesive to the tissue culture substrate and, within the limb environment, to the extracellular matrix and to each other. These results reveal autonomously programmed differences in forelimb and hindlimb cartilage precursors of the limb skeleton are controlled, at least in part, by Pitx1 and suggest this has an important role in generating distinct limb-type morphologies. Our results demonstrate that the micromass culture system is ideally suited to study cues governing morphogenesis of limb skeletal elements in a simple and experimentally tractable in vitro system that reflects in vivo potential.

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.

Limb patterning genes and heterochronic development of the emu wing bud

Background

The forelimb of the flightless emu is a vestigial structure, with greatly reduced wing elements and digit loss. To explore the molecular and cellular mechanisms associated with the evolution of vestigial wings and loss of flight in the emu, key limb patterning genes were examined in developing embryos.

Methods

Limb development was compared in emu versus chicken embryos. Immunostaining for cell proliferation markers was used to analyze growth of the emu forelimb and hindlimb buds. Expression patterns of limb patterning genes were studied, using whole-mount in situ hybridization (for mRNA localization) and RNA-seq (for mRNA expression levels).

Results

The forelimb of the emu embryo showed heterochronic development compared to that in the chicken, with the forelimb bud being retarded in its development. Early outgrowth of the emu forelimb bud is characterized by a lower level of cell proliferation compared the hindlimb bud, as assessed by PH3 immunostaining. In contrast, there were no obvious differences in apoptosis in forelimb versus hindlimb buds (cleaved caspase 3 staining). Most key patterning genes were expressed in emu forelimb buds similarly to that observed in the chicken, but with smaller expression domains. However, expression of Sonic Hedgehog (Shh) mRNA, which is central to anterior–posterior axis development, was delayed in the emu forelimb bud relative to other patterning genes. Regulators of Shh expression, Gli3 and HoxD13, also showed altered expression levels in the emu forelimb bud.

Conclusions

These data reveal heterochronic but otherwise normal expression of most patterning genes in the emu vestigial forelimb. Delayed Shh expression may be related to the small and vestigial structure of the emu forelimb bud. However, the genetic mechanism driving retarded emu wing development is likely to rest within the forelimb field of the lateral plate mesoderm, predating the expression of patterning genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0063-5) contains supplementary material, which is available to authorized users.

ROR-Family Receptor Tyrosine Kinases

ROR-family receptor tyrosine kinases form a small subfamily of receptor tyrosine kinases (RTKs), characterized by a conserved, unique domain architecture. ROR RTKs are evolutionary conserved throughout the animal kingdom and act as alternative receptors and coreceptors of WNT ligands. The intracellular signaling cascades activated downstream of ROR receptors are diverse, including but not limited to ROR-Frizzled-mediated activation of planar cell polarity signaling, RTK-like signaling, and antagonistic regulation of WNT/β-Catenin signaling. In line with their diverse repertoire of signaling functions, ROR receptors are involved in the regulation of multiple processes in embryonic development such as development of the axial and paraxial mesoderm, the nervous system and the neural crest, the axial and appendicular skeleton, and the kidney. In humans, mutations in the ROR2 gene cause two distinct developmental syndromes, recessive Robinow syndrome (RRS; MIM 268310) and dominant brachydactyly type B1 (BDB1; MIM 113000). In Robinow syndrome patients and animal models, the development of multiple organs is affected, whereas BDB1 results only in shortening of the distal phalanges of fingers and toes, reflecting the diversity of functions and signaling activities of ROR-family RTKs. In this chapter, we give an overview on ROR receptor structure and function. We discuss their signaling functions and role in vertebrate embryonic development with a focus on those developmental processes that are affected by mutations in the ROR2 gene in human patients.

Could we also be regenerative superheroes, like salamanders?

Development of methods to reawaken the semi-dormant regenerative potential that lies within adult human tissues would hold promise for the restoration of diseased or damaged organs and tissues. While most of the regeneration potential is suppressed in many vertebrates, including humans, during adult life, urodele amphibians (salamanders) retain their regenerative ability throughout adulthood. Studies in newts and axolotls, two salamander models, have provided significant knowledge about adult limb regeneration. In this review, we present a comparative analysis of salamander and mammalian regeneration and discuss how evolutionarily altered properties of the regenerative environment can be exploited to restore full regenerative potential in the human body.

Pitchfork and Gprasp2 Target Smoothened to the Primary Cilium for Hedgehog Pathway Activation

The seven-transmembrane receptor Smoothened (Smo) activates all Hedgehog (Hh) signaling by translocation into the primary cilia (PC), but how this is regulated is not well understood. Here we show that Pitchfork (Pifo) and the G protein-coupled receptor associated sorting protein 2 (Gprasp2) are essential components of an Hh induced ciliary targeting complex able to regulate Smo translocation to the PC. Depletion of Pifo or Gprasp2 leads to failure of Smo translocation to the PC and lack of Hh target gene activation. Together, our results identify a novel protein complex that is regulated by Hh signaling and required for Smo ciliary trafficking and Hh pathway activation.

Echoes of the embryo: using the developmental biology toolkit to study cancer

The hallmark of embryonic development is regulation - the tendency for cells to find their way into organized and 'well behaved' structures - whereas cancer is characterized by dysregulation and disorder. At face value, cancer biology and developmental biology would thus seem to have little to do with each other. But if one looks beneath the surface, embryos and cancers share a number of cellular and molecular features. Embryos arise from a single cell and undergo rapid growth involving cell migration and cell-cell interactions: features that are also seen in the context of cancer. Consequently, many of the experimental tools that have been used to study embryogenesis for over a century are well-suited to studying cancer. This article will review the similarities between embryogenesis and cancer progression and discuss how some of the concepts and techniques used to understand embryos are now being adapted to provide insight into tumorigenesis, from the origins of cancer cells to metastasis.

Xenopus Limb bud morphogenesis

Xenopus laevis, the South African clawed frog, is a well-established model organism for the study of developmental biology and regeneration due to its many advantages for both classical and molecular studies of patterning and morphogenesis. While contemporary studies of limb development tend to focus on models developed from the study of chicken and mouse embryos, there are also many classical studies of limb development in frogs. These include both fate and specification maps, that, due to their age, are perhaps not as widely known or cited as they should be. This has led to some inevitable misinterpretations- for example, it is often said that Xenopus limb buds have no apical ectodermal ridge, a morphological signalling centre located at the distal dorsal/ventral epithelial boundary and known to regulate limb bud outgrowth. These studies are valuable both from an evolutionary perspective, because amphibians diverged early from the amniote lineage, and from a developmental perspective, as amphibian limbs are capable of regeneration. Here, we describe Xenopus limb morphogenesis with reference to both classical and molecular studies, to create a clearer picture of what we know, and what is still mysterious, about this process.

From limbs to leaves: common themes in evolutionary diversification of organ form

An open problem in biology is to derive general principles that capture how morphogenesis evolved to generate diverse forms in different organisms. Here we discuss recent work investigating the morphogenetic basis for digit loss in vertebrate limbs and variation in form of marginal outgrowths of angiosperm (flowering plant) leaves. Two pathways underlie digit loss in vertebrate limbs. First, alterations to digit patterning arise through modification of expression of the Patched 1 receptor, which senses the Sonic Hedgehog morphogen and limits its mobility in the limb bud. Second, evolutionary changes to the degree of programmed cell death between digits influence their development after their initiation. Similarly, evolutionary modification of leaf margin outgrowths occurs via two broad pathways. First, species-specific transcription factor expression modulates outgrowth patterning dependent on regulated transport of the hormone auxin. Second, species-specific expression of the newly discovered REDUCED COMPLEXITY homeodomain transcription factor influences growth between individual outgrowths after their initiation. These findings demonstrate that in both plants and animals tinkering with either patterning or post-patterning processes can cause morphological change. They also highlight the considerable flexibility of morphological evolution and indicate that it may be possible to derive broad principles that capture how morphogenesis evolved across complex eukaryotes.

IFT81, encoding an IFT-B core protein, as a very rare cause of a ciliopathy phenotype

Background

Bidirectional intraflagellar transport (IFT) consists of two major protein complexes, IFT-A and IFT-B. In contrast to the IFT-B complex, all components of IFT-A have recently been linked to human ciliopathies when defective. We therefore hypothesised that mutations in additional IFT-B encoding genes can be found in patients with multisystemic ciliopathies.

Methods

We screened 1628 individuals with reno-ocular ciliopathies by targeted next-generation sequencing of ciliary candidate genes, including all IFT-B encoding genes.

Results

Consequently, we identified a homozygous mutation in IFT81 affecting an obligatory donor splice site in an individual with nephronophthisis and polydactyly. Further, we detected a loss-of-stop mutation with extension of the deduced protein by 10 amino acids in an individual with neuronal ceroid lipofuscinosis-1. This proband presented with retinal dystrophy and brain lesions including cerebellar atrophy, a phenotype to which the IFT81 variant might contribute. Cultured fibroblasts of this latter affected individual showed a significant decrease in ciliated cell abundance compared with controls and increased expression of the transcription factor GLI2 suggesting deranged sonic hedgehog signalling.

Conclusions

This work describes identification of mutations of IFT81 in individuals with symptoms consistent with the clinical spectrum of ciliopathies. It might represent the rare case of a core IFT-B complex protein found associated with human disease. Our data further suggest that defects in the IFT-B core are an exceedingly rare finding, probably due to its indispensable role for ciliary assembly in development.

The Relationship between Gene Network Structure and Expression Variation among Individuals and Species

Variation among individuals is a prerequisite of evolution by natural selection. As such, identifying the origins of variation is a fundamental goal of biology. We investigated the link between gene interactions and variation in gene expression among individuals and species using the mammalian limb as a model system. We first built interaction networks for key genes regulating early (outgrowth; E9.5-11) and late (expansion and elongation; E11-13) limb development in mouse. This resulted in an Early (ESN) and Late (LSN) Stage Network. Computational perturbations of these networks suggest that the ESN is more robust. We then quantified levels of the same key genes among mouse individuals and found that they vary less at earlier limb stages and that variation in gene expression is heritable. Finally, we quantified variation in gene expression levels among four mammals with divergent limbs (bat, opossum, mouse and pig) and found that levels vary less among species at earlier limb stages. We also found that variation in gene expression levels among individuals and species are correlated for earlier and later limb development. In conclusion, results are consistent with the robustness of the ESN buffering among-individual variation in gene expression levels early in mammalian limb development, and constraining the evolution of early limb development among mammalian species.

The avian embryo as a model system for skeletal myogenesis

This review will focus on the use of the chicken and quail as model systems to analyze myogenesis and as such will emphasize the experimental approaches that are strongest in these systems-the amenability of the avian embryo to manipulation and in ovo observation. During somite differentiation, a wide spectrum of developmental processes occur such as cellular differentiation, migration, and fusion. Cell lineage studies combined with recent advancements in cell imaging allow these biological phenomena to be readily observed and hypotheses tested extremely rapidly-a strength that is restricted to the avian system. A clear weakness of the chicken in the past has been genetic approaches to modulate gene function. Recent advances in the electroporation of expression vectors, siRNA constructs, and use of tissue specific reporters have opened the door to increasingly sophisticated experiments that address questions of interest not only to the somite/muscle field in particular but also fundamental to biology in general. Importantly, an ever-growing body of evidence indicates that somite differentiation in birds is indistinguishable to that of mammals; therefore, these avian studies complement the complex genetic models of the mouse.

Liebenberg syndrome is caused by a deletion upstream to the PITX1 gene resulting in transformation of the upper limbs to reflect lower limb characteristics

Liebenberg syndrome (MIM 186550) is a very rare autosomal dominant condition characterized by three main features: dysplasia of all of the bony components of the elbow joint, abnormalities in the shape of carpal bones, and brachydactyly. In this paper, we report a Saudi Arabian family with Liebenberg syndrome. Comparative genomic hybridization (CGH) revealed a 275-kb deletion within the cytogenetic band 5q31.1 which contains the H2AFY gene and 190,428bp of its downstream region. The deleted region is upstream to the PITX1 gene. The radiological features in the upper limbs of all affected members of the family were almost identical to the phenotype in the mouse model with ectopic expression of Pitx1 in the forelimbs. We therefore re-define the phenotype of Liebenberg syndrome as a transformation of the upper limbs to reflect lower limb characteristics and speculate that the area of deletion contains a regulatory sequence that suppresses the expression of PITX1 in the upper limb buds.

Characteristics and associated anomalies in radial ray deficiencies in Finland--a population-based study

Upper-limb defects with deficiencies of the radial ray have varying etiologies, with a low proportion of true Mendelian disorders. We carried out a population-based study to elucidate the birth prevalence and clinical spectrum of radial ray deficiencies in Finland. We identified all births with radial ray deficiency reported to the Finnish Register of Congenital Malformations in 1993-2005. Altogether 138 cases were identified (123 live births), with a birth prevalence of 1.83 per 10,000 births and a live birth prevalence of 1.64 per 10,000 live births. The proportion of infant deaths was as high as 35%. The majority of the cases were associated with known syndromes or multiple anomalies; only 13% were true isolated radial ray deficiencies. The most common syndrome was trisomy 18, and the most common in multiple anomalies was VACTERL association. In 8.7% of cases an association between radial ray deficiency and heart anomaly was observed. The high proportion of cases with associated major anomalies indicates that radial ray deficiency can be regarded isolated only after thorough assessment of the various organ systems in an affected infant.

Mutations in mouse Ift144 model the craniofacial, limb and rib defects in skeletal ciliopathies

Mutations in components of the intraflagellar transport (IFT) machinery required for assembly and function of the primary cilium cause a subset of human ciliopathies characterized primarily by skeletal dysplasia. Recently, mutations in the IFT-A gene IFT144 have been described in patients with Sensenbrenner and Jeune syndromes, which are associated with short ribs and limbs, polydactyly and craniofacial defects. Here, we describe an N-ethyl-N-nitrosourea-derived mouse mutant with a hypomorphic missense mutation in the Ift144 gene. The mutant twinkle-toes (Ift144(twt)) phenocopies a number of the skeletal and craniofacial anomalies seen in patients with human skeletal ciliopathies. Like other IFT-A mouse mutants, Ift144 mutant embryos display a generalized ligand-independent expansion of hedgehog (Hh) signalling, in spite of defective ciliogenesis and an attenuation of the ability of mutant cells to respond to upstream stimulation of the pathway. This enhanced Hh signalling is consistent with cleft palate and polydactyly phenotypes in the Ift144(twt) mutant, although extensive rib branching, fusion and truncation phenotypes correlate with defects in early somite patterning and may reflect contributions from multiple signalling pathways. Analysis of embryos harbouring a second allele of Ift144 which represents a functional null, revealed a dose-dependent effect on limb outgrowth consistent with the short-limb phenotypes characteristic of these ciliopathies. This allelic series of mouse mutants provides a unique opportunity to uncover the underlying mechanistic basis of this intriguing subset of ciliopathies.

Pitx1 is necessary for normal initiation of hindlimb outgrowth through regulation of Tbx4 expression and shapes hindlimb morphologies via targeted growth control

The forelimbs and hindlimbs of vertebrates are morphologically distinct. Pitx1, expressed in the hindlimb bud mesenchyme, is required for the formation of hindlimb characteristics and produces hindlimb-like morphologies when misexpressed in forelimbs. Pitx1 is also necessary for normal expression of Tbx4, a transcription factor required for normal hindlimb development. Despite the importance of this protein in these processes, little is known about its mechanism of action. Using a transgenic gene replacement strategy in a Pitx1 mutant mouse, we have uncoupled two discrete functions of Pitx1. We show that, firstly, this protein influences hindlimb outgrowth by regulating Tbx4 expression levels and that, subsequently, it shapes hindlimb bone and soft tissue morphology independently of Tbx4. We provide the first description of how Pitx1 sculpts the forming hindlimb skeleton by localised modulation of the growth rate of discrete elements.

Polydactyly: how many disorders and how many genes? 2010 update

Limb development is clinically and biologically important. Polydactyly is common and caused by aberrant anterior-posterior patterning. Human disorders that include polydactyly are diverse. To facilitate an understanding of the biology of limb development, cataloging the genes that are mutated in patients with polydactyly would be useful. In 2002, I characterized human phenotypes that included polydactyly. Subsequently, many advances have occurred with refinement of clinical entities and identification of numerous genes. Here, I update human polydactyly entities by phenotype and mutated gene. This survey demonstrates phenotypes with overlapping manifestations, genetic heterogeneity, and distinct phenotypes generated from mutations in single genes. Among 310 clinical entities, 80 are associated with mutations in 99 genes. These results show that knowledge of limb patterning genetics is improving rapidly. Soon, we will have a comprehensive toolkit of genes important for limb development, which will lead to regenerative therapies for limb anomalies.

Pleiotropic patterning response to activation of Shh signaling in the limb apical ectodermal ridge

Sonic hedgehog (Shh) signaling in the limb plays a central role in coordination of limb patterning and outgrowth. Shh expression in the limb is limited to the cells of the zone of polarizing activity (ZPA), located in posterior limb bud mesoderm. Shh is not expressed by limb ectoderm or apical ectodermal ridge (AER), but recent studies suggest a role for AER-Shh signaling in limb patterning. Here, we have examined the effects of activation of Shh signaling in the AER. We find that targeted expression of Shh in the AER activates constitutive Shh signaling throughout the AER and subjacent limb mesoderm, and causes a range of limb patterning defects with progressive severity from mild polydactyly, to polysyndactyly with proximal defects, to severe oligodactyly with phocomelia and partial limb ventralization. Our studies emphasize the importance of control of the timing, level and location of Shh pathway signaling for limb anterior-posterior, proximal-distal, and dorsal-ventral patterning.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

Developmental and genetic origins of murine long bone length variation

If we wish to understand whether development influences the rate or direction of morphological evolution, we must first understand the developmental bases of morphological variation within species. However, quantitative variation in adult morphology is the product of molecular and cellular processes unfolding from embryonic development through juvenile growth to maturity. The Atchley-Hall model provides a useful framework for dissecting complex morphologies into their component parts as a way of determining which developmental processes contribute to variation in adult form. We have examined differences in postnatal allometry and the patterns of genetic correlation between age-specific traits for ten recombinant inbred strains of mice generated from an intercross of LG/J and SM/J. Long bone length is closely tied to body size, but variation in adult morphology is more closely tied to differences in growth rate between 3 and 5 weeks of age. These analyses show that variation generated during early development is overridden by variation generated later in life. To more precisely determine the cellular processes generating this variation we then examined the cellular dynamics of long bone growth plates at the time of maximum elongation rate differences in the parent strains. Our analyses revealed that variation in long bone length is the result of faster elongation rates of the LG/J stain. The developmental bases for these differences in growth rate involve the rate of cell division and chondrocyte hypertrophy in the growth plate.

The secreted integrin ligand nephronectin is necessary for forelimb formation in Xenopus tropicalis

While limb regeneration has been extensively studied in amphibians, little is known about the initial events in limb formation in metamorphosing anurans. The small secreted integrin ligand nephronectin (npnt) is necessary for development of the metanephros in mouse. Although expressed in many tissues, its role in other developmental processes is not well-studied. Here we show that a transgene insertion that disrupts this gene ablates forelimb formation in Xenopus tropicalis. Our results suggest a novel role for integrin signalling in limb development, and represent the first insertional phenotype to be cloned in amphibians.

Vertebrate skeletogenesis

Vertebrate skeletogenesis consists in elaborating an edifice of more than 200 pieces of bone and cartilage. Each skeletal piece is crafted at a distinct location in the body, is articulated with others, and reaches a specific size, shape, and tissue composition according to both species instructions and individual determinants. This complex, customized body frame fulfills multiple essential tasks. It confers morphological features, allows controlled postures and movements, protects vital organs, houses hematopoiesis, stores minerals, and adsorbs toxins. This review provides an overview of the multiple facets of this ingenious process for experts as well as nonexperts of skeletogenesis. We explain how the developing vertebrate uses both specific and ubiquitously expressed genes to generate multipotent mesenchymal cells, specify them to a skeletogenic fate, control their survival and proliferation, and direct their differentiation into cartilage, bone, and joint cells. We review milestone discoveries made toward uncovering the intricate networks of regulatory factors that are involved in these processes, with an emphasis on signaling pathways and transcription factors. We describe numerous skeletal malformation and degeneration diseases that occur in humans as a result of mutations in regulatory genes, and explain how these diseases both help and motivate us to further decipher skeletogenic processes. Upon discussing current knowledge and gaps in knowledge in the control of skeletogenesis, we highlight ultimate research goals and propose research priorities and approaches for future endeavors.