Lmx1b expression during joint and tendon formation: localization and evaluation of potential downstream targets

Sox, Fox, and Lmx1b binding sites differentially regulate a Gdf5-Associated regulatory region during elbow development

Introduction: The articulating ends of limb bones have precise morphology and asymmetry that ensures proper joint function. Growth differentiation factor 5 (Gdf5) is a secreted morphogen involved in cartilage and bone development that contributes to the architecture of developing joints. Dysregulation of Gdf5 results in joint dysmorphogenesis often leading to progressive joint degeneration or osteoarthritis (OA). The transcription factors and cis-regulatory modules (CRMs) that regulate Gdf5 expression are not well characterized. We previously identified a Gdf5-associated regulatory region (GARR) that contains predicted binding sites for Lmx1b, Osr2, Fox, and the Sox transcription factors. These transcription factors are recognized factors involved in joint morphogenesis and skeletal development. Methods: We used in situ hybridization to Gdf5, Col2A1, and the transcription factors of interest in developing chicken limbs to determine potential overlap in expression. We further analyzed scRNA-seq data derived from limbs and knees in published mouse and chicken datasets, identifying cells with coexpression of Gdf5 and the transcription factors of interest. We also performed site-directed mutatgenesis of the predicted transcription factor binding sites in a GARR-reporter construct and determined any change in activity using targeted regional electroporation (TREP) in micromass and embryonic chicken wing bioassays. Results: Gdf5 expression overlapped the expression of these transcription factors during joint development both by in situ hybridization (ISH) and scRNA-seq analyses. Within the GARR CRM, mutation of two binding sites common to Fox and Sox transcripstion factors reduced enhancer activity to background levels in micromass cultures and in ovo embryonic chicken wing bioassays, whereas mutation of two Sox-only binding sites caused a significant increase in activity. These results indicate that the Fox/Sox binding sites are required for activity, while the Sox-only sites are involved in repression of activity. Mutation of Lmx1b binding sites in GARR caused an overall reduction in enhancer activity in vitro and a dorsal reduction in ovo. Despite a recognized role for Osr2 in joint development, disruption of the predicted Osr2 site did not alter GARR activity. Conclusion: Taken together, our data indicates that GARR integrates positive, repressive, and asymmetrical inputs to fine-tune the expression of Gdf5 during elbow joint development.

Digits in a dish: An in vitro system to assess the molecular genetics of hand/foot development at single-cell resolution

In vitro models allow for the study of developmental processes outside of the embryo. To gain access to the cells mediating digit and joint development, we identified a unique property of undifferentiated mesenchyme isolated from the distal early autopod to autonomously re-assemble forming multiple autopod structures including: digits, interdigital tissues, joints, muscles and tendons. Single-cell transcriptomic analysis of these developing structures revealed distinct cell clusters that express canonical markers of distal limb development including: Col2a1, Col10a1, and Sp7 (phalanx formation), Thbs2 and Col1a1 (perichondrium), Gdf5, Wnt5a, and Jun (joint interzone), Aldh1a2 and Msx1 (interdigital tissues), Myod1 (muscle progenitors), Prg4 (articular perichondrium/articular cartilage), and Scx and Tnmd (tenocytes/tendons). Analysis of the gene expression patterns for these signature genes indicates that developmental timing and tissue-specific localization were also recapitulated in a manner similar to the initiation and maturation of the developing murine autopod. Finally, the in vitro digit system also recapitulates congenital malformations associated with genetic mutations as in vitro cultures of Hoxa13 mutant mesenchyme produced defects present in Hoxa13 mutant autopods including digit fusions, reduced phalangeal segment numbers, and poor mesenchymal condensation. These findings demonstrate the robustness of the in vitro digit system to recapitulate digit and joint development. As an in vitro model of murine digit and joint development, this innovative system will provide access to the developing limb tissues facilitating studies to discern how digit and articular joint formation is initiated and how undifferentiated mesenchyme is patterned to establish individual digit morphologies. The in vitro digit system also provides a platform to rapidly evaluate treatments aimed at stimulating the repair or regeneration of mammalian digits impacted by congenital malformation, injury, or disease.

En1 and Lmx1b do not recapitulate embryonic dorsal-ventral limb patterning functions during mouse digit tip regeneration

The mouse digit tip regenerates following amputation. How the regenerate is patterned is unknown, but a long-standing hypothesis proposes developmental patterning mechanisms are re-used during regeneration. The digit tip bone exhibits dorsal-ventral (DV) polarity, so we focus on En1 and Lmx1b, two factors necessary for DV patterning during limb development. We investigate whether they are re-expressed during regeneration in a developmental-like pattern and whether they direct DV morphology of the regenerate. We find that both En1 and Lmx1b are expressed in the regenerating digit tip epithelium and mesenchyme, respectively, but without DV polarity. Conditional genetics and quantitative analysis of digit tip bone morphology determine that genetic deletion of En1 or Lmx1b in adult digit tip regeneration modestly reduces bone regeneration but does not affect DV patterning. Collectively, our data suggest that, while En1 and Lmx1b are re-expressed during mouse digit tip regeneration, they do not define the DV axis during regeneration.

Radiological characteristics of the knee joint in nail patella syndrome

Aim

Nail patella syndrome (NPS) is a skeletal dysplasia with patellofemoral dysfunction as a key symptom. We present the first in-depth radiological evaluation of the knee in a large series of NPS patients and describe the typical malformations.

Patients and Methods

Conventional radiological examination of 95 skeletally mature patients with NPS was performed. Patellar morphology was classified according to the Wiberg classification as modified by Baumgartl and Ficat criteria, and trochlear shape was classified according to the Dejour classification.

Results

Patellar aplasia was present in 4/90 (4%), and patellar hypoplasia in 77/90 (86%) of patients. The prevailing patellar shapes were type III, type IV and Hunter’s cap. No patellar shape genotype-phenotype association could be found. The malformations of the distal femur comprised shortening of the lateral femoral condyle in 46 out of 84 patients (55%), with a prominent anterior surface of the lateral femoral condyle in 47 out of 84 patients (56%) and a flat anterior surface of the medial femoral condyle in 78 out of 85 patients (92%). The trochlea was type A1 according to the Dejour classification in 79 out of 85 patients (93%).

Conclusion

An easily recognisable characteristic quartet of malformations consisting of patellar aplasiaor hypoplasia and the malformations of the distal femur was found in 22 out of 81 patients (27%), with the majority displaying at least three malformations. Take home message: The distinct malformations of the knee in nail patella syndrome are easily recognisable on conventional radiographs and lead to the correct interpretation of the aberrant morphology which is essential in the treatment of these patellofemoral disorders. Cite this article: Bone Joint J 2016;98-B:483–9.

Skeletal gene expression in the temporal region of the reptilian embryos: implications for the evolution of reptilian skull morphology

Reptiles have achieved highly diverse morphological and physiological traits that allow them to exploit various ecological niches and resources. Morphology of the temporal region of the reptilian skull is highly diverse and historically it has been treated as an important character for classifying reptiles and has helped us understand the ecology and physiology of each species. However, the developmental mechanism that generates diversity of reptilian skull morphology is poorly understood. We reveal a potential developmental basis that generates morphological diversity in the temporal region of the reptilian skull by performing a comparative analysis of gene expression in the embryos of reptile species with different skull morphology. By investigating genes known to regulate early osteoblast development, we find dorsoventrally broadened unique expression of the early osteoblast marker, Runx2, in the temporal region of the head of turtle embryos that do not form temporal fenestrae. We also observe that Msx2 is also uniquely expressed in the mesenchymal cells distributed at the temporal region of the head of turtle embryos. Furthermore, through comparison of gene expression pattern in the embryos of turtle, crocodile, and snake species, we find a possible correlation between the spatial patterns of Runx2 and Msx2 expression in cranial mesenchymal cells and skull morphology of each reptilian lineage. Regulatory modifications of Runx2 and Msx2 expression in osteogenic mesenchymal precursor cells are likely involved in generating morphological diversity in the temporal region of the reptilian skull.

Molecular and cellular changes associated with the evolution of novel jaw muscles in parrots

Vertebrates have achieved great evolutionary success due in large part to the anatomical diversification of their jaw complex, which allows them to inhabit almost every ecological niche. While many studies have focused on mechanisms that pattern the jaw skeleton, much remains to be understood about the origins of novelty and diversity in the closely associated musculature. To address this issue, we focused on parrots, which have acquired two anatomically unique jaw muscles: the ethmomandibular and the pseudomasseter. In parrot embryos, we observe distinct and highly derived expression patterns for Scx, Bmp4, Tgfβ2 and Six2 in neural crest-derived mesenchyme destined to form jaw muscle connective tissues. Furthermore, immunohistochemical analysis reveals that cell proliferation is more active in the cells within the jaw muscle than in surrounding connective tissue cells. This biased and differentially regulated mode of cell proliferation in cranial musculoskeletal tissues may allow these unusual jaw muscles to extend towards their new attachment sites. We conclude that the alteration of neural crest-derived connective tissue distribution during development may underlie the spatial changes in jaw musculoskeletal architecture found only in parrots. Thus, parrots provide valuable insights into molecular and cellular mechanisms that may generate evolutionary novelties with functionally adaptive significance.

Detection of genes regulated by Lmx1b during limb dorsalization

Lmx1b is a homeodomain transcription factor that regulates dorsal identity during limb development. Lmx1b knockout (KO) mice develop distal ventral-ventral limbs. Although induction of Lmx1b is linked to Wnt7a expression in the dorsal limb ectoderm, the downstream targets of Lmx1b that accomplish limb dorsalization are unknown. To identify genes targeted by Lmx1b, we compared gene arrays from Lmx1b KO and wild type mouse limbs during limb dorsalization, i.e., 11.5, 12.5, and 13.5 days post coitum. We identified 54 target genes that were differentially expressed in all three stages. Several skeletal targets, including Emx2, Matrilin1 and Matrilin4, demonstrated a loss of scapular expression in the Lmx1b KO mice, supporting a role for Lmx1b in scapula development. Furthermore, the relative abundance of extracellular matrix-related soft tissue targets regulated by Lmx1b, such as collagens and proteoglycans, suggests a mechanism that includes changes in the extracellular matrix composition to accomplish limb dorsalization. Our study provides the most comprehensive characterization of genes regulated by Lmx1b during limb development to-date and provides targets for further investigation.

Identification of genes controlled by LMX1B in E13.5 mouse limbs

During limb development, the dorsal limb mesenchyme expression of the transcription factor LMX1B is required for dorsoventral limb patterning. In mice, Lmx1b mutations result in the mirror-image duplication of ventral limb structures and loss of dorsal limb structures. Heterozygous LMX1B mutations in humans cause the Nail-Patella Syndrome characterized by limb, kidney, and eye developmental defects. We used DNA microarrays to compare the mRNAs in E13.5 mouse Lmx1b mutant and wild-type limbs. We report 14 genes that require Lmx1b for their normal expression in the dorsal limb or the restriction of their expression to the ventral limb.

Uncoupling skeletal and connective tissue patterning: conditional deletion in cartilage progenitors reveals cell-autonomous requirements for Lmx1b in dorsal-ventral limb patterning

Integration of muscle, connective tissue and skeletal patterning during development is essential for proper functioning of the musculoskeletal system. How this integration is achieved is poorly understood. There is ample evidence suggesting that skeletal pattern is programmed autonomously, whereas muscle pattern is, for the most part, programmed non-cell-autonomously. Connective tissues depend upon both muscle and skeletal tissues for their proper survival and development. Here, we employed a novel approach to dissect the coordination of musculoskeletal patterning during mouse limb development. Using both conditional gain- and loss-of-function approaches, we selectively deleted or activated the LIM-homeodomain transcription factor Lmx1b in skeletal progenitors using a Sox9-Cre knock-in allele. As Lmx1b is both necessary and sufficient to specify dorsal pattern, this approach allowed us to investigate the effect of selectively deleting or activating Lmx1b in skeletal progenitors on muscle, connective and skeletal tissues during limb development. Our results indicate that whereas Lmx1b activity is required autonomously in skeletal progenitors to direct dorsal pattern, loss or gain of Lmx1b activity in skeletal progenitors has no effect on muscle or connective tissue patterning. Hence, we show for the first time that skeletal and connective tissue patterning can be uncoupled, indicating a degree of autonomy in the formation of the musculoskeletal system.

Lmx1b-expressing cells in the mouse limb bud define a dorsal mesenchymal lineage compartment

The LIM-homeodomain transcription factor lmx1b is a critical regulator of vertebrate dorsal-ventral limb patterning. In the mouse embryo, lmx1b is initially transcribed throughout most, if not all, limb bud cells at early stages, but then rapidly becomes restricted specifically to dorsal mesenchymal cells with a sharp boundary between the dorsal-positive and ventral-negative expression domains. How this expression pattern is initially established is not well understood, nor are mechanism(s) that maintain a sharp dorsal-ventral boundary between lmx1b expressing and nonexpressing cells. Here, we employ a genetic fate mapping approach to establish that the transition from a broad expression domain of lmx1b to a restricted dorsal domain of expression involves selective loss of lmx1b expression in presumptive ventral cells. In addition, we show that once lmx1b expression becomes restricted to dorsal mesenchyme cells, these cells form a lineage-based compartment that prevents mixing between dorsal and ventral cells, consistent with recent fate mapping experiments carried out the chick and mouse (Pearse et al.,2007, Dev Biol 310:388-400; Arques et al.,2007, Development 134:3713-3722). Moreover, lmx1b activity is required to maintain, but not to establish the dorsal mesenchymal compartment likely through a mechanism involving differential cell adhesion. Taken together, our results indicate that lmx1b expressing cell define a dorsal limb bud mesenchymal lineage compartment and that maintenance of this compartment depends on lmx1b function.

The LIM-homeodomain transcription factor LMX1B regulates expression of NF-kappa B target genes

LMX1B is a LIM-homeodomain transcription factor essential for development. Putative LMX1B target genes have been identified through mouse gene targeting studies, but their identity as direct LMX1B targets remains hypothetical. We describe here the first molecular characterization of LMX1B target gene regulation. Microarray analysis using a tetracycline-inducible LMX1B expression system in HeLa cells revealed that a subset of NF-kappaB target genes, including IL-6 and IL-8, are upregulated in LMX1B-expressing cells. Inhibition of NF-kappaB activity by short interfering RNA-mediated knock-down of p65 impairs, while activation of NF-kappaB activity by TNF-alpha synergizes induction of NF-kappaB target genes by LMX1B. Chromatin immunoprecipitation demonstrated that LMX1B binds to the proximal promoter of IL-6 and IL-8 in vivo, in the vicinity of the characterized kappaB site, and that LMX1B recruitment correlates with increased NF-kappaB DNA association. IL-6 promoter-reporter assays showed that the kappaB site and an adjacent putative LMX1B binding motif are both involved in LMX1B-mediated transcription. Expression of NF-kappaB target genes is affected in the kidney of Lmx1b(-/-) knock-out mice, thus supporting the biological relevance of our findings. Together, these data demonstrate for the first time that LMX1B directly regulates transcription of a subset of NF-kappaB target genes in cooperation with nuclear p50/p65 NF-kappaB.

Identification of genes controlled by LMX1B in the developing mouse limb bud

In the developing limb, dorsal-ventral patterning is controlled by the transcription factor LMX1B, expressed in the dorsal mesenchyme. Loss of Lmx1b function in mice or humans results in the loss of dorsal limb structures and Nail-Patella syndrome, but the effectors through which LMX1B controls limb patterning are virtually unknown. Using microarrays to analyze the differential expression of mRNAs in wild-type vs. Lmx1b(-/-) limb buds, we have identified hundreds of genes as putative LMX1B targets. Analysis of a subset of these candidates by in situ mRNA localization has identified eight genes previously unknown to require Lmx1b for their dorsal-ventral restriction of expression in the limb. Furthermore, our results suggest that LMX1B controls different targets along the proximal-distal axis of the limb, and suggest the existence of a dorsal proximal limb region that is rich in mRNAs requiring Lmx1b for their expression.

Brachy-syndactyly caused by loss of Sfrp2 function

Wnt signaling pathways are regulated both at the intracellular and extracellular levels. During embryogenesis, the in vivo effects of the secreted frizzled-related protein (Sfrp) family of Wnt inhibitors are poorly understood. Here, we show that inactivation of Sfrp2 results in subtle limb defects in mice with mesomelic shortening and consistent shortening of all autopodal elements that is clinically manifested as brachydactyly. In addition, there is soft-tissue syndactyly of the hindlimb. The brachydactyly is caused by decreased chondrocyte proliferation and delayed differentiation in distal limb chondrogenic elements. These data suggest that Sfrp2 can regulate both chondrogenesis and regression of interdigital mesenchyme in distal limb. Sfrp2 can also repress canonical Wnt signaling by Wnt1, Wnt9a, and Wnt4 in vitro. Sfrp2-/- and TOPGAL/Sfrp2-/- mice have a mild increase in beta-catenin and beta-galactosidase staining, respectively, in some phalangeal elements. This however does not exclude a potential concurrent effect on non-canonical Wnt signaling in the growth plate. In combination with what is known about BMP and Wnt signaling in human brachydactylies, our data establish a critical role for Sfrp2 in proper distal limb formation and suggest SFPR2 could be a novel candidate gene for human brachy-syndactyly defects.

Molecular targets for tendon neoformation

Tendons and ligaments are unique forms of connective tissue that are considered an integral part of the musculoskeletal system. The ultimate function of tendon is to connect muscles to bones and to conduct the forces generated by muscle contraction into movements of the joints, whereas ligaments connect bone to bone and provide joint stabilization. Unfortunately, the almost acellular and collagen I-rich structure of tendons and ligaments makes them very poorly regenerating tissues. Injured tendons and ligaments are considered a major clinical challenge in orthopedic and sports medicine. This Review discusses the several factors that might serve as molecular targets that upon activation can enhance or lead to tendon neoformation.

Cell tracing reveals a dorsoventral lineage restriction plane in the mouse limb bud mesenchyme

Regionalization of embryonic fields into independent units of growth and patterning is a widespread strategy during metazoan development. Compartments represent a particular instance of this regionalization, in which unit coherence is maintained by cell lineage restriction between adjacent regions. Lineage compartments have been described during insect and vertebrate development. Two common characteristics of the compartments described so far are their occurrence in epithelial structures and the presence of signaling regions at compartment borders. Whereas Drosophila compartmental organization represents a background subdivision of embryonic fields that is not necessarily related to anatomical structures, vertebrate compartment borders described thus far coincide with, or anticipate, anatomical or cell-type discontinuities. Here, we describe a general method for clonal analysis in the mouse and use it to determine the topology of clone distribution along the three limb axes. We identify a lineage restriction boundary at the limb mesenchyme dorsoventral border that is unrelated to any anatomical discontinuity, and whose lineage restriction border is not obviously associated with any signaling center. This restriction is the first example in vertebrates of a mechanism of primordium subdivision unrelated to anatomical boundaries. Furthermore, this is the first lineage compartment described within a mesenchymal structure in any organism, suggesting that lineage restrictions are fundamental not only for epithelial structures, but also for mesenchymal field patterning. No lineage compartmentalization was found along the proximodistal or anteroposterior axes, indicating that patterning along these axes does not involve restriction of cell dispersion at specific axial positions.

Forced chondrocyte expression of sonic hedgehog impairs joint formation affecting proliferation and apoptosis

Proliferation and apoptosis are two fundamental processes that occur during limb development, and in particular in joint formation. To study the role of hedgehog proteins in limbs, we have misexpressed Sonic Hedgehog specifically in chondrocytes. We found that the appendicular skeleton was severely misshapen while pelvic and shoulder girdles developed normally. In particular, we detected fusion of the elbow/knee joint, no definite carpal/tarsal, metacarpal/metatarsal bones and absence of distinct phalanges, fused in a continuous cartilaginous rod. Molecular markers of joints, such as Gdf5 and sFrp2 were absent at presumptive joint sites and Tenascin C, a molecule associated with joint formation and expressed in permanent cartilage, was expressed in a wider region in transgenic animals as compared to the wild type. The ratio of proliferating to non-proliferating chondrocytes was about two times higher in transgenic developing cartilage as compared to the wild type. Accordingly, the proapoptotic gene Bax was barely detectable in the growth plate of transgenic mice and Tunel assay showed the absence of apoptosis in presumptive joints at E15.5. Taken together, these results suggest that misexpression of Sonic Hedgehog causes apoptosis and proliferation defects leading to the lack of joint cavity and fusion of selected limb skeletal elements.

Nail patella syndrome revisited: 50 years after linkage

Nail Patella Syndrome (NPS; OMIM #161200) is a pleiotropic condition, with a classical clinical tetrad of involvement of the nails, knees, elbows and the presence of iliac horns. Kidney disease and glaucoma are now recognised as part of the syndrome. Fifty years ago, James Renwick chose NPS to develop methods of linkage analysis in humans and revealed the third linkage group identified in man--that between NPS and the ABO blood group loci. After a fallow period of some forty years, the gene mutated in NPS has been identified (LMX1B) and the condition serves as a model for understanding the complex relationships between disease loci, modifier genes and the resultant clinical phenotype.

LIM-homeodomain genes in mammalian development and human disease

The human and mouse genomes each contain at least 12 genes encoding LIM homeodomain (LIM-HD) transcription factors. These gene regulatory proteins feature two LIM domains in their amino termini and a characteristic DNA binding homeodomain. Studies of mouse models and human patients have established that the LIM-HD factors are critical for the development of specialized cells in multiple tissue types, including the nervous system, skeletal muscle, the heart, the kidneys, and endocrine organs such as the pituitary gland and the pancreas. In this article, we review the roles of the LIM-HD proteins in mammalian development and their involvement in human diseases.

Human syndromes with congenital patellar anomalies and the underlying gene defects

Genetic disorders characterized by congenital patellar aplasia or hypoplasia belong to a clinically diverse and genetically heterogeneous group of lower limb malformations. Patella development involves different molecular and cellular mechanisms regulating dorso-ventral patterning, cartilage and bone formation along endochondral ossification pathways, and growth. Several human genes that are important for patella development have been uncovered by the study of human limb malformation syndromes, yet causative genes for many more such disorders await to be identified and their complex interactions in the developmental pathways deciphered. Mutant animal models of congenital patellar aplasia or hypoplasia are certainly instrumental to create more insight into this aspect of limb development. Moreover, investigation of the complete phenotype of human syndromes and animal models may reveal novel insights into the pleiotropic roles of the responsible genes in the normal developmental of other organ systems. In this review, the phenotype and gene defects of syndromes with congenital patellar aplasia or hypoplasia will be discussed, including the nail patella syndrome, small patella syndrome, isolated patella aplasia hypoplasia, Meier-Gorlin syndrome, RAPADILINO syndrome, and genitopatellar syndrome.

Genotype–phenotype studies in nail-patella syndrome show that LMX1B mutation location is involved in the risk of developing nephropathy

Nail-patella syndrome (NPS) is characterized by developmental defects of dorsal limb structures, nephropathy, and glaucoma and is caused by heterozygous mutations in the LIM homeodomain transcription factor LMX1B. In order to identify possible genotype-phenotype correlations, we performed LMX1B mutation analysis and comprehensive investigations of limb, renal, ocular, and audiological characteristics in 106 subjects from 32 NPS families. Remarkable phenotypic variability at the individual, intrafamilial, and interfamilial level was observed for different NPS manifestations. Quantitative urinanalysis revealed proteinuria in 21.3% of individuals. Microalbuminuria was detected in 21.7% of subjects without overt proteinuria. Interestingly, nephropathy appeared significantly more frequent in females. A significant association was established between the presence of clinically relevant renal involvement in an NPS patient and a positive family history of nephropathy. We identified normal-tension glaucoma (NTG) and sensorineural hearing impairment as new symptoms associated with NPS. Sequencing of LMX1B revealed 18 different mutations, including six novel variants, in 28 families. Individuals with an LMX1B mutation located in the homeodomain showed significantly more frequent and higher values of proteinuria compared to subjects carrying mutations in the LIM domains. No clear genotype-phenotype association was apparent for extrarenal manifestations. This is the first study indicating that family history of nephropathy and mutation location might be important in precipitating individual risks for developing NPS renal disease. We suggest that the NPS phenotype is broader than previously described and that NTG and hearing impairment are part of NPS. Further studies on modifier factors are needed to understand the mechanisms underlying phenotypic heterogeneity.

Six1 is not involved in limb tendon development, but is expressed in limb connective tissue under Shh regulation

Mice deficient for the homeobox gene Six1 display defects in limb muscles consistent with the Six1 expression in myogenic cells. In addition to its myogenic expression domain, Six1 has been described as being located in digit tendons and as being associated with connective tissue patterning in mouse limbs. With the aim of determining a possible involvement of Six1 in tendon development, we have carefully characterised the non-myogenic expression domain of the Six1 gene in mouse and chick limbs. In contrast to previous reports, we found that this non-myogenic domain is distinct from tendon primordia and from tendons defined by scleraxis expression. The non-myogenic domain of Six1 expression establishes normally in the absence of muscle, in Pax3-/- mutant limbs. Moreover, the expression of scleraxis is not affected in early Six1-/- mutant limbs. We conclude that the expression of the Six1 gene is not related to tendons and that Six1, at least on its own, is not involved in limb tendon formation in vertebrates. Finally, we found that the posterior domain of Six1 in connective tissue is adjacent to that of the secreted factor Sonic hedgehog and that Sonic hedgehog is necessary and sufficient for Six1 expression in posterior limb regions.