Signals regulating tendon formation during chick embryonic development

Connecting muscles to tendons: tendons and musculoskeletal development in flies and vertebrates

The formation of the musculoskeletal system represents an intricate process of tissue assembly involving heterotypic inductive interactions between tendons, muscles and cartilage. An essential component of all musculoskeletal systems is the anchoring of the force-generating muscles to the solid support of the organism: the skeleton in vertebrates and the exoskeleton in invertebrates. Here, we discuss recent findings that illuminate musculoskeletal assembly in the vertebrate embryo, findings that emphasize the reciprocal interactions between the forming tendons, muscle and cartilage tissues. We also compare these events with those of the corresponding system in the Drosophila embryo, highlighting distinct and common pathways that promote efficient locomotion while preserving the form of the organism.

Bmp signaling at the tips of skeletal muscles regulates the number of fetal muscle progenitors and satellite cells during development

Muscle progenitors, labeled by the transcription factor Pax7, are responsible for muscle growth during development. The signals that regulate the muscle progenitor number during myogenesis are unknown. We show, through in vivo analysis, that Bmp signaling is involved in regulating fetal skeletal muscle growth. Ectopic activation of Bmp signaling in chick limbs increases the number of fetal muscle progenitors and fibers, while blocking Bmp signaling reduces their numbers, ultimately leading to small muscles. The Bmp effect that we observed during fetal myogenesis is diametrically opposed to that previously observed during embryonic myogenesis and that deduced from in vitro work. We also show that Bmp signaling regulates the number of satellite cells during development. Finally, we demonstrate that Bmp signaling is active in a subpopulation of fetal progenitors and satellite cells at the extremities of muscles. Overall, our results show that Bmp signaling plays differential roles in embryonic and fetal myogenesis.

Development and progress of engineering of skeletal muscle tissue

Engineering skeletal muscle tissue remains still a challenge, and numerous studies have indicated that this technique may be of great importance in medicine in the near future. This article reviews some of the recent findings resulting from tissue engineering science related to the contractile behavior and the phenotypes of muscle tissue cells in different three-dimensional environment, and discusses how tissue engineering could be used to create and regenerate skeletal muscle, as well as the extended applications and the related patents concerned with engineered skeletal muscle.

Fgf signaling components are associated with muscles and tendons during limb development

Muscle-tendon interactions are important for the establishment of a functional musculoskeletal system. Fgf4 and Fgf8 are expressed in muscle and tendon boundary regions during limb development, suggesting a potential role for Fgf signaling pathway in muscle and tendon interactions. We have examined the expression of Fgf syn-expression group components during muscle and tendon formation of vertebrate limb development. We observed that the transcriptional effector of Fgf signaling, Pea3, and the modulators of Fgf signal, Sprouty1 and 2, were expressed in muscles and tendons and that their expression was enhanced at the myotendinous junctions in chick and mouse limbs. Analysis of Pea3 and Sprouty gene expression in muscleless limbs of Pax3 mutant mice indicated a major expression in muscles but also revealed that the Pea3 and Sprouty expression in tendons depended on muscles. Finally, our data showed that Fgf4 positively regulated Pea3, Sprouty1, and 2 expression in chick limb mesenchyme.

Recruitment and maintenance of tendon progenitors by TGFbeta signaling are essential for tendon formation

Tendons and ligaments mediate the attachment of muscle to bone and of bone to bone to provide connectivity and structural integrity in the musculoskeletal system. We show that TGFbeta signaling plays a major role in the formation of these tissues. TGFbeta signaling is a potent inducer of the tendon progenitor (TNP) marker scleraxis both in organ culture and in cultured cells, and disruption of TGFbeta signaling in Tgfb2(-/-);Tgfb3(-/-) double mutant embryos or through inactivation of the type II TGFbeta receptor (TGFBR2; also known as TbetaRII) results in the loss of most tendons and ligaments in the limbs, trunk, tail and head. The induction of scleraxis-expressing TNPs is not affected in mutant embryos and the tendon phenotype is first manifested at E12.5, a developmental stage in which TNPs are positioned between the differentiating muscles and cartilage, and in which Tgfb2 or Tgfb3 is expressed both in TNPs and in the differentiating muscles and cartilage. TGFbeta signaling is thus essential for maintenance of TNPs, and we propose that it also mediates the recruitment of new tendon cells by differentiating muscles and cartilage to establish the connections between tendon primordia and their respective musculoskeletal counterparts, leading to the formation of an interconnected and functionally integrated musculoskeletal system.

Developmental origins of species-specific muscle pattern

Vertebrate jaw muscle anatomy is conspicuously diverse but developmental processes that generate such variation remain relatively obscure. To identify mechanisms that produce species-specific jaw muscle pattern we conducted transplant experiments using Japanese quail and White Pekin duck, which exhibit considerably different jaw morphologies in association with their particular modes of feeding. Previous work indicates that cranial muscle formation requires interactions with adjacent skeletal and muscular connective tissues, which arise from neural crest mesenchyme. We transplanted neural crest mesenchyme from quail to duck embryos, to test if quail donor-derived skeletal and muscular connective tissues could confer species-specific identity to duck host jaw muscles. Our results show that duck host jaw muscles acquire quail-like shape and attachment sites due to the presence of quail donor neural crest-derived skeletal and muscular connective tissues. Further, we find that these species-specific transformations are preceded by spatiotemporal changes in expression of genes within skeletal and muscular connective tissues including Sox9, Runx2, Scx, and Tcf4, but not by alterations to histogenic or molecular programs underlying muscle differentiation or specification. Thus, neural crest mesenchyme plays an essential role in generating species-specific jaw muscle pattern and in promoting structural and functional integration of the musculoskeletal system during evolution.

Tubulin polymerization-promoting protein family member 3, Tppp3, is a specific marker of the differentiating tendon sheath and synovial joints

Tppp3, a member of the Tubulin polymerization-promoting protein family, is an intrinsically unstructured protein that induces tubulin polymerization. We show that Tppp3 is a distinct marker in the developing musculoskeletal system. In tendons, Tppp3 is expressed in cells at the circumference of the developing tendons, likely the progenitors of connective tissues that surround tendons: the tendon sheath, epitenon, and paratenon. These tissues form an elastic sleeve around tendons and provide lubrication to minimize friction between tendons and surrounding tissues. Tppp3 is the first molecular marker of the tendon sheath, opening the door for direct examination of these tissues. Tppp3 is also expressed in forming synovial joints. The onset of Tppp3 expression in joints coincides with cavitation, representing a molecular marker that can be used to indicate this stage in joint transition in joint differentiation. In late embryonic stages, Tppp3 expression highlights other demarcation lines that surround differentiating tissues in the forelimb.

Relationship between neural crest cells and cranial mesoderm during head muscle development

Background

In vertebrates, the skeletal elements of the jaw, together with the connective tissues and tendons, originate from neural crest cells, while the associated muscles derive mainly from cranial mesoderm. Previous studies have shown that neural crest cells migrate in close association with cranial mesoderm and then circumscribe but do not penetrate the core of muscle precursor cells of the branchial arches at early stages of development, thus defining a sharp boundary between neural crest cells and mesodermal muscle progenitor cells. Tendons constitute one of the neural crest derivatives likely to interact with muscle formation. However, head tendon formation has not been studied, nor have tendon and muscle interactions in the head.

Methodology/Principal Findings

Reinvestigation of the relationship between cranial neural crest cells and muscle precursor cells during development of the first branchial arch, using quail/chick chimeras and molecular markers revealed several novel features concerning the interface between neural crest cells and mesoderm. We observed that neural crest cells migrate into the cephalic mesoderm containing myogenic precursor cells, leading to the presence of neural crest cells inside the mesodermal core of the first branchial arch. We have also established that all the forming tendons associated with branchiomeric and eye muscles are of neural crest origin and express the Scleraxis marker in chick and mouse embryos. Moreover, analysis of Scleraxis expression in the absence of branchiomeric muscles in Tbx1^−/− mutant mice, showed that muscles are not necessary for the initiation of tendon formation but are required for further tendon development.

Conclusions/Significance

This results show that neural crest cells and muscle progenitor cells are more extensively mixed than previously believed during arch development. In addition, our results show that interactions between muscles and tendons during craniofacial development are similar to those observed in the limb, despite the distinct embryological origin of these cell types in the head.

Spatiotemporal protein distribution of TGF-betas, their receptors, and extracellular matrix molecules during embryonic tendon development

Tendon is one of the least understood tissues of the musculoskeletal system in terms of development and morphogenesis. Collagen fibrillogenesis has been the most studied aspect of tendon development, focusing largely on the role of matrix molecules such as collagen type III and decorin. While involvement of matrix molecules in collagen fibrillogenesis during chick tendon development is well understood, the role of growth factors has yet to be elucidated. This work examines the expression patterns of transforming growth factor (TGF) -beta1, -beta2, and -beta3, and their receptors with respect to expression patterns of collagen type III, decorin, and fibronectin. We focus on the intermediate stages of tendon development in the chick embryo, a period during which the tendon micro- and macro-architecture are being established. Our findings demonstrate for the first time that TGF-beta1, -beta2, and -beta3 have distinct spatiotemporal developmental protein localization patterns in the developing tendon and strongly suggest that these isoforms have independent roles in tendon development.

Twenty-five years of tendon and ligament research

Twenty-five years ago, the Journal of Orthopaedic Research published its first volume, which included five articles covering topics in tendon and ligament research. Since then, the body of tendon and ligament research has continued to increase exponentially. This review summarizes major advancements in tendon and ligament research since the initial publication of this journal. The purpose of this article is not to provide an in-depth review of all of tendon and ligament research, but instead to provide a concise literature review of some of the major and recurring areas of research. The general topics covered over the last 25 years include tissue properties, tendinopathy, healing, and engineered scaffolds.

Tendons of myostatin-deficient mice are small, brittle, and hypocellular

Tendons play a significant role in the modulation of forces transmitted between bones and skeletal muscles and consequently protect muscle fibers from contraction-induced, or high-strain, injuries. Myostatin (GDF-8) is a negative regulator of muscle mass. Inhibition of myostatin not only increases the mass and maximum isometric force of muscles, but also increases the susceptibility of muscle fibers to contraction-induced injury. We hypothesized that myostatin would regulate the morphology and mechanical properties of tendons. The expression of myostatin and the myostatin receptors ACVR2B and ACVRB was detectable in tendons. Surprisingly, compared with wild type (MSTN(+/+)) mice, the tendons of myostatin-null mice (MSTN(-/-)) were smaller and had a decrease in fibroblast density and a decrease in the expression of type I collagen. Tendons of MSTN(-/-) mice also had a decrease in the expression of two genes that promote tendon fibroblast proliferation: scleraxis and tenomodulin. Treatment of tendon fibroblasts with myostatin activated the p38 MAPK and Smad2/3 signaling cascades, increased cell proliferation, and increased the expression of type I collagen, scleraxis, and tenomodulin. Compared with the tendons of MSTN(+/+) mice, the mechanical properties of tibialis anterior tendons from MSTN(-/-) mice had a greater peak stress, a lower peak strain, and increased stiffness. We conclude that, in addition to the regulation of muscle mass and force, myostatin regulates the structure and function of tendon tissues.

BMP and FGF regulatory pathways in semilunar valve precursor cells

In the developing atrioventricular (AV) valve, limb bud, and somites, cartilage cell lineage differentiation is regulated by bone morphogenetic protein (BMP), while fibroblast growth factor (FGF) controls tendon cell fate. We observed aggrecan and sox9, characteristic of cartilage cell types, and scleraxis and tenascin, characteristic of tendon cell types, in developing avian semilunar valves. Addition of BMP4 to outflow tract (OFT) precursor cells of young (E4.5) but not older (E6) chick embryos activated Smad1/5/8 and induced sox9 and aggrecan expression, while FGF4 treatment increased phosphorylated MAPK (dpERK) signaling and promoted expression of scleraxis and tenascin. These results identify BMP and FGF pathways that promote expression of cartilage- or tendon-like characteristics in semilunar valve precursor cells. In contrast to AV valve precursor cells, which diversify into leaflets (cartilage-like) or chordae tendineae (tendon-like), semilunar valve cells exhibit both cartilage- and tendon-like characteristics in the developing and mature valve cusp.

Involvement of vessels and PDGFB in muscle splitting during chick limb development

Muscle formation and vascular assembly during embryonic development are usually considered separately. In this paper, we investigate the relationship between the vasculature and muscles during limb bud development. We show that endothelial cells are detected in limb regions before muscle cells and can organize themselves in space in the absence of muscles. In chick limbs, endothelial cells are detected in the future zones of muscle cleavage, delineating the cleavage pattern of muscle masses. We therefore perturbed vascular assembly in chick limbs by overexpressing VEGFA and demonstrated that ectopic blood vessels inhibit muscle formation, while promoting connective tissue. Conversely, local inhibition of vessel formation using a soluble form of VEGFR1 leads to muscle fusion. The endogenous location of endothelial cells in the future muscle cleavage zones and the inverse correlation between blood vessels and muscle suggests that vessels are involved in the muscle splitting process. We also identify the secreted factor PDGFB (expressed in endothelial cells) as a putative molecular candidate mediating the muscle-inhibiting and connective tissue-promoting functions of blood vessels. Finally, we propose that PDGFB promotes the production of extracellular matrix and attracts connective tissue cells to the future splitting site, allowing separation of the muscle masses during the splitting process.

Scleraxis and NFATc regulate the expression of the pro-alpha1(I) collagen gene in tendon fibroblasts

The combinatorial action of separate cis-acting elements controls the cell-specific expression of type I collagen genes. In particular, we have shown that two short elements located between -3.2 and -2.3 kb and named TSE1 and TSE2 are needed for expression of the mouse COL1a1 gene in tendon fibroblasts. In this study, we analyzed the trans-acting factors binding to TSE1 and TSE2. Gel shift experiments showed that scleraxis (SCX), which is a basic helix-loop-helix transcription factor that is expressed selectively in tendon fibroblasts, binds TSE2, preferentially as a SCX/E47 heterodimer. In transfection experiments, overexpression of SCX and E47 strongly enhanced the activity of reporter constructs harboring either four copies of TSE2 cloned upstream of the COL1a1 minimal promoter or a 3.2-kb segment of the COL1a1 proximal promoter. Analysis of TSE1 showed that it contains a consensus binding site for NFATc transcription factors. This led us to show that the NFATc4 gene is expressed in tendons of developing mouse limbs and in TT-D6 cells, a cell line that has characteristics of tendon fibroblasts. In gel shift assays, TSE1 bound NFATc proteins present in nuclear extracts from TT-D6 cells. In transfection experiments, overexpression of NFATc transactivated a reporter construct harboring four copies of TSE1 cloned upstream of the COL1a1 minimal promoter. By contrast, inhibition of the nuclear translocation of NFATc proteins in TT-D6 cells strongly inhibited the expression of the COL1a1 gene. Taken together, these results suggest that SCX and NFATc4 cooperate to activate the COL1a1 gene specifically in tendon fibroblasts.

Sox9 is required for precursor cell expansion and extracellular matrix organization during mouse heart valve development

Heart valve structures derived from mesenchymal cells of the endocardial cushions (ECs) are composed of highly organized cell lineages and extracellular matrix. Sox9 is a transcription factor required for both early and late stages of cartilage formation that is also expressed in the developing valves of the heart. The requirements for Sox9 function during valvulogenesis and adult valve homeostasis in mice were examined by conditional inactivation of Sox9 using Tie2-cre and Col2a1-cre transgenes. Sox9(flox/flox);Tie2-cre mice die before E14.5 with hypoplastic ECs, reduced cell proliferation and altered extracellular matrix protein (ECM) deposition. Sox9(flox/flox);Col2a1-cre mice die at birth with thickened heart valve leaflets, reduced expression of cartilage-associated proteins and abnormal ECM patterning. Thickened valve leaflets and calcium deposits, characteristic of valve disease, are observed in heterozygous adult Sox9(flox/+);Col2a1-cre mice. Therefore, Sox9 is required early in valve development for expansion of the precursor cell population and later is required for normal expression and distribution of valvular ECM proteins. These data indicate that Sox9 is required for early and late stages of valvulogenesis and identify a potential role for Sox9 in valve disease mechanisms.

Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling

Vertebrate muscle morphogenesis is a complex developmental process, which remains quite yet unexplored at cellular and molecular level. In this work, we have found that sculpturing programmed cell death is a key morphogenetic process responsible for the formation of individual foot muscles in the developing avian limb. Muscle fibers are produced in excess in the precursor dorsal and ventral muscle masses of the limb bud and myofibers lacking junctions with digital tendons are eliminated via apoptosis. Microsurgical experiments to isolate the developing muscles from their specific tendons are consistent with a role for tendons in regulating survival of myogenic cells. Analysis of the expression of Raldh2 and local treatments with retinoic acid indicate that this signaling pathway mediates apoptosis in myogenic cells, appearing also involved in tendon maturation. Retinoic acid inhibition experiments led to defects in muscle belly segmentation and myotendinous junction formation. It is proposed that heterogeneous local distribution of retinoids controlled through Raldh2 and Cyp26A1 is responsible for matching the fleshy and the tendinous components of each muscle belly.

Role of RhoC in digit morphogenesis during limb development

Here we report a new role for the small GTPase RhoC in the control of limb chondrogenesis. Expression of rhoC is a precocious marker of the zeugopodial and digit blastemas and is induced by treatments with TGFbetas preceding the formation of ectopic digits. As development progresses, expression of rhoC outlines the growing distal tip of the digits, and marks the regions of interphalangeal joint formation. Functional experiments show that RhoC is a negative regulator of chondrogenesis, which controls digit outgrowth and joint segmentation. These functions appear to be mediated by reorganization of the actin cytoskeleton and modification of the adhesive properties of the mesenchymal cells.

Scleraxis positively regulates the expression of tenomodulin, a differentiation marker of tenocytes

Tenomodulin (TeM) is a type II transmembrane glycoprotein containing a C-terminal anti-angiogenic domain and is predominantly expressed in tendons and ligaments. Here we report that TeM expression is closely associated with the appearance of tenocytes during chick development and is positively regulated by Scleraxis (Scx). At stage 23, when Scx expression in the syndetome has extended to the tail region, TeM was detectable in the anterior eight somites. At stage 25, TeM and Scx were both detectable in the regions adjacent to the myotome. Double positive domains for these genes were flanked by a dorsal TeM single positive and a ventral Scx single positive domain. At stage 28, the expression profile of TeM in the axial tendons displayed more distinct morphological features at different levels of the vertebrae. At stage 32 and later, Scx and TeM showed similar expression profiles in developing tendons. Retroviral expression of Scx resulted in the significant upregulation of TeM in cultured tenocytes, but not in chondrocytes. In addition, the misexpression of RCAS-cScx by electroporation into the hindlimb could not induce the generation of additional tendons, but did result in the upregulation of TeM expression in the tendons at stage 33 and later. These findings suggest that TeM is a late marker of tendon formation and that Scx positively regulates TeM expression in a tendon cell lineage-dependent manner.

BMP and FGF regulatory pathways control cell lineage diversification of heart valve precursor cells

The atrioventricular heart valve leaflets and chordae tendineae are composed of diverse cell lineages and highly organized extracellular matrices that share characteristics with cartilage and tendon cell types in the limb buds and somites. During embryonic chicken valvulogenesis, aggrecan and sox9, characteristic of cartilage cells, are observed in the AV valve leaflets, in contrast to tendon-associated genes scleraxis and tenascin, present in the chordae tendineae. In the limb buds and somites, cartilage cell lineage differentiation is regulated by BMP2, while FGF4 controls tendon cell fate. The ability of BMP2 and FGF4 to induce similar patterns of gene expression in heart valve precursor cells was examined. In multiple assays of cells from prefused endocardial cushions, BMP2 is sufficient to activate Smad1/5/8 phosphorylation and induce sox9 and aggrecan expression, while FGF4 treatment increases phosphorylated MAPK (dpERK) signaling and promotes expression of scleraxis and tenascin. However, these treatments do not alter differentiated lineage gene expression in valve progenitors from fused cushions of older embryos. Together, these studies define regulatory pathways of AV valve progenitor cell diversification into leaflets and chordae tendineae that share inductive interactions and differentiation phenotypes with cartilage and tendon cell lineages.

Hearts and bones: shared regulatory mechanisms in heart valve, cartilage, tendon, and bone development

The mature heart valves are dynamic structures composed of highly organized cell lineages and extracellular matrices. The discrete architecture of connective tissue within valve leaflets and supporting structures allows the valve to withstand life-long functional demands and changes in hemodynamic forces and load. The dysregulation of ECM organization is a common feature of heart valve disease and can often be linked to genetic defects in matrix protein structure or developmental regulation. Recent studies have identified specific regulatory pathways that are active in the developing valve structures and also control cartilage, tendon, and bone development. This review will focus on the regulatory hierarchies that control normal and abnormal heart valve development in parallel with other connective tissue cell types.

Tendon and ligament: development, repair and disease

Tendons and ligaments (T/L) are very similar fibrous tissues that respectively connect muscle to bone and bone to bone. They are comprised of fibroblasts that produce large amounts of extra-cellular matrix, resulting in a dense and hypocellular structure. The complex molecular organization of T/L, together with high water content, are responsible for their viscoelastic properties, hence insuring their mechanical function. We will first review recent work on tendon embryology and discuss ligament formation, which has been less documented. We will next summarize our current knowledge of T/L molecular architecture, alterations of which are a major cause for disease. We will finally focus on T/L repair after injury and on genetic diseases responsible for T/L defects.

New dimensions in tissue engineering: possible models for human physiology

Tissue engineering is a discipline of great promise. In some areas, such as the cornea, tissues engineered in the laboratory are already in clinical use. In other areas, where the tissue architecture is more complex, there are a number of obstacles to manoeuvre before clinically relevant tissues can be produced. However, even in areas where clinically relevant tissues are decades away, the tissues being produced at the moment provide powerful new models to aid the understanding of complex physiological processes. This article provides a personal view of the role of tissue engineering in advancing our understanding of physiology, with specific attention being paid to musculoskeletal tissues.

Fgf8 drives myogenic progression of a novel lateral fast muscle fibre population in zebrafish

Fibroblast growth factors (Fgfs) have long been implicated in regulating vertebrate skeletal muscle differentiation, but their precise role(s) in vivo remain unclear. Here, we show that Fgf8 signalling in the somite is required for myod expression and terminal differentiation of a subset of fast muscle cells in the zebrafish lateral somite. In the absence of Fgf8, lateral somite cells transiently express myf5 but fail to make muscle and remain in a dermomyotome-like state characterised by pax3 and meox expression. Slow muscle fibres form and commence normal migration in the absence of Fgf8, but fail to traverse the expanded undifferentiated lateral somite. The Fgf8-independent residual population of medial fast muscle fibres is not Hedgehog dependent. However, Fgf8-independent medial fast muscle precursors are lacking in floatinghead mutants,suggesting that they require another ventral midline-derived signal. We conclude that Fgf8 drives terminal differentiation of a specific population of lateral muscle precursor cells within the early somite.

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.