Tissue and cellular patterning of the musculature in chick wings

Vitamin A Promotes the Repair of Mice Skeletal Muscle Injury through RARα

Vitamin A (VitA) is an important fat-soluble vitamin which plays an important role in cell growth and individual development. However, the effect of VitA on the repair process of muscle injury and its molecular mechanism are still unclear. In this study, VitA and RA were first added to the culture medium of differentiated cells. We then detected cell differentiation marker proteins and myotube fusion. Moreover, the effects of VitA on RARα expression and nuclear translocation were further examined. The results showed that VitA significantly promoted the differentiation of C2C12, and the expression of RARα was significantly increased. Furthermore, VitA was injected into skeletal muscle injury in mice. HE staining and Western Blot results showed that VitA could significantly accelerate the repair of skeletal muscle injury and VitA increase the expression of RARα in mice. This study provides a theoretical basis for elucidating the regulation mechanism of VitA-mediated muscle development and the development of therapeutic drugs for muscle diseases in animals.

Comparative development of limb musculature in phylogenetically and ecologically divergent lizards

BACKGROUND

Squamate reptiles (lizards, snakes, and amphisbaenians) exhibit incredible diversity in their locomotion, behavior, morphology, and ecological breadth. Although they often are used as models of locomotor diversity, surprisingly little attention has been given to muscle development in squamate reptiles. In fact, the most detailed examination was conducted almost 80 years ago and solely focused on the proximal limb regions. Herein, we present forelimb and hindlimb muscle morphogenesis data for three lizard species with different locomotion and feeding strategies: the desert grassland whiptail lizard, the central bearded dragon, and the veiled chameleon. This study fills critical gaps in our understanding of muscle morphogenesis in squamate reptiles and presents a comparative and temporospatial analysis of muscle development.

RESULTS

Our results reveal a conserved pattern of early muscle development among lizards with different adult morphologies and ecologies. The variations that exist are concentrated in distal regions, particularly the specialized autopodia of chameleons, where differentiation of muscles associated with the digits is delayed.

CONCLUSIONS

The chameleon autopod provides an example of major evolutionary modifications to the skeleton with only minor disruption of the conserved order and pattern of limb muscle development. This robustness of muscle patterning facilitates the evolution of extreme yet functional phenotypes.

Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning

The fossil fish Rhizodus hibberti, a member of the tetrapod stem group, shows a unique skeletal pattern in the pelvic fin. Rather than the highly conserved one-to-two pattern of a femur, tibia, and fibula (seen in all known tetrapods, including the extinct, fishlike members of the group), the fin of Rhizodus comprises a femur articulating distally with three bones, each with a distinct morphology. This reveals an early stage in the evolution of limb development, in which the processes patterning the proximal parts of the embryonic fin/limb (the stylopod and zeugopod) were not constrained in the way seen in living tetrapods and could produce more varied skeletal patterns in the adult.

All living tetrapods have a one-to-two branching pattern in the embryonic proximal limb skeleton, with a single element at the base of the limb (the humerus or femur) that articulates distally with two parallel radials (the ulna and radius or the tibia and fibula). This pattern is also seen in the fossilized remains of stem-tetrapods, including the fishlike members of the group, in which despite the absence of digits, the proximal parts of the fin skeleton clearly resemble those of later tetrapods. However, little is known about the developmental mechanisms that establish and canalize this highly conserved pattern. We describe the well-preserved pelvic fin skeleton of Rhizodus hibberti, a Carboniferous sarcopterygian (lobe-finned) fish, and member of the tetrapod stem group. In this specimen, three parallel radials, each robust with a distinct morphology, articulate with the femur. We review this unexpected morphology in a phylogenetic and developmental context. It implies that the developmental patterning mechanisms seen in living tetrapods, now highly constrained, evolved from mechanisms flexible enough to accommodate variation in the zeugopod (even between pectoral and pelvic fins), while also allowing each element to have a unique morphology.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Retinoic acid maintains human skeletal muscle progenitor cells in an immature state

Muscle satellite cells are resistant to cytotoxic agents, and they express several genes that confer resistance to stress, thus allowing efficient dystrophic muscle regeneration after transplantation. However, once they are activated, this capacity to resist to aggressive agents is diminished resulting in massive death of transplanted cells. Although cell immaturity represents a survival advantage, the signalling pathways involved in the control of the immature state remain to be explored. Here, we show that incubation of human myoblasts with retinoic acid impairs skeletal muscle differentiation through activation of the retinoic-acid receptor family of nuclear receptor. Conversely, pharmacologic or genetic inactivation of endogenous retinoic-acid receptors improved myoblast differentiation. Retinoic acid inhibits the expression of early and late muscle differentiation markers and enhances the expression of myogenic specification genes, such as PAX7 and PAX3. These results suggest that the retinoic-acid-signalling pathway might maintain myoblasts in an undifferentiated/immature stage. To determine the relevance of these observations, we characterised the retinoic-acid-signalling pathways in freshly isolated satellite cells in mice and in siMYOD immature human myoblasts. Our analysis reveals that the immature state of muscle progenitors is correlated with high expression of several genes of the retinoic-acid-signalling pathway both in mice and in human. Taken together, our data provide evidences for an important role of the retinoic-acid-signalling pathway in the regulation of the immature state of muscle progenitors.

Sonic Hedgehog Signaling in Limb Development

The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. Detailed analyses have revealed that Shh has the properties of the long sought after polarizing region morphogen that specifies positional values across the antero-posterior axis (e.g., thumb to little finger axis) of the limb. Shh has also been shown to control the width of the limb bud by stimulating mesenchyme cell proliferation and by regulating the antero-posterior length of the apical ectodermal ridge, the signaling region required for limb bud outgrowth and the laying down of structures along the proximo-distal axis (e.g., shoulder to digits axis) of the limb. It has been shown that Shh signaling can specify antero-posterior positional values in limb buds in both a concentration- (paracrine) and time-dependent (autocrine) fashion. Currently there are several models for how Shh specifies positional values over time in the limb buds of chick and mouse embryos and how this is integrated with growth. Extensive work has elucidated downstream transcriptional targets of Shh signaling. Nevertheless, it remains unclear how antero-posterior positional values are encoded and then interpreted to give the particular structure appropriate to that position, for example, the type of digit. A distant cis-regulatory enhancer controls limb-bud-specific expression of Shh and the discovery of increasing numbers of interacting transcription factors indicate complex spatiotemporal regulation. Altered Shh signaling is implicated in clinical conditions with congenital limb defects and in the evolution of the morphological diversity of vertebrate limbs.

Selective Retinoic Acid Receptor γ Agonists Promote Repair of Injured Skeletal Muscle in Mouse

Retinoic acid signaling regulates several biological events, including myogenesis. We previously found that retinoic acid receptor γ (RARγ) agonist blocks heterotopic ossification, a pathological bone formation that mostly occurs in the skeletal muscle. Interestingly, RARγ agonist also weakened deterioration of muscle architecture adjacent to the heterotopic ossification lesion, suggesting that RARγ agonist may oppose skeletal muscle damage. To test this hypothesis, we generated a critical defect in the tibialis anterior muscle of 7-week-old mice with a cautery, treated them with RARγ agonist or vehicle corn oil, and examined the effects of RARγ agonist on muscle repair. The muscle defects were partially repaired with newly regenerating muscle cells, but also filled with adipose and fibrous scar tissue in both RARγ-treated and control groups. The fibrous or adipose area was smaller in RARγ agonist-treated mice than in the control. In addition, muscle repair was remarkably delayed in RARγ-null mice in both critical defect and cardiotoxin injury models. Furthermore, we found a rapid increase in retinoid signaling in lacerated muscle, as monitored by retinoid signaling reporter mice. Together, our results indicate that endogenous RARγ signaling is involved in muscle repair and that selective RARγ agonists may be beneficial to promote repair in various types of muscle injuries.

Growth of limb muscle is dependent on skeletal-derived Indian hedgehog

During embryogenesis, muscle and bone develop in close temporal and spatial proximity. We show that Indian Hedgehog, a bone-derived signaling molecule, participates in growth of skeletal muscle. In Ihh(-/-) embryos, skeletal muscle development appears abnormal at embryonic day 14.5 and at later ages through embryonic day 20.5, dramatic losses of hindlimb muscle occur. To further examine the role of Ihh in myogenesis, we manipulated Ihh expression in the developing chick hindlimb. Reduction of Ihh in chicken embryo hindlimbs reduced skeletal muscle mass similar to that seen in Ihh(-/-) mouse embryos. The reduction in muscle mass appears to be a direct effect of Ihh since ectopic expression of Ihh by RCAS retroviral infection of chicken embryo hindlimbs restores muscle mass. These effects are independent of bone length, and occur when Shh is not expressed, suggesting Ihh acts directly on fetal myoblasts to regulate secondary myogenesis. Loss of muscle mass in Ihh null mouse embryos is accompanied by a dramatic increase in myoblast apoptosis by a loss of p21 protein. Our data suggest that Ihh promotes fetal myoblast survival during their differentiation into secondary myofibers by maintaining p21 protein levels.

Many routes to the same destination: lessons from skeletal muscle development

The development and differentiation of vertebrate skeletal muscle provide an important paradigm to understand the inductive signals and molecular events controlling differentiation of specific cell types. Recent findings show that a core transcriptional network, initiated by the myogenic regulatory factors (MRFs; MYF5, MYOD, myogenin and MRF4), is activated by separate populations of cells in embryos in response to various signalling pathways. This review will highlight how cells from multiple distinct starting points can converge on a common set of regulators to generate skeletal muscle.

Connective tissue fibroblasts and Tcf4 regulate myogenesis

Muscle and its connective tissue are intimately linked in the embryo and in the adult, suggesting that interactions between these tissues are crucial for their development. However, the study of muscle connective tissue has been hindered by the lack of molecular markers and genetic reagents to label connective tissue fibroblasts. Here, we show that the transcription factor Tcf4 (transcription factor 7-like 2; Tcf7l2) is strongly expressed in connective tissue fibroblasts and that Tcf4(GFPCre) mice allow genetic manipulation of these fibroblasts. Using this new reagent, we find that connective tissue fibroblasts critically regulate two aspects of myogenesis: muscle fiber type development and maturation. Fibroblasts promote (via Tcf4-dependent signals) slow myogenesis by stimulating the expression of slow myosin heavy chain. Also, fibroblasts promote the switch from fetal to adult muscle by repressing (via Tcf4-dependent signals) the expression of developmental embryonic myosin and promoting (via a Tcf4-independent mechanism) the formation of large multinucleate myofibers. In addition, our analysis of Tcf4 function unexpectedly reveals a novel mechanism of intrinsic regulation of muscle fiber type development. Unlike other intrinsic regulators of fiber type, low levels of Tcf4 in myogenic cells promote both slow and fast myogenesis, thereby promoting overall maturation of muscle fiber type. Thus, we have identified novel extrinsic and intrinsic mechanisms regulating myogenesis. Most significantly, our data demonstrate for the first time that connective tissue is important not only for adult muscle structure and function, but is a vital component of the niche within which muscle progenitors reside and is a critical regulator of myogenesis.

Vestigial-like 2 acts downstream of MyoD activation and is associated with skeletal muscle differentiation in chick myogenesis

The co-factor Vestigial-like 2 (Vgl-2), in association with the Scalloped/Tef/Tead transcription factors, has been identified as a component of the myogenic program in the C2C12 cell line. In order to understand Vgl-2 function in embryonic muscle formation, we analysed Vgl-2 expression and regulation during chick embryonic development. Vgl-2 expression was associated with all known sites of skeletal muscle formation, including those in the head, trunk and limb. Vgl-2 was expressed after the myogenic factor MyoD, regardless of the site of myogenesis. Analysis of Vgl-2 regulation by Notch signalling showed that Vgl-2 expression was down-regulated by Delta1-activated Notch, similarly to the muscle differentiation genes MyoD, Myogenin,Desmin, and Mef2c, while the expression of the muscle progenitor markers such as Myf5, Six1 and FgfR4 was not modified. Moreover, we established that the Myogenic Regulatory Factors (MRFs) associated with skeletal muscle differentiation (MyoD, Myogenin and Mrf4) were sufficient to activate Vgl-2 expression, while Myf5 was not able to do so. The Vgl-2 endogenous expression, the similar regulation of Vgl-2 and that of MyoD and Myogenin by Notch signalling, and the positive regulation of Vgl-2 by these MRFs suggest that Vgl-2 acts downstream of MyoD activation and is associated with the differentiation step in embryonic skeletal myogenesis.

Micro-magnetic resonance imaging of avian embryos

Chick embryos are useful models for probing developmental mechanisms including those involved in organogenesis. In addition to classic embryological manipulations, it is possible to test the function of molecules and genes while the embryo remains within the egg. Here we define conditions for imaging chick embryo anatomy and for visualising living quail embryos. We focus on the developing limb and describe how different tissues can be imaged using micro-magnetic resonance imaging and this information then synthesised, using a three-dimensional visualisation package, into detailed anatomy. We illustrate the potential for micro-magnetic resonance imaging to analyse phenotypic changes following chick limb manipulation. The work with the living quail embryos lays the foundations for using micro-magnetic resonance imaging as an experimental tool to follow the consequences of such manipulations over time.

MDGA1, an IgSF molecule containing a MAM domain, heterophilically associates with axon- and muscle-associated binding partners through distinct structural domains

Molecules belonging to the immunoglobulin superfamily (IgSF) are reported to be involved in intercellular communication in the developing nervous system. We have identified a novel GPI-anchored IgSF molecule containing a MAM (meprin, A5 protein, PTPmu) domain, named MDGA1, by screening for genes that are expressed by subpopulations of cells in the embryonic chick spinal cord. MDGA1 is selectively expressed by brachial LMCm motor neurons, some populations of DRG neurons, and interneurons. We found that MDGA1 interacts heterophilically with axon-rich regions, mainly through its MAM domain. Interestingly, MDGA1 also interacts with differentiating muscle through its N-terminal region, which contains Ig domains. These results suggest that MDGA1 functions in MDGA1-expressing nerves en route to and at their target site.

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.

The differentiation and morphogenesis of craniofacial muscles

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

Comparative analysis of the expression patterns of Wnts during chick limb development

Wnts control a number of processes during limb development-from initiating outgrowth and controlling patterning, to regulating cell differentiation in a number of tissues. We analyzed the expression pattern of various Wnts (4, 5a, 5b, 6, 11, and 14) in whole mount in situ hybridization during chick wing development. From HH stage 26, expression of Wnt 4 is observed in the central elbow region and wrist-forming regions, and during later stages, expression is seen in the joint-forming regions of the whole limb. Wnt 5a is expressed throughout the limb mesenchyme during early limb developmental stages, and later, at HH stage 23, it becomes predominantly confined to the distal tip, leaving low expression levels proximally. At HH stage 29, expression at the distal tip is restricted to the interdigital regions, and at day 8, expression is seen in the region surrounding the phalanges. Wnt 5b expression is first observed in the AER at HH stage 20 and later in the dorsal and ventral mesenchyme surrounding the cartilage elements of the limb. Expression of Wnt 6 is observed from HH stage 17 until day 8 in the dorsal and ventral ectoderm and also in the dorsoventral limb boundaries. Expression of Wnt 11 is observed in the proximal dorsal mesenchyme of the limb from HH stage 23 onward and later in the dorsal and ventral subectodermal mesenchyme and in the regions adjacent to the digits at day 8. Weak expression of Wnt 14 is observed at the proximal mesenchyme of the limb at HH stage 23; later, it extends as a transverse strip surrounding the cartilage elements as well as in the interdigital mesenchyme.

The contribution of chicken embryology to the understanding of vertebrate limb development

The chicken is an excellent model organism for studying vertebrate limb development, mainly because of the ease of manipulating the developing limb in vivo. Classical chicken embryology has provided fate maps and elucidated the cell-cell interactions that specify limb pattern. The first defined chemical that can mimic one of these interactions was discovered by experiments on developing chick limbs and, over the last 15 years or so, the role of an increasing number of developmentally important genes has been uncovered. The principles that underlie limb development in chickens are applicable to other vertebrates and there are growing links with clinical genetics. The sequence of the chicken genome, together with other recently assembled chicken genomic resources, will present new opportunities for exploiting the ease of manipulating the limb.

Hoxa-11 and Hoxa-13 are involved in repression of MyoD during limb muscle development

Under the influence of the limb mesenchyme, Hoxa-11 is expressed in migrating and proliferating premyoblasts in the limb field and Hoxa-13 is induced in subdomains of congregated limb muscle masses. To evaluate the roles of Hoxa-11 and Hoxa-13 in myogenesis of the limb, we performed electroporation in ovo to force expression of these Hox genes in limb muscle precursors. In the presence of ectopic Hoxa-11, expression of MyoD was blocked transiently. In C2C12 myoblasts, transfection of Hoxa-11 also repressed the expression of endogenous MyoD. Forced expression of Hoxa-13 resulted in more pronounced repression of MyoD in both limb and C2C12 myoblasts. In contrast, targeted disruption of Hoxa-13 gave rise to enhanced expression of MyoD in the flexor carpi radialis muscle, a forearm muscle that normally expressed Hoxa-13. These results suggest that Hoxa-11 and Hoxa-13 are involved in the negative regulation of MyoD expression in limb muscle precursors.

Myogenic regulatory factors and the specification of muscle progenitors in vertebrate embryos

Embryological and genetic studies of mouse, bird, zebrafish, and frog embryos are providing new insights into the regulatory functions of the myogenic regulatory factors, MyoD, Myf5, Myogenin, and MRF4, and the transcriptional and signaling mechanisms that control their expression during the specification and differentiation of muscle progenitors. Myf5 and MyoD genes have genetically redundant, but developmentally distinct regulatory functions in the specification and the differentiation of somite and head muscle progenitor lineages. Myogenin and MRF4 have later functions in muscle differentiation, and Pax and Hox genes coordinate the migration and specification of somite progenitors at sites of hypaxial and limb muscle formation in the embryo body. Transcription enhancers that control Myf5 and MyoD activation in muscle progenitors and maintain their expression during muscle differentiation have been identified by transgenic analysis. In epaxial, hypaxial, limb, and head muscle progenitors, Myf5 is controlled by lineage-specific transcription enhancers, providing evidence that multiple mechanisms control progenitor specification at different sites of myogenesis in the embryo. Developmental signaling ligands and their signal transduction effectors function both interactively and independently to control Myf5 and MyoD activation in muscle progenitor lineages, likely through direct regulation of their transcription enhancers. Future investigations of the signaling and transcriptional mechanisms that control Myf5 and MyoD in the muscle progenitor lineages of different vertebrate embryos can be expected to provide a detailed understanding of the developmental and evolutionary mechanisms for anatomical muscles formation in vertebrates. This knowledge will be a foundation for development of stem cell therapies to repair diseased and damaged muscles.

Molecular basis of vertebrate limb patterning

Mechanisms of limb development are common to all higher vertebrates. The current understanding of how vertebrate limbs develop comes mainly from studies on chick embryos, which are classical models for experimental manipulation, and mouse embryos, which can be genetically manipulated. Work on chick and mouse embryos is often complementary and has direct implications for human limb development. Analysis has moved to the molecular level, which allows direct links to genetics. Even though genes involved in limb development have been discovered by basic scientists through different routes to that taken by clinical geneticists, many of the same genes have been identified. Thus, the fields of embryology and clinical medicine increasingly converge. The next challenge will be to go back to animal models to begin to dissect how particular gene mutations lead to specific limb phenotypes.

Local extrinsic signals determine muscle and endothelial cell fate and patterning in the vertebrate limb

Both the muscle and endothelium of the vertebrate limb derive from somites. We have used replication-defective retroviral vectors to analyze the lineage relationships of these somite-derived cells in the chick. We find that myogenic precursors in the somites or proximal limb are not committed to forming slow or fast muscle fibers, particular anatomical muscles, or muscles within specific proximal/distal or dorsal/ventral limb regions. Somitic endothelial precursors are uncommitted to forming endothelium in particular proximal/distal or dorsal/ventral limb regions. Surprisingly, we also find that myogenic and endothelial cells are derived from a common somitic precursor. Thus, local extrinsic signals are critical for determining muscle and endothelial patterning as well as cell fate in the limb.

Fgf4 positively regulates scleraxis and tenascin expression in chick limb tendons

In vertebrates, tendons connect muscles to skeletal elements. Surgical experiments in the chick have underlined developmental interactions between tendons and muscles. Initial formation of tendons occurs autonomously with respect to muscle. However, further tendon development requires the presence of muscle. The molecular signals involved in these interactions remain unknown. In the chick limb, Fgf4 transcripts are located at the extremities of muscles, where the future tendons will attach. In this paper, we analyse the putative role of muscle-Fgf4 on tendon development. We have used three general tendon markers, scleraxis, tenascin, and Fgf8 to analyse the regulation of these tendon-associated molecules by Fgf4 under different experimental conditions. In the absence of Fgf4, in muscleless and aneural limbs, the expression of the three tendon-associated molecules, scleraxis, tenascin, and Fgf8, is down-regulated. Exogenous implantation of Fgf4 in normal, aneural, and muscleless limbs induces scleraxis and tenascin expression but not that of Fgf8. These results indicate that Fgf4 expressed in muscle is required for the maintenance of scleraxis and tenascin but not Fgf8 expression in tendons.

Differentiation of avian craniofacial muscles: I. Patterns of early regulatory gene expression and myosin heavy chain synthesis

Myogenic populations of the avian head arise within both epithelial (somitic) and mesenchymal (unsegmented) mesodermal populations. The former, which gives rise to neck, tongue, laryngeal, and diaphragmatic muscles, show many similarities to trunk axial, body wall, and appendicular muscles. However, muscle progenitors originating within unsegmented head mesoderm exhibit several distinct features, including multiple ancestries, the absence of several somite lineage-determining regulatory gene products, diverse locations relative to neuraxial and pharyngeal tissues, and a prolonged and necessary interaction with neural crest cells. The object of this study has been to characterize the spatial and temporal patterns of early muscle regulatory gene expression and subsequent myosin heavy chain isoform appearance in avian mesenchyme-derived extraocular and branchial muscles, and compare these with expression patterns in myotome-derived neck and tongue muscles. Myf5 and myoD transcripts are detected in the dorsomedial (epaxial) region of the occipital somites before stage 12, but are not evident in the ventrolateral domain until stage 14. Within unsegmented head mesoderm, myf5 expression begins at stage 13.5 in the second branchial arch, followed within a few hours in the lateral rectus and first branchial arch myoblasts, then other eye and branchial arch muscles. Expression of myoD is detected initially in the first branchial arch beginning at stage 14.5, followed quickly by its appearance in other arches and eye muscles. Multiple foci of myoblasts expressing these transcripts are evident during the early stages of myogenesis in the first and third branchial arches and the lateral rectus-pyramidalis/quadratus complex, suggesting an early patterned segregation of muscle precursors within head mesoderm. Myf5-positive myoblasts forming the hypoglossal cord emerge from the lateral borders of somites 4 and 5 by stage 15 and move ventrally as a cohort. Myosin heavy chain (MyHC) is first immunologically detectable in several eye and branchial arch myofibers between stages 21 and 22, although many tongue and laryngeal muscles do not initiate myosin production until stage 24 or later. Detectable synthesis of the MyHC-S3 isoform, which characterizes myofibers as having "slow" contraction properties, occurs within 1-2 stages of the onset of MyHC synthesis in most head muscles, with tongue and laryngeal muscles being substantially delayed. Such a prolonged, 2- to 3-day period of regulatory gene expression preceding the onset of myosin production contrasts with the interval seen in muscles developing in axial (approximately 18 hr) and wing (approximately 1-1.5 days) locations, and is unique to head muscles. This finding suggests that ongoing interactions between head myoblasts and their surroundings, most likely neural crest cells, delay myoblast withdrawal from the mitotic pool. These descriptions define a spatiotemporal pattern of muscle regulatory gene and myosin heavy chain expression unique to head muscles. This pattern is independent of origin (somitic vs. unsegmented paraxial vs. prechordal mesoderm), position (extraocular vs. branchial vs. subpharyngeal), and fiber type (fast vs. slow) and is shared among all muscles whose precursors interact with cephalic neural crest populations. Dev Dyn 1999;216:96-112.

Vertebrate limb development and malformations

To understand limb abnormalities it is necessary to understand how the limb develops. The limb is the organ whose development is probably best understood. The limbs develop from small protrusions (the limb buds) that arise from the body wall of the embryo. Positioning and patterning the limb involves cellular interactions both between the ectoderm surrounding the limb bud and between the mesenchymal cells that form the core of the limb bud. As the limb grows out the cells acquire a positional value that relates to their position in the bud with respect to all three axes, proximo-distal, antero-posterior, and dorso-ventral. These positional values largely determine how the cells will develop such as what sort of cartilaginous elements they will form. The positional value of the cells is acquired in the progress zone at the tip of the growing bud. The time spent in the progress zone may determine the positional values along the proximo-distal axis, that is the formation of, for example the humerus, then the radius and ulna. Loss of the progress zone due to damage to the overlying apical ridge leads to truncations, and this progress zone model can also account for the effects of thalidomide. Position along the antero-posterior axis such as the character of the digits is by a signal from the polarizing region at the posterior margin of the limb and involves the signaling protein Sonic hedgehog. A signal from the dorsal ectoderm specifies the dorso-ventral axis. Hox genes that are transcription factors are expressed both along the body axis and in a complex pattern in the limb and may record positional value. Human mutations in these genes lead to limb abnormalities. Muscle cells have a separate origin from the cartilaginous cells and those that form connective tissue and tendons, and they migrate into the bud from the somites and are patterned by the connective tissue. Cell death separates the digits.

Local signals in the chick limb bud can override myoblast lineage commitment: induction of slow myosin heavy chain in fast myoblasts

Patterning of fast and slow muscle fibres in limbs is regulated by signals from non-muscle cells. Myoblast lineage has, however, also been implicated in fibre type patterning. Here we test a founder cell hypothesis for the role of myoblast lineage, by implanting characterized fast and slow mouse myoblast clones into chick limb buds. In culture, late foetal mouse myoblast clones are committed to a probability (range 0-0.92) of slow myosin heavy chain (MyHC) expression. In contrast, when implanted into chick limbs, fast mouse myoblast clones express myosin characteristic of their new environment, without fusion to chick muscle cells and in the absence of innervation. Therefore, local signals exist within the chick limb bud during primary myogenesis that can override intrinsic commitment of at least some myoblasts, and induce slow MyHC.

Distinct signaling molecules control Hoxa-11 and Hoxa-13 expression in the muscle precursor and mesenchyme of the chick limb bud

The limb muscles, originating from the ventrolateral portion of the somites, exhibit position-specific morphological development through successive splitting and growth/differentiation of the muscle masses in a region-specific manner by interacting with the limb mesenchyme and the cartilage elements. The molecular mechanisms that provide positional cues to the muscle precursors are still unknown. We have shown that the expression patterns of Hoxa-11 and Hoxa-13 are correlated with muscle patterning of the limb bud (Yamamoto et al., 1998) and demonstrated that muscular Hox genes are activated by signals from the limb mesenchyme. We dissected the regulatory mechanisms directing the unique expression patterns of Hoxa-11 and Hoxa-13 during limb muscle development. HOXA-11 protein was detected in both the myogenic cells and the zeugopodal mesenchymal cells of the limb bud. The earlier expression of HOXA-11 in both the myogenic precursor cells and the mesenchyme was dependent on the apical ectodermal ridge (AER), but later expression was independent of the AER. HOXA-11 expression in both myogenic precursor cells and mesenchyme was induced by fibroblast growth factor (FGF) signal, whereas hepatocyte growth factor/scatter factor (HGF/SF) maintained HOXA-11 expression in the myogenic precursor cells, but not in the mesenchyme. The distribution of HOXA-13 protein expression in the muscle masses was restricted to the posterior region. We found that HOXA-13 expression in the autopodal mesenchyme was dependent on the AER but not on the polarizing region, whereas expression of HOXA-13 in the posterior muscle masses was dependent on the polarizing region but not on the AER. Administration of BMP-2 at the anterior margin of the limb bud induced ectopic HOXA-13 expression in the anterior region of the muscle masses followed by ectopic muscle formation close to the source of exogenous BMP-2. In addition, NOGGIN/CHORDIN, antagonists of BMP-2 and BMP-4, downregulated the expression of HOXA-13 in the posterior region of the muscle masses and inhibited posterior muscle development. These results suggested that HOXA-13 expression in the posterior muscle masses is activated by the posteriorizing signal from the posterior mesenchyme via BMP-2. On the contrary, the expression of HOXA-13 in the autopodal mesenchyme was affected by neither BMP-2 nor NOGGIN/CHORDIN. Thus, mesenchymal HOXA-13 expression was independent of BMP-2 from polarizing region, but was under the control of as yet unidentified signals from the AER. These results showed that expression of Hox genes is regulated differently in the limb muscle precursor and mesenchymal cells.

Sonic hedgehog (SHH) specifies muscle pattern at tissue and cellular chick level, in the chick limb bud

Development of the musculature in chick limbs involves tissue and cellular patterning. Patterning at the tissue level leads to the precise arrangement of specific muscles; at the cellular level patterning gives rise to the fibre type diversity in muscles. Although the data suggests that the information controlling muscle patterning is localised within the limb mesenchyme and not in the somitic myogenic precursor cells themselves, the mechanisms underlying muscle organisation have still to be elucidated. The anterior-posterior axis of the limb is specified by a group of cells in the posterior region of the limb mesenchyme, called the zone of polarizing activity (ZPA). When polarizing-region cells are grafted to the anterior margin of the bud, they cause mirror-image digit duplications to be produced. The effect of ZPA grafts can be reproduced by application of retinoic acid (RA) beads and by grafting sonic hedgehog (SHH)-expressing cells to the anterior margin of the limb. Although most previous studies have looked at changes of the skeletal patterning, ZPA and RA also affect muscle patterning. In this report, we investigated the role of SHH in tissue and cellular patterning of forearm wing muscles. Ectopic application of a localised source of SHH to the anterior margin of the wing, leading to complete digit duplication, is able to transform anterior forearm muscles into muscles with a posterior identity. Moreover, the ectopic source of SHH induces a mirror image duplication of the normal posterior muscles fibre types in the new posterior muscles. The reorganisation of the slow fibres can be detected before muscle mass cleavage has started; suggesting that the appropriate fibre type arrangement is in place before the splitting process can be observed.

Involvement of Wnt-5a in chondrogenic pattern formation in the chick limb bud

Members of the Wnt family are known to play diverse roles in the organogenesis of vertebrates. The full-coding sequences of chicken Wnt-5a were identified and the role it plays in limb development was examined by comparing its expression pattern with that of two other Wnt members, Wnt-4 and Wnt-11, and by misexpressing it with a retrovirus vector in the limb bud. Wnt-5a expression is detected in the limb-forming region at stage 14, and in the apical ectodermal ridge and distal mesenchyme of the limb bud. The signal was graded along the proximal-distal axis at stages 20-28 and also along the anterior-posterior axis during early stages. It disappeared in the cartilage-forming region after stage 26, and was restricted to the region surrounding the phalanges at stage 34. Wnt-4 and Wnt-11, other members of the Wnt-5a-subclass, were expressed with a distinct spatiotemporal pattern during the later phase. Wnt-4 was expressed in the articular structure and Wnt-11 was expressed in the dorsal and ventral mesenchyme adjacent to the ectoderm. Wnt-5a expression was partially reduced after apical ectodermal ridge removal, whereas Wnt-11 expression was down-regulated by dorsal ectoderm removal. Therefore, expression of these Wnt was differentially regulated by the ectodermal signal. Misexpression of Wnt-5a in the limb bud with the retrovirus resulted in truncation of long bones predominantly in the zeugopod because of retarded chondrogenic differentiation. Distal elements, such as the phalanges and metacarpals, were not significantly reduced in size. These results suggest that Wnt-5a is involved in pattern formation along the proximal-distal axis by regulation of chondrogenic differentiation.

Primary myotubes preferentially mature into either the fastest or slowest muscle fibers

Myoblasts and myotubes are heterogeneous, but what is the significance of this heterogeneity? Is it a vital component of the mechanism by which a muscle develops or is it part of the process that generates mature fibers with diverse sizes, speeds of contracture, and metabolisms? We have begun to explore these questions by using BrdU to selectively label rat primary myotubes, thus enabling their mature characteristics to be defined for the first time. In the soleus, the type I fibers of primary myotube origin were 21% larger than those of secondary myotube origin, indicating that the origin of a fiber can affect its mature force production. In the extensor digitorum longus (EDL), the primary myotubes differentiated into all known fibers types, but with marked variation in frequency. In the superficial portion of the EDL, 97% of primary myotubes became IIB fibers, even though approximately 41% of the fibers in this region are IIA or IIX. In the deep portion, primary myotubes preferentially developed into type I fibers. Thus, primary myotubes in the EDL predominantly differentiate into the two most dissimilar fiber types: the slowest, smallest, most oxidative, type I fibers and the largest, fastest, most glycolytic, type IIB fibers. Each of the subtypes of primary myotubes had a different fate. In the EDL, the slow and fast primary myotubes appeared to differentiate into type I and IIB fibers, respectively. This implies that spatial and temporal signals operating in the limb are major determinants of the mature pattern of fiber types and that innervation of a muscle involves a selective matching between the various types of motoneurons and muscle fibers.

Coordinated expression of Hoxa-11 and Hoxa-13 during limb muscle patterning

The limb muscle precursor cells migrate from the somites and congregate into the dorsal and ventral muscle masses in the limb bud. Complex muscle patterns are formed by successive splitting of the muscle masses and subsequent growth and differentiation in a region-specific manner. Hox genes, known as key regulator genes of cartilage pattern formation in the limb bud, were found to be expressed in the limb muscle precursor cells. We found that HOXA-11 protein was expressed in the premyoblasts in the limb bud, but not in the somitic cells or migrating premyogenic cells in the trunk at stage 18. By stage 24, HOXA-11 expression began to decrease from the posterior halves of the muscle masses. HOXA-13 was expressed strongly in the myoblasts of the posterior part in the dorsal/ventral muscle masses and weakly in a few myoblasts of the anterior part of the dorsal muscle mass. Transplantation of the lateral plate of the presumptive wing bud to the flank induced migration of premyoblasts from somites to the graft. Under these conditions, HOXA-11 expression was induced in the migrating premyoblasts in the ectopic limb buds. Application of retinoic acid at the anterior margin of the limb bud causes duplication of the autopodal cartilage and transformation of the radius to the ulna, and at the same time induces duplication of the muscle pattern along the anteroposterior axis. Under these conditions, HOXA-13 was also induced in the anterior region of the ventral muscles in the zeugopod. These results suggest that Hoxa-11 and Hoxa-13 expression in the migrating premyoblasts is under the control of the limb mesenchyme and the polarizing signal(s). In addition, these results indicate that these Hox genes are involved in muscle patterning in the limb buds.

Sonic Hedgehog induces proliferation of committed skeletal muscle cells in the chick limb

Myogenic Regulatory Factors (MRFs) are a family of transcription factors whose expression in a cell reflects the commitment of this cell to a myogenic fate before any cytological sign of muscle differentiation is detectable. Myogenic cells in limb skeletal muscles originate from the lateral half of the somites. Cells that migrate away from the lateral part of the somites to the limb bud do not initially express any member of the MRF family. Expression of MRFs in the muscle precursor cells starts after the migration process is completed. The extracellular signals involved in activating the myogenic programme in muscle precursor cells in the limb in vivo are not known. We wished to investigate whether Sonic Hedgehog (SHH) expressed in the posterior part of the limb bud could be involved in differentiation of the muscle precursor cells in the limb. We found that retrovirally overexpressed SHH in the limb bud induced the extension of the expression domain of the Pax-3 gene, then that of the MyoD gene and finally that of the myosin protein. This led to an hypertrophy of the muscles in vivo. Addition of SHH to primary cultures of myoblasts resulted in an increase in the proportion of myoblasts that incorporate bromodeoxyuridine, resulting in an increase of myotube number. These data show that SHH is able to activate myogenesis in vivo and in vitro in already committed myoblasts and suggest that the stimulation of the myogenic programme by SHH involves activation of cell proliferation.

Mouse Eya genes are expressed during limb tendon development and encode a transcriptional activation function

Vertebrate limb tendons are derived from connective cells of the lateral plate mesoderm. Some of the developmental steps leading to the formation of vertebrate limb tendons have been previously identified; however, the molecular mechanisms responsible for tendinous patterning and maintenance during embryogenesis are largely unknown. The eyes absent (eya) gene of Drosophila encodes a novel nuclear protein of unknown molecular function. Here we show that Eya1 and Eya2, two mouse homologues of Drosophila eya, are expressed initially during limb development in connective tissue precursor cells. Later in limb development, Eya1 and Eya2 expression is associated with cell condensations that form different sets of limb tendons. Eya1 expression is largely restricted to flexor tendons, while Eya2 is expressed in the extensor tendons and ligaments of the phalangeal elements of the limb. These data suggest that Eya genes participate in the patterning of the distal tendons of the limb. To investigate the molecular functions of the Eya gene products, we have analyzed whether the highly divergent PST (proline-serine-threonine)-rich N-terminal regions of Eya1-3 function as transactivation domains. Our results demonstrate that Eya gene products can act as transcriptional activators, and they support a role for this molecular function in connective tissue patterning.

The distal limb environment regulates MyoD accumulation and muscle differentiation in mouse-chick chimaeric limbs

Differentiation of muscle and cartilage within developing vertebrate limbs occurs in a proximodistal progression. To investigate the cues responsible for regulating muscle pattern, mouse myoblasts were implanted into early chick wings prior to endogenous chick muscle differentiation. Fetal myogenic cells originating from transgenic mice carrying a lacZ reporter were readily detected in vivo after implantation and their state of differentiation determined with species-specific antibodies to MyoD and myosin heavy chain. When mouse myogenic cells are implanted at the growing tip of early stage 21 limbs MyoD expression is suppressed and little differentiation of the mouse cells is detected initially. At later stages ectopically implanted mouse cells come to lie within muscle masses, re-express MyoD and differentiate in parallel with differentiating chick myoblasts. However, if mouse cells are implanted either proximally at stage 21 or into the limb tip at stage 24, situations in which mouse cells encounter endogenous differentiating chick myoblasts earlier, MyoD suppression is not detected and a higher proportion of mouse cells differentiate. Mouse cells that remain distal to endogenous differentiating myogenic cells are more likely to remain undifferentiated than myoblasts that lie within differentiated chick muscle. Undifferentiated distal mouse cells are still capable of differentiating if explanted in vitro, suggesting that myoblast differentiation is inhibited in vivo. In vitro, MyoD is suppressed in primary mouse myoblasts by the addition of FGF2 and FGF4 to the culture media. Taken together, our data suggest that the inhibition of myogenic differentiation in the distal limb involves MyoD suppression in myoblasts, possibly through an FGF-like activity.

Homeobox genes and connective tissue patterning

In vertebrates, limb tendons are derived from cells that migrate from the lateral plate mesoderm during early development. While some of the developmental steps leading to the formation of these tissues are known, little is known about the molecular mechanisms controlling them. We have identified two murine homeobox-containing genes, Six 1 and Six 2, which are expressed in a complementary fashion during the development of limb tendons. Transcripts for both genes are found in different sets of phalangeal tendons. Six 1 and Six 2 also are expressed in skeletal and smooth muscle, respectively. These genes may participate in the patterning of the distal tendons of the limb phalanges by setting positional values along the limb axes.