Distinct mechanisms regulate slow-muscle development

New insights into skeletal muscle development and growth in teleost fishes

Recent research has significantly broadened our understanding of how the teleost somite is patterned to achieve embryonic and postembryonic myogenesis. Medial (adaxial) cells and posterior cells of the early epithelial somite generate embryonic superficial slow and deep fast muscle fibers, respectively, whereas anterior somitic cells move laterally to form an external cell layer of undifferentiated Pax7-positive myogenic precursors surrounding the embryonic myotome. In late embryo and in larvae, some of the cells contained in the external cell layer incorporate into the myotome and differentiate into new muscle fibers, thus contributing to medio-lateral expansion of the myotome. This supports the suggestion that the teleost external cell layer is homologous to the amniote dermomyotome. Some of the signalling molecules that promote lateral movement or regulate the myogenic differentiation of external cell precursors have been identified and include stromal cell-derived factor 1 (Sdf1), hedgehog proteins, and fibroblast growth factor 8 (Fgf8). Recent studies have shed light on gene activations that underlie the differentiation and maturation of slow and fast muscle fibers, pointing out that both adaxially derived embryonic slow fibers and slow fibers formed during the myotome expansion of larvae initially and transiently bear features of the fast fiber phenotype.

Graded hedgehog and fibroblast growth factor signaling independently regulate pituitary cell fates and help establish the pars distalis and pars intermedia of the zebrafish adenohypophysis

The vertebrate adenohypophysis forms as a placode at the anterior margin of the neural plate, requiring both hedgehog (Hh) and fibroblast growth factor (Fgf) mediated cell-cell signaling for induction and survival of endocrine cell types. Using small molecule inhibitors to modulate signaling levels during zebrafish development we show that graded Hh and Fgf signaling independently help establish the two subdomains of the adenohypophysis, the anteriorly located pars distalis (PD) and the posterior pars intermedia (PI). High levels of Hh signaling are required for formation of the PD and differentiation of anterior endocrine cell types, whereas lower levels of Hh signaling are required for formation of the PI and differentiation of posterior endocrine cell types. In contrast, high Fgf signaling levels are required for formation of the PI and posterior endocrine cell differentiation, whereas anterior regions require lower levels of Fgf signaling. Based on live observations and marker analyses, we show that the PD forms first at the midline closest to the central nervous system source of Sonic hedgehog. In contrast the PI appears to form from more lateral/posterior cells close to a central nervous system source of Fgf3. Together our data show that graded Hh and Fgf signaling independently direct induction of the PD and PI and help establish endocrine cell fates along the anterior/posterior axis of the zebrafish adenohypophysis. These data suggest that there are distinct origins and signaling requirements for the PD and PI.

Growth in the larval zebrafish pectoral fin and trunk musculature

After initial patterning, muscle in the trunk and fins of teleosts grows extensively. Here, we describe muscle growth in zebrafish, with emphasis on the pectoral fin musculature. In the trunk, slow muscle fibers differentiate first. In contrast, slow muscle does not appear in the pectoral fin until the beginning of the juvenile period. Mosaic hyperplasia contributes to trunk muscle growth, and new fibers are apparent within the muscle as early as 6 mm standard length. In the pectoral fin muscle, mosaic hyperplasia is not evident at any examined stage. Instead, the predominant mode of hyperplasia is stratified. In larval pectoral fin muscle new fibers appear subjacent to the skin, and this correlates with the expression of myogenic genes such as muscle regulatory factors and Pax7. Our results suggest that regulation of fiber type development and muscle growth may differ in the pectoral fin and trunk.

Expression of multiple slow myosin heavy chain genes reveals a diversity of zebrafish slow twitch muscle fibres with differing requirements for Hedgehog and Prdm1 activity

The zebrafish embryo develops a series of anatomically distinct slow twitch muscle fibres that characteristically express genes encoding lineage-specific isoforms of sarcomeric proteins such as MyHC and troponin. We show here that different subsets of these slow fibres express distinct members of a tandem array of slow MyHC genes. The first slow twitch muscle fibres to differentiate, which are specified by the activity of the transcription factor Prdm1 (also called Ubo or Blimp1) in response to Hedgehog (Hh) signalling, express the smyhc1 gene. Subsequently, secondary slow twitch fibres differentiate in most cases independently of Hh activity. We find that although some of these later-forming fibres also express smyhc1, others express smyhc2 or smyhc3. We show that the smyhc1-positive fibres express the ubo (prdm1) gene and adopt fast twitch fibre characteristics in the absence of Prdm1 activity, whereas those that do not express smyhc1 can differentiate independently of Prdm1 function. Conversely, some smyhc2-expressing fibres, although independent of Prdm1 function, require Hh activity to form. The adult trunk slow fibres express smyhc2 and smyhc3, but lack smyhc1 expression. The different slow fibres in the craniofacial muscles variously express smyhc1, smyhc2 and smyhc3, and all differentiate independently of Prdm1.

Other model organisms for sarcomeric muscle diseases

Model organisms are vital to our understanding of human muscle biology and disease. The potential of the nematode Caenorhabditis elegans, the fruitfly, Drosophila melanogaster and the zebrafish, Danio rerio, as model genetic organisms for the study of human muscle disease is discussed by examining their muscle biology, muscle genetics and development. The powerful genetic tools available with each organism are outlined. It is concluded that these organisms have already demonstrated potential in facilitating the study of muscle disease and in screening for therapeutic agents.

The teleost dermomyotome

Recent work in teleosts has renewed interest in the dermomyotome, which was initially characterized in the late 19th century. We review the evidence for the teleost dermomyotome, comparing it to the more well-characterized amniote dermomyotome. We discuss primary myotome morphogenesis, the relationship between the primary myotome and the dermomyotome, the differentiation of axial muscle, appendicular muscle, and dermis from the dermomyotome, and the signaling molecules that regulate myotome growth from myogenic precursors within the dermomyotome. The recognition of a dermomyotome in teleosts provides a new perspective on teleost muscle growth, as well as a fruitful approach to understanding the vertebrate dermomyotome.

MyoD and Myogenin expression during myogenic phases in brown trout: a precocious onset of mosaic hyperplasia is a prerequisite for fast somatic growth

Muscle cell recruitment (hyperplasia) during myogenesis in the vertebrate embryo is known to occur in three consecutive phases. In teleost fish (including zebrafish), however, information on myogenic precursor cell activation is largely fragmentary, and comprehensive characterization of the myogenic phases has only been fully undertaken in a single slow-growing cyprinid species by examination of MEF2D expression. Here, we use molecular techniques to provide a comprehensive characterization of MyoD and Myogenin expression during myogenic cell activation in embryos and larvae of brown trout, a fast-growing salmonid with exceptionally large embryos. Results confirm the three-phase pattern, but also demonstrate that the second and third phases begin simultaneously and progress vigorously, which is different from the previously described consecutive activation of these phases. Furthermore, we suggest that Pax7 is expressed in myogenic progenitor cells that account for second- and third-phase myogenesis. These findings are discussed in relation to teleost myotome development and to teleost growth strategies.

Cellular heterogeneity during vertebrate skeletal muscle development

Although skeletal muscles appear superficially alike at different anatomical locations, in reality there is considerably more diversity than previously anticipated. Heterogeneity is not only restricted to completely developed fibers, but is clearly apparent during development at the molecular, cellular and anatomical level. Multiple waves of muscle precursors with different features appear before birth and contribute to muscular diversification. Recent cell lineage and gene expression studies have expanded our knowledge on how skeletal muscle is formed and how its heterogeneity is generated. This review will present a comprehensive view of relevant findings in this field.

Dynamic somite cell rearrangements lead to distinct waves of myotome growth

The myogenic precursors responsible for muscle growth in amniotes develop from the dermomyotome, an epithelium at the external surface of the somite. In teleosts, the myogenic precursors responsible for growth have not been identified. We have used single cell lineage labeling in zebrafish to show that anterior border cells of epithelial somites are myogenic precursors responsible for zebrafish myotome growth. These cells move to the external surface of the embryonic myotome and express the transcription factor Pax7. Some remain on the external surface and some incorporate into the fast myotome, apparently by moving between differentiated slow fibres. The posterior cells of the somite, by contrast, elongate into medial muscle fibres. The surprising movement of the anterior somite cells to the external somite surface transforms a segmentally repeated arrangement of myogenic precursors into a medio-lateral arrangement similar to that seen in amniotes.

Muscle-specific expression of the smyd1 gene is controlled by its 5.3-kb promoter and 5'-flanking sequence in zebrafish embryos

Zebrafish SmyD1 is a SET and MYND domain-containing protein that plays an important role in myofiber maturation and muscle contraction. SmyD1 is required for myofibril organization and sarcomere assembly during myofiber maturation. Whole-mount in situ hybridization revealed that smyd1 mRNAs are specifically expressed in skeletal and cardiac muscles in zebrafish embryos. However, it is unknown if smyd1 is expressed in other striated muscles, such as cranial and fin muscles, and moreover, the regulatory elements required for its muscle-specific expression. We report here the analyses of smyd1 expression using smyd1-gfp transgenic zebrafish. smyd1-gfp transgenic zebrafish were generated using the 5.3-kb smyd1 promoter and its 5'-flanking sequence. GFP expression was found in the skeletal and cardiac muscles of smyd1-gfp transgenic embryos. GFP expression appeared stronger in slow muscles than fast muscles in transgenic zebrafish larvae. In addition, GFP expression was also detected in cranial and fin muscles of smyd1-gfp transgenic zebrafish larvae. In situ hybridization confirmed smyd1 mRNA expression in these tissues, suggesting that the expression of the smyd1-gfp transgene recapitulated that of the endogenous smyd1 gene. Deletion analysis revealed that the 0.5-kb sequence in the proximal promoter of smyd1 was essential for its muscle specificity. Together, these data indicate that smyd1 is specifically expressed in most, if not all, striated muscles, and the muscle specificity is controlled by the 5.3-kb promoter and flanking sequences.

Signaling networks that regulate muscle development: Lessons from zebrafish

Locomotion mediated by skeletal muscle provides a basis for the behavioral repertoire of most animals. Embryological and genetic studies of mouse, bird, fish and frog embryos are providing insights into the functions of the myogenic regulatory factors (MRFs) and the signaling molecules that regulate activity of MRFs. Nevertheless, our understanding of muscle development remains somewhat limited. Fundamental goals are to elucidate how mesodermal cells are induced during gastrulation to form muscle precursor cells and how muscle precursor cells acquire specific cell fates, such as slow and fast muscle cells. In this review, we focus on studies of zebrafish muscle development that have advanced our understanding of the molecular genetics of muscle cell induction and specification.

Phases of myogenic cell activation and possible role of dermomyotome cells in teleost muscle formation

Present knowledge indicates that fibre recruitment (hyperplasia) in developing teleost fish occurs in three distinct phases. However, the origin and relationship of the myogenic precursors activated during the different phases remains unclear. Here, we address this issue using molecular techniques on embryos and larvae of pearlfish, a large cyprinid species. Results provide comprehensive molecular characterisation of cell recruitment over the three phases of myogenesis, identifying muscle types as they arise. Specifically, we show that the myogenic cells arising during 2nd phase myogenesis are clearly different from the myogenic cells arising during the 3rd phase and that the dermomyotome is a major source of myogenic cells driving 2nd phase hyperplasia. These findings are discussed in relation to their implications for the generality of vertebrate developmental patterns.

Hedgehog acts directly on the zebrafish dermomyotome to promote myogenic differentiation

Vertebrate myogenesis is regulated by signaling proteins secreted from surrounding tissues. One of the most important, Sonic hedgehog, has been proposed to regulate myogenic precursor cell survival, proliferation, and differentiation in a variety of vertebrates. In zebrafish, Hedgehog signaling is both necessary and sufficient for the development of embryonic slow muscle fibers-the earliest differentiating muscle fibers. Here we investigated the function of Hedgehog signaling in another zebrafish myogenic lineage, a dermomyotomal population of cells defined by somitic pax3/7 expression. We found that Hedgehog negatively regulates the number of myogenic precursors expressing pax3/7. Hh also positively regulates the growth of embryonic fast muscle. Unlike Hedgehog's function in regulating the elongation of fast muscle fibers, this regulation is not mediated by embryonic slow muscle fibers. Instead, it is a direct Hedgehog response, cell autonomous to myogenic precursors. The regulation of myogenic precursors and their differentiation into fast fibers have a different critical time period for Hh signaling, and different requirements for specific gli gene family members of Hh activated transcription factors from the earlier promotion of embryonic slow muscle fiber differentiation. We propose that Hedgehog signaling acts at multiple times on different lineages, through different downstream pathways, to promote myogenic differentiation.

Signals and myogenic regulatory factors restrict pax3 and pax7 expression to dermomyotome-like tissue in zebrafish

Pax3/7 paired homeodomain transcription factors are important markers of muscle stem cells. Pax3 is required upstream of myod for lateral dermomyotomal cells in the amniote somite to form particular muscle cells. Later Pax3/7-dependent cells generate satellite cells and most body muscle. Here we analyse early myogenesis from, and regulation of, a population of Pax3-expressing dermomyotome-like cells in the zebrafish. Zebrafish pax3 is widely expressed in the lateral somite and, along with pax7, becomes restricted anteriorly and then to the external cells on the lateral somite surface. Midline-derived Hedgehog signals appear to act directly on lateral somite cells to repress Pax3/7. Both Hedgehog and Fgf8, signals that induce muscle formation within the somite, suppress Pax3/7 and promote expression of myogenic regulatory factors (MRFs) myf5 and myod in specific muscle precursor cell populations. Loss of MRF function leads to loss of myogenesis by specific populations of muscle fibres, with parallel up-regulation of Pax3/7. Myod is required for lateral fast muscle differentiation from pax3-expressing cells. In contrast, either Myf5 or Myod is sufficient to promote slow muscle formation from adaxial cells. Thus, myogenic signals act to drive somite cells to a myogenic fate through up-regulation of distinct combinations of MRFs. Our data show that the relationship between Pax3/7 genes and myogenesis is evolutionarily ancient, but that changes in the MRF targets for particular signals contribute to myogenic differences between species.

Comparison of neurolin (ALCAM) and neurolin-like cell adhesion molecule (NLCAM) expression in zebrafish

Many immunoglobulin (Ig)-superfamily cell adhesion molecules influence skeletal muscle formation. In Drosophila, dumbfounded (duf/kirre), irreC, sticks and stones and hibris encode related Ig-family proteins expressed in subsets of neurons and muscle precursor cells. The family mediates cell migration, axon guidance and fusion of myoblasts. Despite the importance of these genes in invertebrate myogenesis, no obvious functional parallels are known in vertebrate myogenesis. Here we investigate the gene expression pattern and phylogenetic and protein-structural relationships of the duf-related molecules neurolin and neurolin-like cell adhesion molecule (NLCAM), members of the activated leukocyte cell adhesion molecule (ALCAM) sub-family of Ig-molecules. These proteins are among the closest to Duf/Kirre by sequence. During zebrafish development, neurolin is expressed in subsets of somite and muscle cells, heart and numerous sites of neuronal maturation. The new ALCAM-family member, NLCAM, appears to have arisen by duplication of neurolin/ALCAM. NLCAM is expressed widely during gastrulation, particularly in the nascent neural plate, but later becomes predominantly expressed in sites of muscle and nerve maturation and in the fin fold. The expression of each gene is often in groups of cells in similar parts of the embryo; for example, in the region of Rohon Beard neurons, trigeminal ganglion and fusing fast and migrating slow muscle fibres. However, expression can also be distinct and dynamic; for example, muscle pioneer fibres express neurolin but not NLCAM at high level. Both molecules are expressed in subsets of muscle precursors at times prior to fusion.

Molecular cloning and mRNA expression analysis of carp embryonic, slow and cardiac myosin heavy chain isoforms

Three embryonic class II myosin heavy chains (MYHs) were cloned from the common carp (Cyprinus carpio L.), MYHemb1, MYHemb2 and MYHemb3. MYH DNA clones were also isolated from the slow muscle of adult carp acclimated to 10 degrees C (MYHS10) and 30 degrees C (MYHS30). Phylogenetic analysis demonstrated that MYHemb1 and MYHemb2 belonged to the fast skeletal muscle MYH clade. By contrast, the sequence of MYHemb3 was similar to the adult slow muscle isoforms, MYHS10 and MYHS30. MYHemb1 and MYHemb2 transcripts were first detected by northern blot analysis in embryos 61 h post-fertilization (h.p.f.) at the heartbeat stage, with peak expression occurring in 1-month-old juveniles. MYHemb1 continued to be expressed at low levels in 7-month-old juveniles when MYHemb2 was not detectable. MYHemb3 transcripts appeared at almost the same stage as MYHemb1 transcripts did (61 h.p.f.), and these genes showed a similar pattern of expression. Whole mount in situ hybridization analysis revealed that the transcripts of MYHemb1 and MYHemb2 were expressed in the inner part of myotome, whereas MYHemb3 was expressed in the superficial compartment. MYHS10 and MYHS30 mRNAs were first detected at hatching. In adult stages, the expression of slow muscle MYH mRNAs was dependent on acclimation temperature. MYHS10 mRNA was expressed at an acclimation temperature of 10 and 20 degrees C, but not at 30 degrees C. In contrast, MYHS30 mRNA was strongly expressed at all acclimation temperatures. The predominant MYH transcripts found in adult slow muscle and in embryos at hatching were expressed in adult fast muscle at some acclimation temperatures but not others. A MYH DNA clone was isolated from the cardiac muscle of 10 degrees C-acclimated adult fish (MYHcard). MYHcard mRNA was first detected at 61 h.p.f., but strong signals were only observed in the adult myocardium. The present study has therefore revealed a complex pattern of expression of MYH genes in relation to developmental stage, muscle type and acclimation temperature. None of the skeletal muscle MYHs identified so far was strongly expressed during the late juvenile stage, indicating further developmentally regulated members of the MYH II gene family remain to be discovered.

A novel salmonid myoD gene is distinctly regulated during development and probably arose by duplication after the genome tetraploidization

A novel myoD paralogue was characterised in Salmo salar (smyoD1c) and S. trutta (btmyoD1c). SmyoD1c had 78.2/90.6% protein sequence identity to smyoD1a/smyoD1b, respectively. Each paralogue was differentially expressed throughout somitogenesis. In adult fish, smyoD1a was the predominant gene expressed in fast muscle, whereas smyoD1c was 2-3 times upregulated in slow muscle compared to smyoD1a/1b. A maximum likelihood analysis indicated that myoD1c arose by duplication of myoD1b after the salmonid tetraploidization. Another myoD paralogue (myoD2) is present in at least some teleosts, reflecting a more ancient genome duplication. To accommodate these findings we propose a simplified teleost-myoD nomenclature.

Environment and plasticity of myogenesis in teleost fish

Embryonic development in teleosts is profoundly affected by environmental conditions, particularly temperature and dissolved oxygen concentrations. The environment determines the rate of myogenesis, the composition of sub-cellular organelles, patterns of gene expression, and the number and size distribution of muscle fibres. During the embryonic and larval stages, muscle plasticity to the environment is usually irreversible due to the rapid pace of ontogenetic change. In the early life stages, muscle can affect locomotory performance and behaviour, with potential consequences for larval survival. Postembryonic growth involves myogenic progenitor cells (MPCs) that originate in the embryo. The embryonic temperature regime can have long-term consequences for the growth of skeletal muscle in some species, including the duration and intensity of myotube formation in adult stages. In juvenile and adult fish, abiotic (temperature, day-length, water flow characteristics, hypoxia) and biotic factors (food availability, parasitic infection) have complex effects on the signalling pathways regulating the proliferation and differentiation of MPCs, protein synthesis and degradation, and patterns of gene expression. The phenotypic responses observed to the environment frequently vary during ontogeny and are integrated with endogenous physiological rhythms, particularly sexual maturation. Studies with model teleosts provide opportunities for investigating the underlying genetic mechanisms of muscle plasticity that can subsequently be applied to non-model species of more ecological or commercial interest.

Zebrafish cypher is important for somite formation and heart development

Mammalian CYPHER (Oracle, KIA0613), a member of the PDZ-LIM family of proteins (Enigma/LMP-1, ENH, ZASP/Cypher, RIL, ALP, and CLP-36), has been associated with cardiac and muscular myopathies. Targeted deletion of Cypher in mice is neonatal lethal possibly caused by myopathies. To further investigate the role of cypher in development, we have cloned the zebrafish orthologue. We present here the gene, domain structure, and expression pattern of zebrafish cypher during development. Cypher was not present as a maternal mRNA and was absent during early development. Cypher mRNA was first detected at the 3-somite stage in adaxial somites, and as somites matured, cypher expression gradually enveloped the whole somite. Later, cypher expression was also found in the heart, in head and jaw musculature, and in the brain. We further identified 13 alternative spliced forms of cypher from zebrafish heart and skeletal muscle tissue, among them a very short form containing the PDZ domain but lacking the ZM (ZASP-like) motif and the LIM domains. Targeted gene knock-down experiments using cypher antisense morpholinos led to severe defects, including truncation of the embryo, deformation of somites, dilatation of the pericardium, and thinning of the ventricular wall. The phenotype could be rescued by a cypher form, which contains the PDZ domain and the ZM motif, but lacks all three LIM domains. These findings indicate that a PDZ domain protein is important for normal somite formation and in normal heart development. Treatment of zebrafish embryos with cyclopamine, which disrupts hedgehog signaling, abolished cypher expression in 9 somite and 15-somite stage embryos. Taken together, our data suggest that cypher may play a role downstream of sonic hedgehog, in a late stage of somite development, when slow muscle fibers differentiate and migrate from the adaxial cells.

Genomewide expression profiling in the zebrafish embryo identifies target genes regulated by Hedgehog signaling during vertebrate development

Hedgehog proteins play critical roles in organizing the embryonic development of animals, largely through modulation of target gene expression. Little is currently known, however, about the kinds and numbers of genes whose expression is controlled, directly or indirectly, by Hedgehog activity. Using techniques to globally repress or activate Hedgehog signaling in zebrafish embryos followed by microarray-based expression profiling, we have discovered a cohort of genes whose expression responds significantly to loss or gain of Hedgehog function. We have confirmed the Hedgehog responsiveness of a representative set of these genes with whole-mount in situ hybridization as well as real time PCR. In addition, we show that the consensus Gli-binding motif is enriched within the putative regulatory elements of a sizeable proportion of genes that showed positive regulation in our assay, indicating that their expression is directly induced by Hedgehog. Finally, we provide evidence that the Hedgehog-dependent spatially restricted transcription of one such gene, nkx2.9, is indeed mediated by Gli1 through a single Gli recognition site located within an evolutionarily conserved enhancer fragment. Taken together, this study represents the first comprehensive survey of target genes regulated by the Hedgehog pathway during vertebrate development. Our data also demonstrate for the first time the functionality of the Gli-binding motif in the control of Hedgehog signaling-induced gene expression in the zebrafish embryo.

In situhybridisation of a large repertoire of muscle-specific transcripts in fish larvae: the new superficial slow-twitch fibres exhibit characteristics of fast-twitch differentiation

Much of the present information on muscle differentiation in fish concerns the early embryonic stages. To learn more about the maturation and the diversification of the fish myotomal fibres in later stages of ontogeny, we investigated, by means of in situ hybridisation, the developmental expression of a large repertoire of muscle-specific genes in trout larvae from hatching to yolk resorption. At hatching, transcripts for fast and slow muscle protein isoforms, namely myosins, tropomyosins, troponins and myosin binding protein C were present in the deep fast and the superficial slow areas of the myotome, respectively. During myotome expansion that follows hatching, the expression of fast isoforms became progressively confined to the borders of the fast muscle mass, whereas, in contrast, slow muscle isoform transcripts were uniformly expressed in all the slow fibres. Transcripts for several enzymes involved in oxidative metabolism such as citrate synthase, cytochrome oxidase component IV and succinate dehydrogenase, were present throughout the whole myotome of hatching embryos but in later stages became concentrated in slow fibre as well as in lateral fast fibres. Surprisingly, the slow fibres that are added externally to the single superficial layer of the embryonic(original) slow muscle fibres expressed not only slow twitch muscle isoforms but also, transiently, a subset of fast twitch muscle isoforms including MyLC1, MyLC3, MyHC and myosin binding protein C. Taken together these observations show that the growth of the myotome of the fish larvae is associated with complex patterns of muscular gene expression and demonstrate the unexpected presence of fast muscle isoform-expressing fibres in the most superficial part of the slow muscle.

Inhibition of Notch signaling induces myotube hypertrophy by recruiting a subpopulation of reserve cells

During muscle differentiation, a population of quiescent undifferentiated myoblasts (reserve cells) emerges among mature muscle cells. However, the molecular mechanisms underlying such cell segregation and the characterization of this subpopulation of myoblasts remain to be determined. Notch is known to control the behavior and fate of murine muscle stem cells. In this study, we examined the role of Notch in myoblast segregation. We showed that inhibition of Notch activity by either overexpressing Numb or by using a pharmacological gamma-secretase inhibitor (DAPT) enhanced differentiation of murine and human myoblasts. This effect was not restricted to in vitro culture systems since DAPT-treated zebrafish embryos also showed increased differentiation. Using C2.7 myoblasts as a model, we showed that inhibition of Notch induced myotube hypertrophy by recruiting reserve cells that do not normally fuse. We further showed that endogenous Notch-signaling components were differentially expressed and activated in reserve cells with respect to Notch 1 and CD34 expression. We identified CD34 negative reserve cells as the subpopulation of myoblasts recruited to fuse into myotubes during differentiation in response to Notch inhibition. Therefore, we showed here that the activation of Notch 1 is important to maintain a subpopulation of CD34 negative reserve cells in an undifferentiated state.

Evolution and developmental patterning of the vertebrate skeletal muscles: Perspectives from the lamprey

The myotome in gnathostome vertebrates, which gives rise to the trunk skeletal muscles, consists of epaxial (dorsal) and hypaxial (ventral) portions, separated by the horizontal myoseptum. The hypaxial portion contains some highly derived musculature that is functionally as well as morphologically well differentiated in all the gnathostome species. In contrast, the trunk muscles of agnathan lampreys lack these distinctions and any semblance of limb muscles. Therefore, the lamprey myotomes probably represent a primitive condition compared with gnathostomes. In this review, we compare the patterns of expression of some muscle-specific genes between the lamprey and gnathostomes. Although the cellular and tissue morphology of lamprey myotomes seems uniform and undifferentiated, some of the muscle-specific genes are expressed in a spatially restricted manner. The lamprey Pax3/7 gene, a cognate of gnathostome Pax3, is expressed only at the lateral edge of the myotomes and in the hypobranchial muscle, which we presume is homologous to the gnathostome hypobranchial muscle. Thus, the emergence of some part of a hypaxial-specific gene regulatory cascade might have evolved before the agnathan/gnathostome divergence, or before the evolutionary separation of epaxial and hypaxial muscles.

Acetylcholine and calcium signalling regulates muscle fibre formation in the zebrafish embryo

Nerve activity is known to be an important regulator of muscle phenotype in the adult, but its contribution to muscle development during embryogenesis remains unresolved. We used the zebrafish embryo and in vivo imaging approaches to address the role of activity-generated signals, acetylcholine and intracellular calcium, in vertebrate slow muscle development. We show that acetylcholine drives initial muscle contraction and embryonic movement via release of intracellular calcium from ryanodine receptors. Inhibition of this activity-dependent pathway at the level of the acetylcholine receptor or ryanodine receptor did not disrupt slow fibre number, elongation or migration but affected myofibril organisation. In mutants lacking functional acetylcholine receptors myofibre length increased and sarcomere length decreased significantly. We propose that calcium is acting via the cytoskeleton to regulate myofibril organisation. Within a myofibre, sarcomere length and number are the key parameters regulating force generation; hence our findings imply a critical role for nerve-mediated calcium signals in the formation of physiologically functional muscle units during development.

Interactions between muscle fibers and segment boundaries in zebrafish

The most obvious segmental structures in the vertebrate embryo are somites: transient structures that give rise to vertebrae and much of the musculature. In zebrafish, most somitic cells give rise to long muscle fibers that are anchored to intersegmental boundaries. Therefore, this boundary is analogous to the mammalian tendon in that it transduces muscle-generated force to the skeletal system. We have investigated interactions between somite boundaries and muscle fibers. We define three stages of segment boundary formation. The first stage is the formation of the initial epithelial somite boundary. The second "transition" stage involves both the elongation of initially round muscle precursor cells and somite boundary maturation. The third stage is myotome boundary formation, where the boundary becomes rich in extracellular matrix and all muscle precursor cells have elongated to form long muscle fibers. It is known that formation of the initial epithelial somite boundary requires Notch signaling; vertebrate Notch pathway mutants show severe defects in somitogenesis. However, many zebrafish Notch pathway mutants are homozygous viable suggesting that segmentation of their larval and adult body plans at least partially recovers. We show that epithelial somite boundary formation and slow-twitch muscle morphogenesis are initially disrupted in after eight (aei) mutant embryos (which lack function of the Notch ligand, DeltaD); however, myotome boundaries form later ("recover") in a Hedgehog-dependent fashion. Inhibition of Hedgehog-induced slow muscle induction in aei/deltaD and deadly seven (des)/notch1a mutant embryos suggests that slow muscle is necessary for myotome boundary recovery in the absence of initial epithelial somite boundary formation. Because we have previously demonstrated that slow muscle migration triggers fast muscle cell elongation in zebrafish, we hypothesize that migrating slow muscle facilitates myotome boundary formation in aei/deltaD mutant embryos by patterning coordinated fast muscle cell elongation. In addition, we utilized genetic mosaic analysis to show that somite boundaries also function to limit the extent to which fast muscle cells can elongate. Combined, our results indicate that multiple interactions between somite boundaries and muscle fibers mediate zebrafish segmentation.

A genomic approach to reveal novel genes associated with myotube formation in the model teleost,Takifugu rubripes

Little is known about the transcriptional networks that regulate myotube production in vertebrates. In the present study, we have used a genomic approach to discover novel genes associated with myotube formation in fast muscle of the tiger puffer fish, Takifugu rubripes. The number of fast muscle fibers per myotome increased until 1.2 kg body mass, and subsequent growth was by fiber hypertrophy alone. Forward and reverse subtracted cDNA libraries were prepared from a 180-g (myotube +) and a 3.4-kg (myotube −) fish, and 1,452 expressed sequence tags (ESTs) were obtained. After these ESTs were grouped into nonredundant clusters and housekeeping and structural genes were eliminated, 57 genes were selected and quantitative PCR was used to investigate their expression levels in different tissues from independent groups of myotube(−) and myotube(+) fish acclimated to the same environmental conditions and diet. Eleven novel genes were found to be consistently differentially expressed, but only four showed appropriate tissue-specific expression. These four genes were upregulated 5–25 times in fast muscle of myotube(−) relative to myotube(+) growth stages, while their expression remained unchanged in the other tissues studied. The novel genes identified, which are also present in other vertebrate genomes, may play a role in inhibiting myotube formation in vertebrate muscle.

Genetic screens for genes controlling motor nerve-muscle development and interactions

Motor growth cones navigate long and complex trajectories to connect with their muscle targets. Experimental studies have shown that this guidance process critically depends on extrinsic cues. In the zebrafish embryo, a subset of mesodermal cells, the adaxial cells, delineates the prospective path of pioneering motor growth cones. Genetic ablation of adaxial cells causes profound pathfinding defects, suggesting the existence of adaxial cell derived guidance factors. Intriguingly, adaxial cells are themselves migratory, and as growth cones approach they migrate away from the prospective axonal path to the lateral surface of the myotome, where they develop into slow-twitching muscle fibers. Genetic screens in embryos stained with an antibody cocktail identified mutants with specific defects in differentiation and migration of adaxial cells/slow muscle fibers, as well as mutants with specific defects in axonal pathfinding, including exit from the spinal cord and pathway selection. Together, the genes underlying these mutant phenotypes define pathways essential for nerve and muscle development and interactions between these two cell types.

Muscle growth patterns and regulation during fish ontogeny

In fish, the skeletal muscle of the trunk and the tail derives from the somites which form in the paraxial mesoderm in a rostro-caudal sequence. The development of the fish myotome begins with the onset of myogenic regulatory factors expression and continues with the formation of a distinct superficial layer of slow muscle fibres that covers a bulk of fast muscle fibres located in the deep portion of the myotome. Muscle fibres of the slow-twitch lineage originate in fish embryos from adaxial cells, a distinct subpopulation of the paraxial mesoderm that flanks the notochord. During the early maturation of the somite these adaxial cells migrate away from the notochord towards the lateral part of the somite where they form the superficial slow fibres. Lateral presomitic cells that remain deep in the myotome differentiate into fast muscle fibres. Morphogens of the hedgehog family secreted by the notochord have a pivotal role in inducing the slow-twitch lineage. In late embryos, additional fibres are added from discrete germinal zones situated at the ventral and dorsal extremes of the developing myotome. This regionalised process has been termed "stratified hyperplasia." In fish which grow to a large final size this is followed by a mosaic hyperplastic process that leads to the formation of new fibres throughout the whole myotome. Current knowledge about the endocrine and autocrine factors that potentially regulate the proliferation and the differentiation of muscle cells within the embryonic and larval fish myotome is reviewed.

Rapid evolution of muscle fibre number in post-glacial populations of Arctic charr Salvelinus alpinus

Thingvallavatn, the largest and one of the oldest lakes in Iceland, contains four morphs of Arctic charr Salvelinus alpinus. Dwarf benthic (DB), large benthic (LB), planktivorous (PL) and piscivorous (PI) morphs can be distinguished and differ markedly in head morphology, colouration and maximum fork length (FL(max)), reflecting their different resource specialisations within the lake. The four morphs in Thingvallavatn are thought to have been isolated for approximately 10 000 years, since shortly after the end of the last Ice Age. We tested the null hypothesis that the pattern of muscle fibre recruitment was the same in all morphs, reflecting their recent diversification. The cross-sectional areas of fast and slow muscle fibres were measured at 0.7 FL in 46 DB morphs, 23 LB morphs, 24 PL morphs and 22 PI morphs, and the ages of the charr were estimated using sacculus otoliths. In fish larger than 10 g, the maximum fibre diameter scaled with body mass (M(b))(0.18) for both fibre types in all morphs. The number of myonuclei per cm fibre length increased with fibre diameter, but was similar between morphs. On average, at 60 mum diameter, there were 2264 nuclei cm(-1) in slow fibres and 1126 nuclei cm(-1) in fast fibres. The absence of fibres of diameter 4-10 mum was used to determine the FL at which muscle fibre recruitment stopped. Slow fibre number increased with body length in all morphs, scaling with M(b)(0.45). In contrast, the recruitment of fast muscle fibres continued until a clearly identifiable FL, corresponding to 18-19 cm in the dwarf morph, 24-26 cm in the pelagic morph, 32-33 cm in the large benthic morph and 34-35 cm in the piscivorous morph. The maximum fast fibre number (FN(max)) in the dwarf morph (6.97x10(4)) was 56.5% of that found in the LB and PI morphs combined (1.23x10(5)) (P<0.001). Muscle fibre recruitment continued until a threshold body size and occurred at a range of ages, starting at 4+ years in the DB morph and 7+ years in the LB and PI morphs. Our null hypothesis was therefore rejected for fast muscle and it was concluded that the dwarf condition was associated with a reduction in fibre number. We then investigated whether variations in development temperature associated with different spawning sites and periods were responsible for the observed differences in muscle cellularity between morphs. Embryos from the DB, LB and PL morphs were incubated at temperature regimes simulating cold subterranean spring-fed sites (2.2-3.2 degrees C) and the general lakebed (4-7 degrees C). Myogenic progenitor cells (MPCs) were identified using specific antibodies to Paired box protein 7 (Pax 7), Forkhead box protein K1-alpha (FoxK1-alpha), MyoD and Myf-5. The progeny showed no evidence of developmental plasticity in the numbers of either MPCs or muscle fibres. Juveniles and adult stages of the DB and LB morphs coexist and have a similar diet. We therefore conclude that the reduction in FN(max) in the dwarf morph probably has a genetic basis and that gene networks regulating myotube production are under high selection pressure. To explain these findings we propose that there is an optimal fibre size, and hence number, which varies with maximum body size and reflects a trade-off between diffusional constraints on fibre diameter and the energy costs of maintaining ionic gradients. The predictions of the optimal fibre size hypothesis and its consequences for the adaptive evolution of muscle architecture in fishes are briefly discussed.

Hedgehog Signaling Is Required for Adult Blood Stem Cell Formation in Zebrafish Embryos

Studies with embryonic explants and embryonic stem cells have suggested a role for Hedgehog (Hh) signaling in hematopoiesis. However, targeted deletion of Hh pathway components in the mouse has so far failed to provide in vivo evidence. Here we show that zebrafish embryos mutant in the Hh pathway or treated with the Hh signaling inhibitor cyclopamine display defects in adult hematopoietic stem cell (HSC) formation but not in primitive hematopoiesis. Hh is required in the trunk at three consecutive stages during vascular development: for the medial migration of endothelial progenitors of the dorsal aorta (DA), for arterial gene expression, and for the formation of intersomitic vessel sprouts. Interference with Hh signaling during the first two stages also interferes with HSC formation. Furthermore, HSC and DA formation also share Vegf and Notch requirements, which further distinguishes them from primitive hematopoiesis and underlines their close relationship during vertebrate development.

Zebrafish slow muscle cell migration induces a wave of fast muscle morphogenesis

The specification and morphogenesis of slow and fast twitch muscle fibers are crucial for muscle development. In zebrafish, Hedgehog is required for slow muscle fiber specification. However, less is known about signals that promote development of fast muscle fibers, which constitute the majority of somitic cells. We show that when Hedgehog signaling is blocked, fast muscle cell elongation is disrupted. Using genetic mosaics, we show that Hedgehog signal perception is required by slow muscle cells but not by fast muscle cells for fast muscle cell elongation. Furthermore, we show that slow muscle cells are sufficient to pattern the medial to lateral wave of fast muscle fiber morphogenesis even when fast muscle cells cannot perceive the Hedgehog signal. Thus, the medial to lateral migration of slow muscle fibers through the somite creates a morphogenetic signal that patterns fast muscle fiber elongation in its wake.

Vertebrate myotome development

The embryonic myotome generates both the axial musculature and the appendicular muscle of the fins and limbs. Early in embryo development the mesoderm is segmented into somites, and within these the primary myotome forms by a complex series of cellular movements and migrations. A new model of primary myotome formation in amniotes has emerged recently. The myotome also includes the muscle progenitor cells that are known to contribute to the secondary formation of the myotome. The adult myotome contains satellite cells that play an important role in adult muscle regeneration. Recent studies have shed light on how the growth and patterning of the myotome occurs.

Hedgehog signaling is required for commitment but not initial induction of slow muscle precursors

In zebrafish, skeletal muscle precursors can adopt at least three distinct fates: fast, non-pioneer slow, or pioneer slow muscle fibers. Slow muscle fibers develop from adaxial cells and depend on Hedgehog signaling. We analyzed when precursors become committed to their fates and the step(s) along their differentiation pathway affected by Hedgehog. Unexpectedly, we find that embryos deficient in Hedgehog signaling still contain postmitotic adaxial cells that differentiate into fast muscle fibers instead of slow. We show that by the onset of gastrulation, slow and fast muscle precursors are already spatially segregated but uncommitted to their fates until much later, in the segmental plate when slow precursors become independent of Hedgehog. In contrast, pioneer and non-pioneer slow muscle precursors share a common lineage from the onset of gastrulation. Our results demonstrate that slow muscle precursors form independently of Hedgehog signaling and further provide direct evidence for a multipotent muscle precursor population whose commitment to the slow fate depends on Hedgehog at a late stage of development when postmitotic adaxial cells differentiate into slow muscle fibers.

Hedgehog regulation of superficial slow muscle fibres inXenopusand the evolution of tetrapod trunk myogenesis

In tetrapod phylogeny, the dramatic modifications of the trunk have received less attention than the more obvious evolution of limbs. In somites,several waves of muscle precursors are induced by signals from nearby tissues. In both amniotes and fish, the earliest myogenesis requires secreted signals from the ventral midline carried by Hedgehog (Hh) proteins. To determine if this similarity represents evolutionary homology, we have examined myogenesis in Xenopus laevis, the major species from which insight into vertebrate mesoderm patterning has been derived. Xenopus embryos form two distinct kinds of muscle cells analogous to the superficial slow and medial fast muscle fibres of zebrafish. As in zebrafish, Hh signalling is required for XMyf5 expression and generation of a first wave of early superficial slow muscle fibres in tail somites. Thus, Hh-dependent adaxial myogenesis is the likely ancestral condition of teleosts, amphibia and amniotes. Our evidence suggests that midline-derived cells migrate to the lateral somite surface and generate superficial slow muscle. This cell re-orientation contributes to the apparent rotation of Xenopussomites. Xenopus myogenesis in the trunk differs from that in the tail. In the trunk, the first wave of superficial slow fibres is missing,suggesting that significant adaptation of the ancestral myogenic programme occurred during tetrapod trunk evolution. Although notochord is required for early medial XMyf5 expression, Hh signalling fails to drive these cells to slow myogenesis. Later, both trunk and tail somites develop a second wave of Hh-independent slow fibres. These fibres probably derive from an outer cell layer expressing the myogenic determination genes XMyf5, XMyoD and Pax3 in a pattern reminiscent of amniote dermomyotome. Thus, Xenopus somites have characteristics in common with both fish and amniotes that shed light on the evolution of somite differentiation. We propose a model for the evolutionary adaptation of myogenesis in the transition from fish to tetrapod trunk.

Hedgehog can drive terminal differentiation of amniote slow skeletal muscle

BACKGROUND

Secreted Hedgehog (Hh) signalling molecules have profound influences on many developing and regenerating tissues. Yet in most vertebrate tissues it is unclear which Hh-responses are the direct result of Hh action on a particular cell type because Hhs frequently elicit secondary signals. In developing skeletal muscle, Hhs promote slow myogenesis in zebrafish and are involved in specification of medial muscle cells in amniote somites. However, the extent to which non-myogenic cells, myoblasts or differentiating myocytes are direct or indirect targets of Hh signalling is not known.

RESULTS

We show that Sonic hedgehog (Shh) can act directly on cultured C2 myoblasts, driving Gli1 expression, myogenin up-regulation and terminal differentiation, even in the presence of growth factors that normally prevent differentiation. Distinct myoblasts respond differently to Shh: in some slow myosin expression is increased, whereas in others Shh simply enhances terminal differentiation. Exposure of chick wing bud cells to Shh in culture increases numbers of both muscle and non-muscle cells, yet simultaneously enhances differentiation of myoblasts. The small proportion of differentiated muscle cells expressing definitive slow myosin can be doubled by Shh. Shh over-expression in chick limb bud reduces muscle mass at early developmental stages while inducing ectopic slow muscle fibre formation. Abundant later-differentiating fibres, however, do not express extra slow myosin. Conversely, Hh loss of function in the limb bud, caused by implanting hybridoma cells expressing a functionally blocking anti-Hh antibody, reduces early slow muscle formation and differentiation, but does not prevent later slow myogenesis. Analysis of Hh knockout mice indicates that Shh promotes early somitic slow myogenesis.

CONCLUSIONS

Taken together, the data show that Hh can have direct pro-differentiative effects on myoblasts and that early-developing muscle requires Hh for normal differentiation and slow myosin expression. We propose a simple model of how direct and indirect effects of Hh regulate early limb myogenesis.

Myotome meanderings. Cellular morphogenesis and the making of muscle

The formation of muscles within the vertebrate embryo is a tightly orchestrated and complex undertaking. Beyond the initial specification of cells to become muscle are several complex cellular movements and migrations, which lead to the positioning of muscle precursors at specific locations within the embryo. The consequent differentiation, elongation and striation of these cells results in the formation of individual muscles. Investigation of the in vivo morphogenesis of individual vertebrate muscle cells has only recently begun, and is being approached through the use of sophisticated cell labelling and lineage analysis techniques. However, a consensus about the mechanisms involved has yet to be achieved. This review outlines vertebrate embryonic muscle formation in chick, fish and mice, focusing on the embryonic myotome, which generates both the axial musculature and the appendicular muscle of the fins and limbs. We highlight the points of consensus about, and the complexity of, this developmental system, and propose an evolutionary context for the basis of these understandings.

Multiple muscle cell identities induced by distinct levels and timing of hedgehog activity in the zebrafish embryo

BACKGROUND

In the zebrafish embryo, the differentiation of distinct muscle fiber types has been shown to require the activity of members of the Hedgehog (Hh) family of secreted proteins. Evidence from other systems suggests that Hh behaves as a morphogen, inducing cell fates in a concentration-dependent manner. Exactly how Hh signaling contributes to the generation of the correct pattern of cells within the zebrafish myotome, however, has remained obscure.

RESULTS

Here, we distinguish four distinct myotomal cell identities in the zebrafish embryo on the basis of their position, morphology, and gene expression patterns. Using morpholino oligonucleotides (MOs) to diminish the activities of the Hh pathway components Patched (Ptc), Fused (Fu), and Suppressor of Fused (Su(fu)), and the teratogen cyclopamine to inhibit the Hh transducer Smoothened (Smo), we show that the appropriate differentiation of each cell type depends upon the levels and range of Hh signaling within the myotome. In addition, by transiently modulating Hh activity by using cyclopamine and a heat-inducible transgene, we demonstrate that the competence of myotomal cells to respond to Hh changes with time. Finally, we show that the Gli1 and Gli2 transcription factors mediate most of the response of myotomal cells to Hh.

CONCLUSIONS

Hh signaling acts in a dosage-dependent manner to specify cell fate in the zebrafish myotome. Allocation of the correct number of cells to a specific fate depends upon the range of Hh activity. The timing of exposure to Hh determines the response of cells to the signal.

Recapitulation of fast skeletal muscle development in zebrafish by transgenic expression of GFP under the mylz2 promoter

A 1,934-bp muscle-specific promoter from the zebrafish mylz2 gene was isolated and characterized by transgenic analysis. By using a series of 5' promoter deletions linked to the green fluorescent protein (gfp) reporter gene, transient transgenic analysis indicated that the strength of promoter activity appeared to correlate to the number of muscle cis-elements in the promoter and that a minimal -77-bp region was sufficient for a relatively strong promoter activity in muscle cells. Stable transgenic lines were obtained from several mylz2-gfp constructs. GFP expression in the 1,934-bp promoter transgenic lines mimicked well the expression pattern of endogenous mylz2 mRNA in both somitic muscle and nonsomitic muscles, including fin, eye, jaw, and gill muscles. An identical pattern of GFP expression, although at a much lower level, was observed from a transgenic line with a shorter 871-bp promoter. Our observation indicates that there is no distinct cis-element for activation of mylz2 in different skeletal muscles. Furthermore, RNA encoding a dominant negative form of cAMP-dependent protein kinase A was injected into mylz2-gfp transgenic embryos and GFP expression was significantly reduced due to an expanded slow muscle development at the expense of GFP-expressing fast muscle. The mylz2-gfp transgene was also transferred into two zebrafish mutants, spadetail and chordino, and several novel phenotypes in muscle development in these mutants were discovered.

Freshwater environment affects growth rate and muscle fibre recruitment in seawater stages of Atlantic salmon (Salmo salar L.)

The influence of freshwater environment on muscle growth in seawater was investigated in an inbred population of farmed Atlantic salmon (Salmo salar L.). The offspring from a minimum of 64 families per group were incubated at either ambient temperature (ambient treatment) or in heated water (heated treatment). Growth was investigated using a mixed-effect statistical model with repeated measures, which included terms for treatment effect and random fish effects for individual growth rate (alpha) and the instantaneous growth rate per unit change in temperature (gamma). Prior to seawater transfer, fish were heavier in the heated (61.6+/-1.0 g; N=298) than in the ambient (34.1+/-0.4 g; N=206) treatments, reflecting their greater growth opportunity: 4872 degree-days and 4281 degree-days, respectively. However, the subsequent growth rate of the heated group was lower, such that treatments had a similar body mass (3.7-3.9 kg) after approximately 450 days in seawater. The total cross-sectional area of fast muscle and the number (FN) and size distribution of the fibres was determined in a subset of the fish. We tested the hypothesis that freshwater temperature regime affected the rate of recruitment and hypertrophy of muscle fibres. There were differences in FN between treatments and a significant age x treatment interaction but no significant cage effect (ANOVA). Cessation of fibre recruitment was identified by the absence of fibres of <10 micro m diameter. The maximum fibre number was 22.4% more in the ambient (9.3 x 10(5)+/-2.0 x 10(4) than in the heated (7.6 x 10(5)+/-1.5 x 10(4)) treatments (N=44 and 40 fish, respectively; P<0.001). For fish that had completed fibre recruitment, there was a significant correlation between FN and individual growth rate, explaining 35% of the total variation. The density of myogenic progenitor cells was quantified using an antibody to c-met and was approximately 2-fold higher in the ambient than in the heated group, equivalent to 2-3% of the total muscle nuclei. The number of myonuclei in isolated fibre segments showed a linear relationship with fibre diameter. On average, there were 20.6% more myonuclei in 200-microm-diameter fibres isolated from the ambient (3734 myonuclei cm(-1)) than from the heated (3097 myonuclei cm(-1)) treatments. The maximum fibre diameter was greater in heated than in ambient groups, whereas the age x treatment interaction was not significantly different (ANCOVA). There were also no consistent differences in the rate of hypertrophy of muscle fibres between treatments. It was concluded that freshwater temperature regime affected fibre number and the nuclear content of fast muscle in seawater but not the rate of fibre hypertrophy. The mechanisms and life history consequences of developmental plasticity in fibre number are discussed.

Development of the locomotor network in zebrafish

The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.