The onset of myotome formation in the chick

Upper and Lower Motor Neurons and the Skeletal Muscle: Implication for Amyotrophic Lateral Sclerosis (ALS)

The relationships between motor neurons and the skeletal muscle during development and in pathologic contexts are addressed in this Chapter.We discuss the developmental interplay of muscle and nervous tissue, through neurotrophins and the activation of differentiation and survival pathways. After a brief overview on muscular regulatory factors, we focus on the contribution of muscle to early and late neurodevelopment. Such a role seems especially intriguing in relation to the epigenetic shaping of developing motor neuron fate choices. In this context, emphasis is attributed to factors regulating energy metabolism, which may concomitantly act in muscle and neural cells, being involved in common pathways.We then review the main features of motor neuron diseases, addressing the cellular processes underlying clinical symptoms. The involvement of different muscle-associated neurotrophic factors for survival of lateral motor column neurons, innervating MyoD-dependent limb muscles, and of medial motor column neurons, innervating Myf5-dependent back musculature is discussed. Among the pathogenic mechanisms, we focus on oxidative stress, that represents a common and early trait in several neurodegenerative disorders. The role of organelles primarily involved in reactive oxygen species scavenging and, more generally, in energy metabolism-namely mitochondria and peroxisomes-is discussed in the frame of motor neuron degeneration.We finally address muscular involvement in amyotrophic lateral sclerosis (ALS), a multifactorial degenerative disorder, hallmarked by severe weight loss, caused by imbalanced lipid metabolism. Even though multiple mechanisms have been recognized to play a role in the disease, current literature generally assumes that the primum movens is neuronal degeneration and that muscle atrophy is only a consequence of such pathogenic event. However, several lines of evidence point to the muscle as primarily involved in the disease, mainly through its role in energy homeostasis. Data from different ALS mouse models strongly argue for an early mitochondrial dysfunction in muscle tissue, possibly leading to motor neuron disturbances. Detailed understanding of skeletal muscle contribution to ALS pathogenesis will likely lead to the identification of novel therapeutic strategies.

Development of ribs and intercostal muscles in the chicken embryo

In the thorax of higher vertebrates, ribs and intercostal muscles play a decisive role in stability and respiratory movements of the body wall. They are derivatives of the somites, the ribs originating in the sclerotome and the intercostal muscles originating in the myotome. During thorax development, ribs and intercostal muscles extend into the lateral plate mesoderm and eventually contact the sternum during ventral closure. Here, we give a detailed description of the morphogenesis of ribs and thoracic muscles in the chicken embryo (Gallus gallus). Using Alcian blue staining as well as Sox9 and Desmin whole‐mount immunohistochemistry, we monitor synchronously the development of rib cartilage and intercostal muscle anlagen. We show that the muscle anlagen precede the rib anlagen during ventrolateral extension, which is in line with the inductive role of the myotome in rib differentiation. Our studies furthermore reveal the temporary formation of a previously unknown eighth rib in the chicken embryonic thorax.

4D imaging reveals stage dependent random and directed cell motion during somite morphogenesis

Somites are paired embryonic segments that form in a regular sequence from unsegmented mesoderm during vertebrate development. Although transient structures they are of fundamental importance as they generate cell lineages of the musculoskeletal system in the trunk such as cartilage, tendon, bone, endothelial cells and skeletal muscle. Surprisingly, very little is known about cellular dynamics underlying the morphological transitions during somite differentiation. Here, we address this by examining cellular rearrangements and morphogenesis in differentiating somites using live multi-photon imaging of transgenic chick embryos, where all cells express a membrane-bound GFP. We specifically focussed on the dynamic cellular changes in two principle regions within the somite, the medial and lateral domains, to investigate extensive morphological transformations. Furthermore, by using quantitative analysis and cell tracking, we capture for the first time a directed movement of dermomyotomal progenitor cells towards the rostro-medial domain of the dermomyotome, where skeletal muscle formation initiates.

Making skeletal muscle from progenitor and stem cells: development versus regeneration

For locomotion, vertebrate animals use the force generated by contractile skeletal muscles. These muscles form an actin/myosin-based biomachinery that is attached to skeletal elements to affect body movement and maintain posture. The mechanics, physiology, and homeostasis of skeletal muscles in normal and disease states are of significant clinical interest. How muscles originate from progenitors during embryogenesis has attracted considerable attention from developmental biologists. How skeletal muscles regenerate and repair themselves after injury by the use of stem cells is an important process to maintain muscle homeostasis throughout lifetime. In recent years, much progress has been made toward uncovering the origins of myogenic progenitors and stem cells as well as the regulation of these cells during development and regeneration.

The dermomyotome ventrolateral lip is essential for the hypaxial myotome formation

Background

The myotome is the primitive skeletal muscle that forms within the embryonic metameric body wall. It can be subdivided into an epaxial and hypaxial domain. It has been shown that the formation of the epaxial myotome requires the dorsomedial lip of the dermomyotome (DML). Although the ventrolateral lip (VLL) of the dermomyotome is believed to be required for the formation of the hypaxial myotome, experimentally evidence for this statement still needs to be provided. Provision of such data would enable the resolution of a debate regarding the formation of the hypaxial dermomyotome. Two mechanisms have been proposed for this tissue. The first proposes that the intermediate dermomyotome undergoes cellular expansion thereby pushing the ventral lateral lip in a lateral direction (translocation). In contrast, the alternative view holds that the ventral lateral lip grows laterally.

Results

Using time lapse confocal microscopy, we observed that the GFP-labelled ventrolateral lip (VLL) of the dermomyotome grows rather than translocates in a lateral direction. The necessity of the VLL for lateral extension of the myotome was addressed by ablation studies. We found that the hypaxial myotome did not form after VLL ablation. In contrast, the removal of an intermediate portion of the dermomyotome had very little effect of the hypaxial myotome. These results demonstrate that the VLL is required for the formation of the hypaxial myotome.

Conclusion

Our study demonstrates that the dermomyotome ventrolateral lip is essential for the hypaxial myotome formation and supports the lip extension model. Therefore, despite being under independent signalling controls, both the dorsomedial and ventrolateral lip fulfil the same function, i.e. they extend into adjacent regions permitting the growth of the myotome.

Sclerotome-derived Slit1 drives directional migration and differentiation of Robo2-expressing pioneer myoblasts

Pioneer myoblasts generate the first myotomal fibers and act as a scaffold to pattern further myotome development. From their origin in the medial epithelial somite, they dissociate and migrate towards the rostral edge of each somite, from which differentiation proceeds in both rostral-to-caudal and medial-to-lateral directions. The mechanisms underlying formation of this unique wave of pioneer myofibers remain unknown. We show that rostrocaudal or mediolateral somite inversions in avian embryos do not alter the original directions of pioneer myoblast migration and differentiation into fibers, demonstrating that regulation of pioneer patterning is somite-intrinsic. Furthermore, pioneer myoblasts express Robo2 downstream of MyoD and Myf5, whereas the dermomyotome and caudal sclerotome express Slit1. Loss of Robo2 or of sclerotome-derived Slit1 function perturbed both directional cell migration and fiber formation, and their effects were mediated through RhoA. Although myoblast specification was not affected, expression of the intermediate filament desmin was reduced. Hence, Slit1 and Robo2, via RhoA, act to pattern formation of the pioneer myotome through the regulation of cytoskeletal assembly.

Somite cell cycle analysis using somite-staging to measure intrinsic developmental time

Somite stages were employed as units of intrinsic developmental time to measure cell doubling rate and other cell cycle parameters of chick forelimb level somites. Somite cell nuclei doubled over an interval corresponding to approximately 7+ somite stages (7+ ss; approximately 11 hr) and approximately 24 new primary myotome cells are born per somite stage ( approximately 16/hr). FACS analysis of DNA content in dissociated paraxial mesoderm cells indicated that slightly more than half are in G1/G0 phase of the cell cycle and that the average combined length of the S phase and G2 phase intervals is approximately 3 ss ( approximately 4.5 hr). A wavefront of increased mitotic nuclei per segment coincident with somite budding potentially reflects a surge in the number of cells entering S phase 3 ss earlier as each PSM segment becomes unresponsive to FGF signaling as it passes through the determination front.

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.

Myogenesis in the mouse

The first striated muscle to form during mouse embryogenesis is the heart followed by skeletal muscle which is derived from the somites. The expression of genes encoding muscle structural proteins and myogenic regulatory sequences of the MyoD1 family has been examined using 35S-labelled riboprobes. In the cardiac tube, actin and myosin genes are expressed together from an early stage, whereas in the myotome, the earliest skeletal muscle, they are activated asynchronously over days. They are not expressed in the somite prior to myotome formation. One potential muscle marker, carbonic anhydrase III, is expressed in early mesoderm and subsequently in the notochord, similarly to the Brachyury gene. The myogenic sequences are not detectable in the heart. In the myotome they show distinct patterns of expression; this is discussed in the context of their role as muscle transcription factors. myf-5 is the only myogenic factor sequence present in the somite prior to muscle formation and thus is potentially involved in an earlier step of muscle determination. It is also present in the early limb bud, but the status of myogenic precursor cells in the limb in this context is less clear.

Amniote somite derivatives

Somites are segments of paraxial mesoderm that give rise to a multitude of tissues in the vertebrate embryo. Many decades of intensive research have provided a wealth of data on the complex molecular interactions leading to the formation of various somitic derivatives. In this review, we focus on the crucial role of the somites in building the body wall and limbs of amniote embryos. We give an overview on the current knowledge on the specification and differentiation of somitic cell lineages leading to the development of the vertebral column, skeletal muscle, connective tissue, meninges, and vessel endothelium, and highlight the importance of the somites in establishing the metameric pattern of the vertebrate body.

Mechanisms of lineage segregation in the avian dermomyotome

The somite and its intermediate derivatives, sclerotome and dermomyotome (DM), are composed of distinct subdomains based on lineage analysis and gene expression patterns. This sets the grounds for elucidating the mechanisms underlying differential cell specification and morphogenesis. By examining the in vivo roles of N-cadherin on discrete domains of the somitic epithelium at various times, our recent studies highlight the existence of a regional and temporal heterogeneity in cellular responsiveness. As examples of this assortment, we document a coupling between asymmetric cell division and fate segregation in the DM sheet, sequential effects of N-cadherin-mediated adhesion on early myogenic specification compared to later myofiber patterning, and a differential behavior of pioneer myoblasts compared to later myogenic waves.

Differential effects of N-cadherin-mediated adhesion on the development of myotomal waves

Myotomal fibers form by a first wave of pioneer myoblasts from the medial epithelial somite, and by a second wave from all four lips of the dermomyotome. Then, a third wave of mitotic progenitors colonizes the myotome,initially stemming from the extreme lips and, later, from the central dermomyotome sheet. In vitro studies have suggested that N-cadherin plays a role in myogenesis, but its role in vivo remains poorly understood. We find that during the growth phase of the dermomyotome sheet, when the orientation of mitotic spindles is parallel to the mediolateral extent of the epithelium,N-cadherin protein is inherited by both daughter cells. Prior to dermomyotome dissociation into dermis and muscle progenitors, when mitoses become perpendicularly oriented, N-cadherin remains associated only with the apical cell located in apposition to the myotome, generating molecular asymmetry between basal and apical progeny. Local gene missexpression confirms that N-cadherin-mediated adhesion is sufficient to promote myotome colonization,whereas its absence drives cells towards the subectodermal domain, hence coupling the asymmetric distribution of N-cadherin to a shift in mitotic orientation and to fate segregation. Site-directed electroporation to additional, discrete somite regions, further reveals that N-cadherin-mediated adhesion is necessary for maintaining the epithelial configuration of all dermomyotome domains while promoting the onset of Myod transcription and the translocation into the myotome of myofibers and/or of Pax-positive progenitors. By contrast, N-cadherin has no effect on migration or differentiation of the first wave of myotomal pioneers. Altogether, we show for the first time that the asymmetric localization of N-cadherin during mitosis indirectly influences fate segregation by differentially driving the allocation of progenitors to muscle versus dermal primordia, that the adhesive domain of N-cadherin maintains the integrity of the dermomyotome epithelium,which is necessary for myogenic specification, and that different molecular mechanisms underlie the establishment of pioneer and later myotomal waves.

The pattern of MyoD and contractile protein localization in primary epaxial myotome reflects the dynamic progression of nascent myoblast differentiation

The localization of contractile and regulatory proteins in early stages of epaxial primary myotome development was analyzed by immunofluorescence microscopy. Contractile proteins that appear in an ordered sequence in the rostro-caudal axis of somite development were found to reiterate that sequence in the dorso-medial-to-ventro-lateral axis of primary epaxial myotome development. Pair-wise localization of MyoD-titin, desmin-titin, and desmin-myosin defined three zones extending from the dermomyotome dorso-medial lip (DML) into the primary myotome layer. Zones M1 and M2, which were positive for MyoD + titin and MyoD + titin + desmin, respectively, were restricted to the dorso-medial-most extremity of the myotome layer and did not expand during the course of myotome development. Zone M3 was positive for MyoD, desmin, titin, myosin, and cardiac troponin T and was the only zone that expanded during primary myotome development. Myotome fibers in zone M3 were unit-length, spanning the full rostro-caudal axis of the myotome while fibers in zones M1 and M2 were shorter than unit length. Anti-myoD immunofluorescence, when detected in cells lacking contractile-protein-positive cytoplasm, was restricted to the DML and nascent myotome cells immediately subjacent to the DML. These results demonstrate a dynamic spatio-temporal sequence in the differentiation program of nascent myotome cells as they emerge from the DML; zones M1 and M2 reflect standing waves of sequential contractile protein activation during the maturation of nascent myotomal myoblasts, while the expanding zone M3 reflects the accumulation of mature myotome fibers expressing a full cohort contractile proteins.

Expression patterns of engrailed-like proteins in the chick embryo

The protein products of both of the identified chick engrailed-like (En) genes, chick En-1 and chick En-2, are localized in cells of the developing brain, mandibular arch, spinal cord, dermatome, and ventral limb bud ectoderm, as demonstrated by labeling with the polyclonal antiserum alpha Enhb-1 developed by Davis et al. (Development 111:281-298, 1991). A subpopulation of cephalic neural crest cells is also En-protein-positive. The monoclonal antibody 4D9 recognizes the chick En-2 gene product exclusively (Patel et al.: Cell 58:955-968, 1989; Davis et al., 1991) and colocalizes with chick En-2 mRNA in the developing head region of the chick embryo as shown by in situ hybridization (Gardner et al.: J. Neurosci. Res. 21:426-437, 1988). In the present study we examine the pattern of alpha Enhb-1 and 4D9 localization throughout the chick embryo from the first appearance of antibody (Ab)-positive cells at stage 8 (Hamburger and Hamilton: J. Morphol. 88:49-92, 1951) through stage 28 (1-5.5 days). We compare the localization patterns of the two Abs to each other, as well as to the localization of the monoclonal Ab, HNK-1, which recognizes many neural crest cells, using double- and triple-label fluorescence immunohistochemistry. Most En protein-positive cells in the path of neural crest cell migration are not HNK-1 positive. In detailed examination of alpha Enhb-1 and 4D9 localization, we find previously undetected patterns of En protein localization in the prechordal plate, hindbrain, myotome, ventral body-wall mesoderm, and extraembryonic membranes. Based upon these observations we propose: 1) that En expression in the mesoderm may be induced through interaction with En expressing cells in the neuroectoderm; 2) that En expression in the head mesenchyme is associated with somitomere 4; and 3) that En expression may be involved in epithelial-mesenchymal cell transformations.

Highly restricted pattern of connexin36 expression in chick somite development

The gap junction protein connexin36 (CX36) has been well studied in the mature central nervous system, but there has been little information regarding its possible roles in embryonic development. We report here the isolation of the full-length chick CX36 coding sequence (predicted M(r) 35.1 kDa) and its strikingly restricted pattern of gene expression in the mesoderm of the chick embryo. In situ hybridization experiments demonstrated CX36 expression in somites by embryonic day 2. The transcripts first appeared dorsomedially within the somite and expanded ventrolaterally to form stripes in the middle of each somite. The CX36 stripes fell within somitic territories enriched in MYOD and FGF8 expression and impoverished in PAX3 transcripts, establishing that CX36 mRNA is expressed in the myotome. We compared the somitic expression pattern of CX36 with those of three other connexins, CX42, CX43, and CX45. At embryonic day 4, CX42 transcripts were localized to the myotome in a pattern resembling that of CX36. In contrast, CX43 was enriched in the dermomyotome, and CX45 was detected in both the myotome and the dermomyotome. Immunoblotting using Cx36 antibodies demonstrated bands of identical electrophoretic mobilities in trunk and retinal homogenates, and Cx36 immunostaining detected punctate immunoreactivity in the myotome. These results demonstrate that some connexins in the developing mesoderm are broadly expressed whereas others are highly localized, and suggest that CX36, CX42, and CX45 are involved in intercellular communication among developing muscle cells.

Integrins in the mouse myotome: Developmental changes and differences between the epaxial and hypaxial lineage

Integrins are cellular adhesion receptors that mediate signaling and play key roles in the development of multicellular organisms. However, their role in the cellular events leading to myotome formation is completely unknown. Here, we describe the expression patterns of the alpha1, alpha4, alpha5, alpha6, and alpha7 integrin subunits in the mouse myotome and correlate them with the expression of several differentiation markers. Our results indicate that these integrin subunits may be differentially involved in the various phases of myogenic determination and differentiation. A detailed characterization of the myogenic cell types expressing the alpha4 and alpha6 subunits showed a regionalization of the myotome and dermomyotome based on cell-adhesion properties. We conclude that alpha6beta1 may be an early marker of epaxial myogenic progenitor cells. In contrast, alpha4beta1 is up-regulated in the intercalated myotome after myocyte differentiation. Furthermore, alpha4beta1 is expressed in the hypaxial dermomyotome and is maintained by early hypaxial myogenic progenitor cells colonizing the myotome.

A Two-Step Mechanism for Myotome Formation in Chick

The study of the morphogenetic cell movements underlying myotome formation in the chick embryo has led to the emergence of highly controversial models. Here we report a real-time cell lineage analysis of myotome development using electroporation of a GFP reporter in newly formed chick somites. Confocal analysis of cell movements demonstrates that myotome formation involves two sequential steps. In a first phase, incremental myotome growth results from a contribution of myocytes derived solely from the medial border of the dermomyotome. In a second phase, myocytes are produced from all four borders of the dermomyotome. The relative distribution of myocytes demonstrates that the medial and the lateral borders of the somite generate exclusively epaxial and hypaxial muscles. This analysis also identified five myotomal regions, characterized by the origin of the myocytes that constitute them. Together, our results provide a comprehensive model describing the morphogenesis of the early myotome in higher vertebrates.

Precocious terminal differentiation of premigratory limb muscle precursor cells requires positive signalling

The timing of myogenic differentiation of hypaxial muscle precursor cells in the somite lags behind that of epaxial precursors. Two hypotheses have been proposed to explain this delay. One attributes the delay to the presence of negative-acting signals from the lateral plate mesoderm adjacent to the hypaxial muscle precursor cells located in the ventrolateral lip of the somitic dermomyotome (Pourquié et al. [1995] Proc. Natl. Acad. Sci. USA 92:3219-3223). The second attributes the delay to an absence of positive-acting inductive signals, similar to those from the axial structures that induce epaxial myotome development (Pownall et al. [1996] Development 122:1475-1488). Because both studies relied principally upon changes in the expression pattern of mRNAs specific to early muscle precursor cell markers, we revisited these experiments using two methods to assess muscle terminal differentiation. First, injection of fluorescent dyes before surgery was used to determine whether ventrolateral lip cells transform from epithelial cells to elongated myocytes. Second, an antibody to a terminal differentiation marker and a new monoclonal antibody that recognises avian and mammalian Pax3 were used for immunohistochemistry to assess the transition from precursor cell to myocyte. The results support both hypotheses and show further that placing axial structures adjacent to the somite ventrolateral lip induces an axial pattern of myocyte terminal differentiation and elongation.

The roles of cell migration and myofiber intercalation in patterning formation of the postmitotic myotome

We have previously found that the postmitotic myotome is formed by two successive waves of myoblasts. A first wave of pioneer cells is generated from the dorsomedial region of epithelial somites. A second wave originates from all four edges of the dermomyotome but cells enter the myotome only from the rostral and caudal lips. We provide new evidence for the existence of these distinctive waves. We show for the first time that when the somite dissociates, pioneer myotomal progenitors migrate as mesenchymal cells from the medial side towards the rostral edge of the segment. Subsequently, they generate myofibers that elongate caudally. Pioneer myofiber differentiation then progresses in a medial-to-lateral direction with fibers reaching the lateralmost region of each segment. At later stages, pioneers participate in the formation of multinucleated fibers during secondary myogenesis by fusing with younger cells. We also demonstrate that subsequent to primary myotome formation by pioneers, growth occurs by uniform cell addition along the dorsoventral myotome. At this stage, the contributing cells arise from multiple sources as the myotome keeps growing even in the absence of the dorsomedial lip. Moreover, as opposed to suggestions that myotome growth is driven primarily and directly by the medial and lateral edges, we demonstrate that there is no direct fiber generation from the dorsomedial lip. Instead, we find that added fibers elongate from the extreme edges. Altogether, the integration between both myogenic waves results in an even pattern of dorsoventral growth of the myotome which is accounted for by progressive cell intercalation of second wave cells between preexisting pioneer fibers.

The Mef2c gene is a direct transcriptional target of myogenic bHLH and MEF2 proteins during skeletal muscle development

Members of the MEF2 family of transcription factors are upregulated during skeletal muscle differentiation and cooperate with the MyoD family of myogenic basic helix-loop-helix (bHLH) transcription factors to control the expression of muscle-specific genes. To determine the mechanisms that regulate MEF2 gene expression during skeletal muscle development, we analyzed the mouse Mef2c gene for cis-regulatory elements that direct expression in the skeletal muscle lineage in vivo. We describe a skeletal muscle-specific control region for Mef2c that is sufficient to direct lacZ reporter gene expression in a pattern that recapitulates that of the endogenous Mef2c gene in skeletal muscle during pre- and postnatal development. This control region is a direct target for the binding of myogenic bHLH and MEF2 proteins. Mutagenesis of the Mef2c control region shows that a binding site for myogenic bHLH proteins is essential for expression at all stages of skeletal muscle development, whereas an adjacent MEF2 binding site is required for maintenance but not for initiation of Mef2c transcription. Our findings reveal the existence of a regulatory circuit between these two classes of transcription factors that induces, amplifies and maintains their expression during skeletal muscle development.

The third wave of myotome colonization by mitotically competent progenitors: regulating the balance between differentiation and proliferation during muscle development

The myotome is formed by a first wave of pioneer cells originating from the entire dorsomedial region of epithelial somites and a second wave that derives from all four lips of the dermomyotome but generates myofibers from only the rostral and caudal edges. Because the precedent progenitors exit the cell cycle upon myotome colonization, subsequent waves must account for consecutive growth. In this study, double labeling with CM-DiI and BrdU revealed the appearance of a third wave of progenitors that enter the myotome as mitotically active cells from both rostral and caudal dermomyotome edges. These cells express the fibroblast growth factor (FGF) receptor FREK and treatment with FGF4 promotes their proliferation and redistribution towards the center of the myotome. Yet, they are negative for MyoD, Myf5 and FGF4, which are, however, expressed in myofibers.

The proliferating progenitors first appear around the 30-somite stage in cervical-level myotomes and their number continuously increases, making up 85% of total muscle nuclei by embryonic day (E)4. By this stage, generation of second-wave myofibers, which also enter from the extreme lips is still under way. Formation of the latter fibers peaks at 30 somites and progressively decreases with age until E4. Thus, cells in these dermomyotome lips generate simultaneously distinct types of muscle progenitors in changing proportions as a function of age. Consistent with a heterogeneity in the cellular composition of the extreme lips, MyoD is normally expressed in only a subset of these epithelial cells. Treatment with Sonic hedgehog drives most of them to become MyoD positive and then to become myofibers, with a concurrent reduction in the proportion of proliferating muscle precursors.

The dermomyotome dorsomedial lip drives growth and morphogenesis of both the primary myotome and dermomyotome epithelium

The cellular and molecular mechanisms that govern early muscle patterning in vertebrate development are unknown. The earliest skeletal muscle to organize, the primary myotome of the epaxial domain, is a thin sheet of muscle tissue that expands in each somite segment in a lateral-to-medial direction in concert with the overlying dermomyotome epithelium. Several mutually contradictory models have been proposed to explain how myotome precursor cells, which are known to reside within the dermomyotome, translocate to the subjacent myotome layer to form this first segmented muscle tissue of the body. Using experimental embryology to discriminate among these models, we show here that ablation of the dorsomedial lip (DML) of the dermomyotome epithelium blocks further primary myotome growth while ablation of other dermomyotome regions does not. Myotome growth and morphogenesis can be restored in a DML-ablated somite of a host embryo by transplantation of a second DML from a donor embryo. Chick-quail marking experiments show that new myotome cells in such recombinant somites are derived from the donor DML and that cells from other regions of the somite are neither present nor required. In addition to the myotome, the transplanted DML also gives rise to the dermomyotome epithelium overlying the new myotome growth region and from which the mesenchymal dermatome will later emerge. These results demonstrate that the DML is a cellular growth engine that is both necessary and sufficient to drive the growth and morphogenesis of the primary myotome and simultaneously drive that of the dermomyotome, an epithelium containing muscle, dermis and possibly other potentialities.

Morphogenetic cell movements in the middle region of the dermomyotome dorsomedial lip associated with patterning and growth of the primary epaxial myotome

The morphogenetic cell movements responsible for growth and morphogenesis in vertebrate embryos are poorly understood. Myotome precursor cells undergo myotomal translocation; a key morphogenetic cell movement whereby myotomal precursor cells leave the dermomyotome epithelium and enter the subjacent myotome layer where myogenic differentiation ensues. The precursors to the embryonic epaxial myotome are concentrated in the dorsomedial lip (DML) of the somite dermomyotome (W. F. Denetclaw, B. Christ and C. P. Ordahl (1997) Development 124, 1601–1610), a finding recently substantiated through surgical transplantation studies (C. P. Ordahl, E. Berdougo, S. J. Venters and W. F. Denetclaw, Jr (2001) Development 128, 1731–1744). Confocal microscopy was used here to analyze the location and pattern of myotome cells whose precursors had earlier been labeled by fluorescent dye injection into the middle region of the DML, a site that maximizes the potential to discriminate among experimental outcomes. Double-dye injection experiments conducted at this site demonstrate that cells fated to form myotome do not involute around the recurved epithelium of the DML but rather are displaced laterally where they transiently intermingle with cells fated to enter the central epithelial sheet region of the dermomyotome. Time- and position-dependent labeling experiments demonstrated that myotome precursor cells translocate directly from the middle region of the DML without prior intra-epithelial ‘translational’ movements of precursor cells to either the cranial or caudal lips of the dermomyotome epithelium, nor were any such translational movements evident in these experiments. The morphogenetic cell movements demonstrated here to be involved in the directional growth and segmental patterning of the myotome and dermomyotome bear interesting similarities with those of other morphogenetic systems.

Molecular and cellular biology of avian somite development

Much of our understanding of early vertebrate embryogenesis derives from experimental work done with the chick embryo. Studies of the avian somite have played a key role in elucidating the developmental history of this important structure, the source of most muscle and bone in the organism. Here we review the development of the avian somite including morphological and molecular data on the origin of paraxial mesoderm, maturation of the segmental plate, specification and formation of somite compartments, and somite cell differentiation into cartilage and skeletal muscle.

Somite formation and patterning

As a consequence of their segmented arrangement and the diversity of their tissue derivatives, somites are key elements in the establishment of the metameric body plan in vertebrates. This article aims to largely review what is known about somite development, from the initial stages of somite formation through the process of somite regionalization along the three major body axes. The role of both cell intrinsic mechanisms and environmental cues are evaluated. The periodic and bilaterally synchronous nature of somite formation is proposed to rely on the existence of a developmental clock. Molecular mechanisms underlying these events are reported. The importance of an antero-posterior somitic polarity with respect to somite formation on one hand and body segmentation on the other hand is discussed. Finally, the mechanisms leading to the regionalization of somites along the dorso-ventral and medio-lateral axes are reviewed. This somitic compartmentalization is believed to underlie the segregation of dermis, skeleton, and dorsal and appendicular musculature.

Fate restriction in limb muscle precursor cells precedes high-level expression of MyoD family member genes

The mechanisms by which pluripotent embryonic cells generate unipotent tissue progenitor cells during development are unknown. Molecular/genetic experiments in cultured cells have led to the hypothesis that the product of a single member of the MyoD gene family (MDF) is necessary and sufficient to establish the positive aspects of the determined state of myogenic precursor cells: i.e., the ability to initiate and maintain the differentiated state (Weintraub, H., Davis, R., Tapscott, S., Thayer, M., Krause, M., Benezra, R., Blackwell, T. K., Turner, D., Rupp, R., Hollenberg, S. et al. (1991) Science 251, 761–766). Embryonic cell type determination also involves negative regulation, such as the restriction of developmental potential for alternative cell types, that is not directly addressed by the MDF model. In the experiments reported here, phenotypic restriction in myogenic precursor cells is assayed by an in vivo ‘notochord challenge’ to evaluate their potential to ‘choose’ between two alternative cell fate endpoints: cartilage and muscle (Williams, B. A. and Ordahl, C. P. (1997) Development 124, 4983–4997). Two separate myogenic precursor cell populations were found to be phenotypically restricted while expressing the Pax3 gene and prior to MDF gene activation. Therefore, while MDF family members act positively during myogenic differentiation, phenotypic restriction, the negative aspect of cell specification, requires cellular and molecular events and interactions that precede MDF expression in myogenic precursor cells. The qualities of muscle formed by the determined myogenic precursor cells in these experiments further indicate that their developmental potential is intermediate between that of myoblastic stem cells taken from fetal or adult tissue (which lack mitotic and morphogenetic potential when tested in vivo) and embryonic stem cells (which are multipotent). We hypothesize that such embryonic myogenic progenitor cells represent a distinct class of determined embryonic cell, one that is responsible for both tissue growth and tissue morphogenesis.

Isolation of a spontaneously fusing BC3H1 muscle cell line: fusion alters the response to serum stimulation

Differentiation of skeletal muscle cells involves two distinct events: exit from the cell cycle and expression of muscle-specific contractile genes and formation of multinucleated myocytes. Although many studies have shown that growth factors regulate the initial step of differentiation, little is known about regulation of fusion. BC3H1 cells are a skeletal muscle cell line characterized by a nonfusing phenotype and an ability to dedifferentiate. When subjected to serum or growth factors, differentiated BC3H1 cells lose muscle-specific gene expression and re-enter the cell cycle. In this study, we describe a spontaneously fusing clone of BC3H1 cells. We demonstrate that this fusion capability is not due to altered muscle regulatory factor or adhesion molecule expression. Furthermore, we show that fusion inhibits dedifferentiation. Multinucleated BC3H1 cells do not lose myosin expression, nor do they re-enter the cell cycle. Fused BC3HI cells react to serum stimulation with a hypertrophic response. Our results suggest that the state of differentiation, mono- or multi-nucleated, is essential to how myocytes react to growth stimulation and may provide a mechanism for how differentiation, fusion, and hypertrophy are regulated in vivo.

Identification of proliferating cells in chicken embryos using 5-bromo- 2'-deoxyuridine immunohistochemical detection

Chicken embryos were incubated with BrdU, embedded in plastic resin, sectioned and screened immunohistochemically to identify proliferating cells in the neural tube and somites. Fixation in 4% paraformaldehyde for 1 h was essential for detecting specific colorimetric signals of BrdU incorporation into cells during the S phase of the cell cycle. Transverse sections of the neural tube showed that the nuclei of proliferating cells (BrdU positive) had a uniform and centralized distribution, whereas unstained nuclei were found only along the extremities of the neural tube. Transverse sections of differentiated somites showed proliferating cells in the scleratome and dermatome. However, no incorporation of BrdU was observed in myotomic cells, which give rise to axial skeletal muscle. In spite of their proximity, the dermatome and myotome showed marked differences in cell proliferation. The excellent preservation of morphological characteristics in the embryonic tissues facilitated identification of variations in BrdU incorporation.

The growth of the dermomyotome and formation of early myotome lineages in thoracolumbar somites of chicken embryos

Myotome formation in the epaxial and hypaxial domains of thoraco-lumbar somites was analyzed using fluorescent vital dye labeling of dermomyotome cells and cell-fate assessment by confocal microscopy. Muscle precursor cells for the epaxial and hypaxial myotomes are predominantly located in the dorsomedial and ventrolateral dermomyotome lips, respectively, and expansion of the dermomyotome is greatest along its mediolateral axis coincident with the dorsalward and ventralward growth directions of the epaxial and hypaxial myotomes. Measurements of the dermomyotome at different stages of development shows that myotome growth begins earlier in the epaxial than in the hypaxial domain, but that after an initial lag phase, both progress at the same rate. A combination of dye injection and/or antibody labeling of early and late-expressed muscle contractile proteins confirms the myotome mediolateral growth directions, and shows that the myotome thickness increases in a superficial (near dermis) to deep (near sclerotome) growth direction. These findings also provide a basis for predicting the following gene expression sequence program for the earliest muscle precursor lineages in mouse embryos: Pax-3 (stem cells), myf-5 (myoblast cells) and myoD (myocytes). The movements and mitotic activity of early muscle precursor cells lead to the conclusion that patterning and growth in the myotome specifically, and in the epaxial and hypaxial domains of the body generally, are governed by morphogenetic cell movements.

Evolution and development of distinct cell lineages derived from somites

In the vertebrate embryo, the somites arise from the paraxial mesoderm as paired mesodermal units in a craniocaudal sequence. Segmentation is also the underlying principle of the body plan in annelids and arthropods. Genes controlling segmentation have been identified that are highly conserved in organisms belonging to different phyla. Segmentation facilitates movement and regionalization of the vertebrate body. Its traces in humans are, for example, vertebral bodies, intervertebral disks, ribs, and spinal nerves. Somite research has a history of at least three centuries. Detailed morphological data have accumulated on the development of the avian somite. Especially in connection with the quailchick interspecific marker system, progress was made toward an understanding of underlying mechanisms. At first each somite consists of an outer epithelium and a mesenchymal core. Later, the ventral portion of the somite undergoes de-epithelialization and gives rise to the sclerotome, whereas the dorsal portion forms the dermomyotome. The dermomyotome is the source of myotomal muscle cells and the dermis of the back. It also yields the hypaxial muscle buds at flank level and the myogenic cells invading the limb buds. The dorsal and ventral somitic domains express different sets of developmental control genes, for example, those of the Pax family. During later stages of development, the sclerotomes undergo a new arrangement called "resegmentation" leading to the fusion of the caudal half of one sclerotome with the cranial half of the following sclerotome. Further somitic derivatives include fibroblasts, smooth muscle, and endothelial cells. While sclerotome formation is controlled by the notochord, signals from the dorsal neural tube and ectoderm support the development of the dermomyotome. Myogenic precursor cells for the limb bud are recruited from the dermomyotome by the interaction of c-met with its ligand scatter factor (SF/HGF). In the evolution of metamerism in vertebrates, the first skeletal elements were primitive parts of neural arches, while axial elements developed only later in teleosts as pleurocentra and hypocentra.

Filamin isogene expression during mouse myogenesis

The developmental pattern of filamin gene expression has been studied in mouse embryos by using in situ hybridization. The probes used were isoform specific, (35)S-labeled antisense complementary ribonucleic acids (cRNAs) to the 3; untranslated region (3; UTR) of muscle-specific and nonmuscle-specific filamin genes. Northern blot and in situ hybridization results showed that nonmuscle-specific filamin transcripts had a size of 9.5 kb and were expressed in all nonmuscle tissues. Labeling was most intense in tissues containing a substantial proportion of epithelial and smooth muscle cells. Muscle-specific filamin transcripts had a size of 10 kb and were expressed primarily in cardiac and skeletal muscle. The expression of muscle-specific filamin messenger ribonucleicacids (mRNAs) was detected in heart at 8.0 days after coitum, whereas that in the myotomes of somites was not detected until 10.5 days after coitum. The expression of muscle-specific filamin mRNAs in heart and in skeletal muscle continued through the subsequent days of myogenesis. The results showed that muscle-specific filamin gene transcripts are detected before the formation of myotubes in vivo. This is the first study of filamin gene expression at the early stages of skeletal muscle development. Dev Dyn 2000;217:99-108.

Segmentation of the paraxial mesoderm and vertebrate somitogenesis

Somites are the most obviously segmented features of the vertebrate embryo. Although the way segmentation is achieved in the fly is now well described, little was known about the molecular mechanisms underlying vertebrate somitogenesis. Through the recent identification of genes important for vertebrate somitogenesis and the analysis of their function, several theoretical models accounting for somitogenesis such as the clock and wavefront model, which have been proposed over the past 20 years, are now starting to receive experimental support. A molecular clock linked to somitogenesis has been identified which might act as a periodicity generator in the presomitic cells. This temporal periodicity is then translated into a tightly controlled spatial periodicity which is revealed by the expression of several genes. Analysis of mouse mutants in the Notch-Delta pathway suggest that this signaling mechanism might play an important role at this level. The final step of the cascade is to translate these genetically specified segments into morphological units: the somites. Importantly, these studies have helped in dissociating the segmentation and the somitogenesis processes in vertebrates. In addition, although segmentation was classically thought to have arisen independently in protostomes and deuterostomes, recent evidence suggests that part of the segmentation machinery might actually have been conserved. The conservation of segmentation mechanisms reported in the fly such as the pair-rule pattern, however, remain a subject of controversy.

Differential regulation of epaxial and hypaxial muscle development by paraxis

In vertebrates, skeletal muscle is derived from progenitor cell populations located in the epithelial dermomyotome compartment of the each somite. These cells become committed to the myogenic lineage upon delamination from the dorsomedial and dorsolateral lips of the dermomyotome and entry into the myotome or dispersal into the periphery. Paraxis is a developmentally regulated transcription factor that is required to direct and maintain the epithelial characteristic of the dermomyotome. Therefore, we hypothesized that Paraxis acts as an important regulator of early events in myogenesis. Expression of the muscle-specific myogenin-lacZ transgene was used to examine the formation of the myotome in the paraxis−/− background. Two distinct types of defects were observed that mirrored the different origins of myoblasts in the myotome. In the medial myotome, where the expression of the myogenic factor Myf5 is required for commitment of myoblasts, the migration pattern of committed myoblasts was altered in the absence of Paraxis. In contrast, in the lateral myotome and migratory somitic cells, which require the expression of MyoD, expression of the myogenin-lacZ transgene was delayed by several days. This delay correlated with an absence of MyoD expression in these regions, indicating that Paraxis is required for commitment of cells from the dorsolateral dermomyotome to the myogenic lineage. In paraxis−/−/myf5−/− neonates, dramatic losses were observed in the epaxial and hypaxial trunk muscles that are proximal to the vertebrae in the compound mutant, but not those at the ventral midline or the non-segmented muscles of the limb and tongue. In this genetic background, myoblasts derived from the medial (epaxial) myotome are not present to compensate for deficiencies of the lateral (hypaxial) myotome. Our data demonstrate that Paraxis is an important regulator of a subset of the myogenic progenitor cells from the dorsolateral dermomyotome that are fated to form the non-migratory hypaxial muscles.

Early development of the myotome in the mouse

The structure and development of the myotome has been extensively studied in birds and amphibians but few studies have systematically addressed its development in mammals. We have used a transgenic mouse carrying an nLacZ marker coupled to a myosin light chain 3F promoter to describe the structure of the developing mammalian myotome. Through studies of transgene expression pattern, coupled with immunohistochemistry for the muscle structural proteins desmin and slow myosin heavy chain we describe a gradient of maturity for the cells within the developing myotome. Our results show that the earliest myocytes of the mammalian myotome span the rostrocaudal extent of the somite and have single large nuclei which localise centrally within the myotome. Throughout the period of study the myotome is more mature ventrally than dorsally and cells comprising the medial aspect of the myotome are younger than those lying laterally. Immunohistochemistry for the earliest expressed muscle regulatory factor (myf-5) is used to define areas of the myotome contributing new myogenic cells. In the early myotome small, round, myf-5-expressing cells are found extensively within the dorsomedial aspect of the dermamyotome and also within the entire rostral and caudal dermamyotomal lips. They subsequently appear within the central zone of the myotome, adjacent to the medially curled rostral and caudal dermamyotomal lips, and there begin to elongate symmetrically. As the myotome enlarges, myf-5 expression is always restricted to the most medial aspect of the myotome, adjacent to the least mature myocytes, marking the site of addition of new myogenic cells. Together, these results allow development of a model of mammalian myotome formation where growth occurs medially by addition of new cells from both rostral and caudal dermamyotome lips, while more mature myocytes are displaced laterally. Furthermore, early myotomal myocytes differentiate in the absence of MyoD expression, unlike later myotomal myocytes. This, along with their distinct morphology, suggests these cells may form a separate lineage of pioneer myogenic cells.

A molecular mechanism enabling continuous embryonic muscle growth - a balance between proliferation and differentiation

Embryonic muscle growth requires a fine balance between proliferation and differentiation. In this study we have investigated how this balance is achieved during chick development. Removal of ectoderm from trunk somites results in the down-regulation of Pax-3 expression and cell division of myogenic precursors is halted. This initially leads to an up-regulation of MyoD expression and to a burst in terminal differentiation but further muscle growth is arrested. Locally applied bone morphogenetic protein-4 (BMP-4) to somites mimics the effect of the ectoderm and stimulates Pax-3 expression which eventually results in excessive muscle growth in somites. Surprisingly, BMP-4 up-regulates expression of noggin which encodes a BMP-4 antagonist. This suggests that the proliferation enhancing activity of BMP-4 can be limited via up-regulation of noggin and that myogenic cells differentiate, as an intrinsic property, when deprived of BMP-4 influence. In contrast to BMP-4, Sonic hedgehog (Shh) locally applied to somites arrests muscle growth by down-regulation of Pax-3 and immediate up-regulation of MyoD expression. Such premature muscle differentiation in somites at tongue and limb levels prevents myogenic migration and thus tongue and limb muscle are not formed. Therefore, precise limitation of differentiation, executed by proliferative and Pax-3 promoting signals, is indispensable for continuous embryonic muscle growth.

The cellular mechanism by which the dermomyotome contributes to the second wave of myotome development

We have shown that a subset of early postmitotic progenitors that originates along the medial part of the epithelial somite gives rise to the primary myotome (Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998). Mech. Dev. 74, 59–73). Because of its postmitotic nature, further myotome expansion must be achieved by cell addition from extrinsic sources. Here we investigate the mechanism whereby the dermomyotome contributes to this process. Using several different methods we found that cell addition occurs from both rostral and caudal edges of the dermomyotome, but not directly from its dorsomedial lip (DML). First, labeling of quail embryos with [3H]thymidine revealed a time-dependent entry of radiolabeled nuclei into the myotome from the entire rostral and caudal lips of the dermomyotome, but not from the DML. Second, fluorescent vital dyes were injected at specific sites in the dermomyotome lips and the fate of dye-labeled cells followed by confocal microscopy. Consistent with the nucleotide labeling experiments, dye-labeled myofibers directly emerged from injected epithelial cells from either rostral or caudal lips. In contrast, injected cells from the DML first translocated along the medial boundary, reached the rostral or caudal dermomyotome lips and only then elongated into the myotome. These growing myofibers had always one end attached to either lip from which they elongated in the opposite direction. Third, following establishment of the primary myotome, cells along the extreme dermomyotome edges, but not the DML, expressed QmyoD, supporting the notion that rostral and caudal boundaries generate myofibers. Fourth, ablation of the DML had only a limited effect on myotomal cell number. Thus, cells deriving from the extreme dermomyotome lips contribute to uniform myotome growth in the dorsoventral extent of the myotome. They also account for its expansion in the transverse plane and this is achieved by myoblast addition in a lateral to medial direction (from the dermal to the sclerotomal sides), restricting the pioneer myofibers to the dermal side of the myotome. Taken together, the data suggest that myotome formation is a multistage process. A first wave of pioneers establishes the primary structure. A second wave generated from specific dermomyotome lips contributes to its expansion. Because dermomyotome lip progenitors are mitotically active within the epithelia of origin but exit the cell cycle upon myotome colonization, they can only provide for limited myotome growth and subsequent waves must take over to ensure further muscle development.

The origin and fate of pioneer myotomal cells in the avian embryo

The ontogeny of the myotome was investigated using [3H]thymidine or Brdu treatment in conjunction with 1,1', di-octadecyl-3, 3, 3', 3',-tetramethylindo-carbocyanine perchlorate (DiI) labeling and expression of specific markers. We have identified a subset of early post-mitotic cells that is present in the dorsomedial aspect of epithelial somites and is homogeneously distributed along their entire rostrocaudal extent. The post-mitotic quality of this cell subset enabled us to trace their fate in time-course experiments. Following initial somite dissociation, this epithelial post-mitotic layer bends underneath the medial portion of the nascent dermomyotome. Then, these cells progressively lose epithelial arrangement and migrate in a rostral direction where they accumulate temporarily. Subsequently, these early post-mitotic precursors extend processes that reach both rostral and caudal edges of each segment. Medial somite-derived myofibers also fill the entire mediolateral extent of the segment and reach the dorsomedial lip of the dermomyotome, thus forming the primary myotome. During this process, their large nuclei localize to a narrow stripe in the middle of the nascent myotome. Consistent with the proliferation studies, DiI labeling of the medial epithelial somite cells gave rise to a primary myotomal structure, and continuous pulsing of the DiI-injected embryos with radioactive thymidine revealed that these fibers indeed developed from post-mitotic progenitors. As these early post-mitotic cells that arise prior to somite dissociation are the first wave of progenitors that constitutes the myotome, we have termed them avian muscle pioneers. We propose that the primary myotome formed by the muscle pioneers constitutes a longitudinal scaffold that serves as a substrate for the addition of subsequent waves of myotomal cells.

The generation and interpretation of positional information within the vertebrate myotome

How somitic cells become restricted to the muscle fate has been investigated on a number of levels. Classical embryological manipulations have attempted to define the source of inductive signals that control the formation of the myotome. Recently, these studies have converged with others dissecting the role of secreted proteins in embryonic patterning to demonstrate a role for specific peptides in inducing individual cell types of the myotome. Collectively, these investigations have implicated the products of the Wnt, Hedgehog (Hh) and Bone morphogenetic protein (Bmp) gene families as key myogenic regulators; simultaneously controlling both the initiation of myogenesis and the fate of individual myoblasts.

Establishing myogenic identity during somitogenesis

Over the past year, interest has focused on identifying signalling molecules--including Wnts, Sonic hedgehog, BMP-4, and noggin--that divert somitic mesodermal cells into the muscle lineage, either by induction or derepression. New mouse mutants have also provided insights into somite formation and differentiation, as well as pointing to novel differences between head, trunk, and limb myogenic programmes. In addition, recent genetic, embryological, and molecular studies have shed new light on somite formation and the establishment of muscle progenitor cells.

Synergistic interactions between bFGF and a TGF-beta family member may mediate myogenic signals from the neural tube

Development of the myotome within somites depends on unknown signals from the neural tube. The present study tested the ability of basic fibroblast growth factor (bFGF), transforming growth factor-beta1 (TGF-beta1) and dorsalin-1 (dsl-1) to promote myogenesis in stage 10–14 chick paraxial mesoderm utilizing 72 hour explant cultures. Each of these factors alone and the combination of bFGF with dsl-1 had limited to no myogenic-promoting activity, but the combination of bFGF with TGF-beta1 demonstrated a potent dose-dependent effect. In addition, bFGF enhanced the survival/proliferation of somite cells. 98% of stage 10–11 caudal segmental plate explants treated with bFGF plus TGF-beta1, exhibited myosin heavy chain (MHC)-positive cells (avg.=60 per explant), whereas only 15% of similarly treated somites responded with an average of 5 MHC-positive cells. Thus at stage 10–11, there are rostrocaudal differences in myogenic responsiveness with the caudal (more ‘immature’) paraxial mesoderm being more myogenically responsive to these factors than are somites. It was also discovered that 17% of stage 10–11 caudal segmental plate explants exhibited several MHC-positive cells even when cultured without added growth factors, further demonstrating a different myogenic potential of the caudal paraxial mesoderm. Stage 13–14 paraxial mesoderm also exhibited a myogenic response to bFGF/TGF-beta1 but, unlike stage 10–11 embryos, both somites and segmental plate exhibited a strong response. A two-step mechanism for the bFGF/TGF-beta1 effect is suggested by the finding that only TGF-beta1 was required during the first 12 hours of culture, whereas bFGF plus a TGF-beta-like factor were required for the remainder of the culture. The biological relevance of the findings with bFGF is underscored by the observation that a monoclonal antibody to bFGF inhibited myogenic signaling from the dorsal neural tube. However, a monoclonal antibody that can neutralize the three factors TGF-beta1, TGF-beta2 and TGF-beta3 did not block myogenic signals from the neural tube, raising the possibility that another TGF-beta family member may be involved in vivo.

Regulation of paraxis expression and somite formation by ectoderm- and neural tube-derived signals

During vertebrate embryogenesis, the paraxial mesoderm becomes segmented into somites, which form as paired epithelial spheres with a periodicity that reflects the segmental organization of the embryo. As a somite matures, the ventral region gives rise to a mesenchymal cell population, the sclerotome, that forms the axial skeleton. The dorsal region of the somite remains epithelial and is called dermomyotome. The dermomyotome gives rise to the trunk and limb muscle and to the dermis of the back. Epaxial and hypaxial muscle precursors can be attributed to distinct somitic compartments which are laid down prior to overt somite differentiation. Inductive signals from the neural tube, notochord, and overlying ectoderm have been shown to be required for patterning of the somites into these different compartments. Paraxis is a basic helix-loop-helix transcription factor expressed in the unsegmented paraxial mesoderm and throughout epithelial somites before becoming restricted to epithelial cells of the dermomyotome. To determine whether paraxis might be a target for inductive signals that influence somite patterning, we examined the influence of axial structures and surface ectoderm on paraxis expression by performing microsurgical operations on chick embryos. These studies revealed two distinct phases of paraxis expression, an early phase in the paraxial mesoderm that is dependent on signals from the ectoderm and independent of the neural tube, and a later phase that is supported by redundant signals from the ectoderm and neural tube. Under experimental conditions in which paraxis failed to be expressed, cells from the paraxial mesoderm failed to epithelialize and somites were not formed. We also performed an RT-PCR analysis of combined tissue explants in vitro and confirmed that surface ectoderm is sufficient to induce paraxis expression in segmental plate mesoderm. These results demonstrate that somite formation requires signals from adjacent cell types and that the paraxis gene is a target for the signal transduction pathways that regulate somitogenesis.

Location and growth of epaxial myotome precursor cells

The skeletal muscle progenitor cells of the vertebrate body originate in the dermomyotome epithelium of the embryonic somites. To precisely locate myotome precursor cells, fluorescent vital dyes were iontophoretically injected at specific sites in the dermomyotome in ovo and the fates of dye-labeled cells monitored by confocal microscopy. Dye-labeled myotome myofibers were generated from cells injected along the entire medial boundary and the medial portion of the cranial boundary of the dermomyotome, regions in close proximity to the dorsal region of the neural tube where myogenic-inducing factors are thought to be produced. Other injected regions of the dermomyotome did not give rise to myotome fibers. Analysis of nascent myotome fibers showed that they elongate along the embryonic axis in cranial and caudal directions, or in both directions simultaneously, until they reach the margins of the dermomyotome. Finally, deposition of myotome fibers and expansion of the dermomyotome epithelium occurs in a lateral-to-medial direction. This new model for early myotome formation has implications for myogenic specification and for growth of the epaxial domain during early embryonic development.

Ectopic Pax-3 activates MyoD and Myf-5 expression in embryonic mesoderm and neural tissue

To understand how the skeletal muscle lineage is induced during vertebrate embryogenesis, we have sought to identify the regulatory molecules that mediate induction of the myogenic regulatory factors MyoD and Myf-5. In this work, we demonstrate that either signals from the overlying ectoderm or Wnt and Sonic hedgehog signals can induce somitic expression of the paired box transcription factors, Pax-3 and Pax-7, concomitant with expression of Myf-5 and prior to that of MyoD. Moreover, infection of embryonic tissues in vitro with a retrovirus encoding Pax-3 is sufficient to induce expression of MyoD, Myf-5, and myogenin in both paraxial and lateral plate mesoderm in the absence of inducing tissues as well as in the neural tube. Together, these findings imply that Pax-3 may mediate activation of MyoD and Myf-5 in response to muscle-inducing signals from either the axial tissues or overlying ectoderm and identify Pax-3 as a key regulator of somitic myogenesis.

Sclerotome development and peripheral nervous system segmentation in embryonic zebrafish

Vertebrate embryos display segmental patterns in many trunk structures, including somites and peripheral nervous system elements. Previous work in avian embryos suggests a role for somite-derived sclerotome in segmental patterning of the peripheral nervous system. We investigated sclerotome development and tested its role in patterning motor axons and dorsal root ganglia in embryonic zebrafish. Individual somite cells labeled with vital fluorescent dye revealed that some cells of a ventromedial cell cluster within each somite produced mesenchymal cells that migrated to positions expected for sclerotome. Individual somites showed anterior/posterior distinctions in several aspects of development: (1) anterior ventromedial cluster cells produced only sclerotome, (2) individual posterior ventromedial cluster cells produced both sclerotome and muscle, and (3) anterior sclerotome migrated earlier and along a more restricted path than posterior sclerotome. Vital labeling showed that anterior sclerotome colocalized with extending identified motor axons and migrating neural crest cells. To investigate sclerotome involvement in peripheral nervous system patterning, we ablated the ventromedial cell cluster and observed subsequent development of peripheral nervous system elements. Primary motor axons were essentially unaffected by sclerotome ablation, although in some cases outgrowth was delayed. Removal of sclerotome did not disrupt segmental pattern or development of dorsal root ganglia or peripheral nerves to axial muscle. We propose that peripheral nervous system segmentation is established through interactions with adjacent paraxial mesoderm which develops as sclerotome in some vertebrate species and myotome in others.

Gene targeting the myf-5 locus with nlacZ reveals expression of this myogenic factor in mature skeletal muscle fibres as well as early embryonic muscle

We have introduced the nlacZ reporter gene into the locus of the myogenic factor gene myf-5 by homologous recombination in embryonic stem (ES) cells. Targeted ES clones were injected into precompaction morula, and the beta-galactosidase expression pattern was monitored. These mice permit the sensitive visualization of myf-5 expression throughout the embryo, and provide a standard for comparing it with that seen with different myf-5/nlacZ transgenes. Thus, in a comparison using ES cells in chimaeric embryos containing the targeted or randomly integrated myf-5/nlacZ construct, we demonstrate that 5.5 kbp of myf-5 upstream flanking sequence including exon1 and most of intron1 directs some skeletal muscle expression, but this is neither qualitatively nor quantitatively equivalent to that of the endogenous gene. Myf-5 is expressed early, before terminal myogenesis takes place in the medial half of the somite, and subsequently it is a major myogenic factor as skeletal muscle forms. All skeletal muscle shows beta-galactosidase activity, even after birth, indicating that myf-5 expression is not confined to primary myotubes, which are derived from embryonic myoblasts, but is also present in muscles containing different adult fibre types. The presence of myf-5 transcripts from the endogenous gene in older muscle was confirmed by in situ hybridization. These results suggest that the myf-5 gene is not activated in only a subset of muscle cells and are consistent with the results on the MyoD knockout mice.