Differentiation of fiber types in wing muscles during embryonic development: effect of neural tube removal

Development of the trigeminal motor neurons in parrots: implications for the role of nervous tissue in the evolution of jaw muscle morphology

Vertebrates have succeeded to inhabit almost every ecological niche due in large part to the anatomical diversification of their jaw complex. As a component of the feeding apparatus, jaw muscles carry a vital role for determining the mode of feeding. Early patterning of the jaw muscles has been attributed to cranial neural crest-derived mesenchyme, however, much remains to be understood about the role of nonneural crest tissues in the evolution and diversification of jaw muscle morphology. In this study, we describe the development of trigeminal motor neurons in a parrot species with the uniquely shaped jaw muscles and compare its developmental pattern to that in the quail with the standard jaw muscles to uncover potential roles of nervous tissue in the evolution of vertebrate jaw muscles. In parrot embryogenesis, the motor axon bundles are detectable within the muscular tissue only after the basic shape of the muscular tissue has been established. This supports the view that nervous tissue does not primarily determine the spatial pattern of jaw muscles. In contrast, the trigeminal motor nucleus, which is composed of somata of neurons that innervate major jaw muscles, of parrot is more developed compared to quail, even in embryonic stage where no remarkable interspecific difference in both jaw muscle morphology and motor nerve branching pattern is recognized. Our data suggest that although nervous tissue may not have a large influence on initial patterning of jaw muscles, it may play an important role in subsequent growth and maintenance of muscular tissue and alterations in cranial nervous tissue development may underlie diversification of jaw muscle morphology.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Dual constraints on synapse formation and regression in schizophrenia: neuregulin, neuroligin, dysbindin, DISC1, MuSK and agrin

During adolescence there is a loss of approximately 30% of the synapses formed in the cortex during childhood. Comprehensive studies of the visual cortex show that this loss of synapses does not occur as a consequence of less appropriate projections being eliminated in favour of more appropriate ones. Rather it seems that synapses with low efficacy for transmission are eliminated in favour of those with higher efficacy. The loss of low-efficacy synapses is known, on theoretical grounds, to enhance the function of neural networks, but large synapse losses lead to failure of network function. In the dorsolateral prefrontal cortex (DLPC) of those suffering from schizophrenia the number of synapses is relatively very low, approximately 60% lower than that observed in normal childhood. It is not known if this is due to an additional loss over that during normal adolescence or whether it results from a failure to form a normal complement of synapses during childhood. The first study of synapse loss in the mammalian nervous system was made on the neuromuscular junction at Sydney University in 1974. Since then this junction has provided principal insights into the molecular basis of synapse formation and regression, so providing a paradigm for investigations of these phenomena in the DLPC. For example the molecules muscle-specific receptor tyrosine kinase (MuSK), agrin and neuregulin have been identified and their critical roles in the formation and maintenance of synapses elucidated. Loss of function of MuSK or agrin leads to failure of neuromuscular synapse formation as well as a loss of approximately 30% of excitatory synapses in the cortex. Similar synapse loss occurs on failure of neuregulin in vitro and of neuroligin in vivo. It is suggested that three important questions need to be answered: first, over what development period are the synapse numbers in DLPC of subjects with schizophrenia lower than normal; second, what are the relative importance of MuSK/agrin, neuregulin/ErB and neurexin/neuroligin in synapse formation and regression in the DLPC; and third, to what extent have these molecules gone awry in schizophrenia.

Hormonal control of myosin heavy chain genes during development of skeletal muscles

A pattern of myosin heavy chain (MHC) switching is a hallmark of developing muscles. Factors responsible for these changes in gene expression include endogenous signals, motoneurons and hormones, especially thyroid hormones. After perturbing the innervation and/or thyroid hormone levels we have examined the neonatal-IIb MHC transition during rat development. First, denervation does not qualitatively affect the transition at either the transcriptional or translational level. Second, hypothyroidism prevents the appearance of IIb MHC and its mRNA in the innervated limb; in the denervated hypothyroid limb IIb MHC is synthesized at moderately high levels. Third, hyperthyroidism causes a precocious increase in IIb MHC in both innervated and denervated muscles. These results suggest that the transition from neonatal to adult IIb myosin synthesis is endogenously programmed during development, but is closely orchestrated by the changing neuronal and hormonal status of the animal. Thyroid hormone may exert its influence by effects both on the muscle fibre and on the developing motoneuron. In the guinea-pig the temporalis muscle is sexually dimorphic: it contains a fast-red MHC in the female but a fast-white MHC in the male. This dimorphism has been shown to be mediated by testosterone, since the castrated male synthesizes the fast-red MHC while the testosterone-supplemented female contains the fast-white MHC. During development male and female muscles initially synthesize the fast-red isoform. The male switches to the fast-white form at puberty.

Transferrin as a muscle trophic factor

Muscle cells grow by proliferation and protein accumulation. During the initial stages of development the participation of nerves is not always required. Myoblasts and satellite cells proliferate, fusing to form myotubes which further differentiate to muscle fibers. Myotubes and muscle fibers grow by protein accumulation and fusion with other myogenic cells. Muscle fibers finally reach a quasi-steady state which is then maintained for a long period. The mechanism of maintenance is not well understood. However, it is clear that protein metabolism plays a paramount role. The role played by satellite cells in the maintenance of muscle fibers is not known. Growth and maintenance of muscle cells are under the influence of various tissues and substances. Among them are Tf and the motor nerve, the former being the main object of this review and essential for both DNA and protein synthesis. Two sources of Tf have been proposed, i.e., the motor nerve and the tissue fluid. The first proposal is that the nervous trophic influence on muscle cells is mediated by Tf which is released from the nerve terminals. In this model, the sole source of Tf which is donated to muscle cells should be the nerve, and Tf should not be provided for muscle fiber at sites other than the synaptic region; otherwise, denervation atrophy would not occur, since Tf provided from TfR located at another site would cancel the effect of denervation. The second proposal is that Tf is provided from tissue fluid. This implies that an adequate amount of Tf is transferred from serum to tissue fluid; in this case TfR may be distributed over the entire surface of the cells. The trophic effects of the motor neuron have been studied in vivo, but its effects of myoblast proliferation have not been determined. There are few experiments on its effects on myotubes. Most work has been made on muscle fibers, where innervation is absolutely required for their maintenance. Without it, muscle fibers atrophy, although they do not degenerate. In contrast, almost all the work on Tf has been performed in vitro. Its effects on myoblast proliferation and myotube growth and maintenance have been established; myotubes degenerate following Tf removal. But its effects on mature muscle fibers in vivo are not well understood. Muscle fibers possess TfR all over on their cell surface and contain a variety of Fe-binding proteins, such as myoglobin. It is entirely plausible that muscle fibers require an amount of Tf, and that this is provided by TfR scattered on the cell surface.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Expression and regulation of the retinoic acid synthetic enzyme RALDH-2 in the embryonic chicken wing

Retinaldehyde dehydrogenase type 2 (RALDH-2) is a major retinoic acid (RA) generating enzyme in the embryo. Here, we report immunolocalization of this enzyme (RALDH-2-IR) in the developing wings of stage 17-30 chicken embryos. RALDH-2-IR is located in the area of the presumptive muscle masses, although it is not colocalized with developing muscle cells. RALDH-2-IR is located in tendon precursor cells and may be present in muscular connective tissue. We show that motor neurons and blood vessels, tissues showing RALDH-2-IR as they enter the limb, are capable of synthesizing and releasing RA in culture. RALDH-2-IR in the limb mesenchyme is under the control of both the vasculature and the motor innervation; it is decreased with denervation and increased with hypervascularization. RALDH-2-IR is present in the motor neuron pool of the brachial spinal cord, but this expression pattern is apparently not under the control of limb target tissues, RA in the periphery, or somitic factors. RA is known to be a potent inducer of cellular differentiation; we propose that locally synthesized RA may be involved in aspects of wing tissue specification, including cartilage condensation and outgrowth, skeletal muscle differentiation, and recruitment of smooth muscle cells to the vasculature.

Patterning of fast and slow fibers within embryonic muscles is established independently of signals from the surrounding mesenchyme

During embryonic development, and before functional innervation, a highly stereotypic pattern of slow- and fast-contracting primary muscle fibers is established within individual muscles of the limbs, from distinct populations of myoblasts. A difference between the fiber-type pattern found within chicken and quail pectoral muscles was exploited to investigate the contributions of somite-derived myogenic precursors and lateral plate-derived mesenchymal stroma to the establishment of muscle fiber-type patterns. Chimeric chicken/quail embryos were constructed by reciprocal transplantation of somites or lateral plate mesoderm at stages prior to muscle formation. Muscle fibers derived from quail myogenic precursors that had migrated into chicken stroma showed a quail pattern of mixed fast- and slow-contracting muscle fibers. Conversely, chicken myogenic precursors that had migrated into quail stroma showed a chicken pattern of nearly exclusive fast muscle fiber formation. These results demonstrate in vivo an intrinsic commitment to fiber-type on the part of the myoblast, independent of extrinsic signals it receives from the mesenchymal stroma in which it differentiates.

Evolutionary and physiological variation in cardiac troponin C in relation to thermal strategies of fish

Striated muscle contraction is initiated when troponin C (TnC) binds Ca(2+), which activates actinomyosin ATPase. We investigated (i) the variation between cardiac TnC (cTnC) primary structure within teleost fish and (ii) the pattern of TnC expression in response to temperature acclimation. There were few differences between rainbow trout (Oncorhynchus mykiss), yellowfin tuna (Thunnus albacares), yellow perch (Perca flavescens), goldfish (Carassius auratus), white sucker (Catostomus commersoni), and icefish (Chaenocephalus aceratus) in cTnC amino acid sequence. No variation existed in the regulatory Ca(2+)-binding site (site 2). The site 3 and 4 substitutions were limited to residues not directly involved in Ca(2+) coordination. Fish cTnC primary structure was highly conserved between species (93%-98%) and collectively divergent from the highly conserved sequence seen in birds and mammals. Northern blots and polymerase chain reaction showed that thermal acclimation of trout (3 degrees, 18 degrees C) did not alter the TnC isoform pattern. While cardiac and white muscle had the expected isoforms-cTnC and fast troponin C (fTnC), respectively-red muscle unexpectedly expressed primarily ftnC. Cold acclimation did not alter myofibrillar ATPase Ca(2+) sensitivity, but maximal velocity increased by 60%. We found no evidence that TnC variants, arising between species or in response to thermal acclimation, play a major role in mitigating the effects of temperature on contractility of the adult fish heart.

Evolutionary significance of myosin heavy chain heterogeneity in birds

This article reviews the complexity, expression, genetics, regulation, function, and evolution of the avian myosin heavy chain (MyHC). The majority of pertinent studies thus far published have focussed on domestic chicken and, to a much lesser extent, Japanese quail. Where possible, information available about wild species has also been incorporated into this review. While studies of additional species might modify current interpretations, existing data suggest that some fundamental properties of myosin proteins and genes in birds are unique among higher vertebrates. We compare the characteristics of myosins in birds to those of mammals, and discuss potential molecular mechanisms and evolutionary forces that may explain how avian MyHCs acquired these properties.

The pattern of avian intramuscular nerve branching is determined by the innervating motoneuron and its level of polysialic acid

Most skeletal muscles are composed of a heterogeneous population of fast and slow muscle fibers that are selectively innervated during development by fast and slow motoneurons, respectively. It is well recognized that, in both birds and mammals, fast and slow motoneurons have substantially different intramuscular branching patterns, a difference critical for proper motor function. However, the cellular mechanisms regulating these differences in motoneuron branching are unknown. In a previous study, we showed that the fast and slow pattern of intramuscular branching, in a chick muscle containing distinct fast and slow muscle regions, was remarkably similar to normal when formed by foreign motoneurons. Whether this was attributable to some property of the innervating "fast" or "slow" motoneurons or to some property of the developing fast-slow muscle fibers was not determined. To distinguish between these two possibilities, we performed chick-quail hindlimb chimeras to force slow chick plantaris motoneurons to innervate a fast quail plantaris muscle. The pattern of intramuscular nerve branching in the fast plantaris of these chimeras closely resembled the slow branching pattern normally observed in chick slow plantaris muscles. Enzymatic removal of polysialic acid (PSA) from nerve and muscle during normal quail plantaris development dramatically changed the normal fast pattern to more closely resemble a slow pattern. In contrast, removal of PSA from chick plantaris motoneurons and muscle fibers had little effect on the pattern of nerve branching. Together, these results indicate that the pattern of intramuscular nerve branching is determined by the level of PSA on the innervating motoneurons.

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

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

Myotube heterogeneity in developing chick craniofacial skeletal muscles

Avian skeletal muscles consist of myotubes that can be categorized according to contraction and fatigue properties, which are based largely on the types of myosins and metabolic enzymes present in the cells. Most mature muscles in the head are mixed, but they display a variety of ratios and distributions of fast and slow muscle cells. We examine the development of all head muscles in chick and quail embryos, using immunohistochemical assays that distinguish between fast and slow myosin heavy chain (MyHC) isoforms. Some muscles exhibit the mature spatial organization from the onset of primary myotube differentiation (e.g., jaw adductor complex). Many other muscles undergo substantial transformation during the transition from primary to secondary myogenesis, becoming mixed after having started as exclusively slow (e.g., oculorotatory, neck muscles) or fast (e.g., mandibular depressor) myotube populations. A few muscles are comprised exclusively of fast myotubes throughout their development and in the adult (e.g., the quail quadratus and pyramidalis muscles, chick stylohyoideus muscles). Most developing quail and chick head muscles exhibit identical fiber type composition; exceptions include the genioglossal (chick: initially slow, quail: mixed), quadratus and pyramidalis (chick: mixed, quail: fast), and stylohyoid (chick: fast, quail: mixed). The great diversity of spatial and temporal scenarios during myogenesis of head muscles exceeds that observed in the limbs and trunk, and these observations, coupled with the results of precursor mapping studies, make it unlikely that a lineage based model, in which individual myoblasts are restricted to fast or slow fates, is in operation. More likely, spatiotemporal patterning of muscle fiber types is coupled with the interactions that direct the movements of muscle precursors and subsequent segregation of individual muscles from common myogenic condensations. In the head, most of these events are facilitated by connective tissue precursors derived from the neural crest. Whether these influences act upon uncommitted, or biased but not restricted, myogenic mesenchymal cells remains to be tested.

Synapse formation molecules in muscle and autonomic ganglia: the dual constraint hypothesis

In 1970 it was thought that if the motor-nerve supply to a muscle was interrupted and then allowed to regenerate into the muscle, motor-synaptic terminals most often formed presynaptic specializations at random positions over the surface of the constituent muscle fibres, so that the original spatial pattern of synapses was not restored. However, in the early 1970s a systematic series of experiments were carried out showing that if injury to muscles was avoided then either reinnervation or cross-reinnervation reconstituted the pattern of synapses on the muscle fibres according to an analysis using the combined techniques of electrophysiology, electronmicroscopy and histology on the muscles. It was thus shown that motor-synaptic terminals are uniquely restored to their original synaptic positions. This led to the concept of the synaptic site, defined as that region on a muscle fibre that contains molecules for triggering synaptic terminal formation. However, nerves in developing muscles were found to form connections at random positions on the surface of the very short muscle cells, indicating that these molecules are not generated by the muscle but imprinted by the nerves themselves; growth in length of the cells on either side of the imprint creates the mature synaptic site in the approximate middle of the muscle fibres. This process is accompanied at first by the differentiation of an excess number of terminals at the synaptic site, and then the elimination of all but one of the terminals. In the succeeding 25 years, identification of the synaptic site molecules has been a major task of molecular neurobiology. This review presents an historical account of the developments this century of the idea that synaptic-site formation molecules exist in muscle. The properties that these molecules must possess if they are to guide the differentiation and elimination of synaptic terminals is considered in the context of a quantitative model of this process termed the dual-constraint hypothesis. It is suggested that the molecules agrin, ARIA, MuSK and S-laminin have suitable properties according to the dual-constraint hypothesis to subserve this purpose. The extent to which there is evidence for similar molecules at neuronal synapses such as those in autonomic ganglia is also considered.

Selective fasciculation and divergent pathfinding decisions of embryonic chick motor axons projecting to fast and slow muscle regions

Proper motor function requires the precise matching of motoneuron and muscle fiber properties. The lack of distinguishing markers for early motoneurons has made it difficult to determine whether this matching is established by selective innervation during development or later via motoneuron-muscle fiber interactions. To examine whether chick motoneurons selectively innervate regions of their target containing either fast or slow muscle fibers, we backlabeled neurons from each of these regions with lipophilic dyes. We found that motor axons projecting to fast and slow muscle regions sorted into separate but adjacent fascicles proximally in the limb, long before they reached the muscle. More distally, these fascicles made divergent pathfinding decisions to course directly to the appropriate muscle fiber region. In contrast, axons projecting to different areas of an all-fast muscle did not fasciculate separately and became more intermingled as they coursed through the limb. Selective fasciculation of fast- and slow-projecting motoneurons was similar both before and after motoneuron cell death, suggesting that motoneurons specifically recognized and fasciculated with axons growing to muscle regions containing the appropriate muscle fiber type. Taken together, these results strongly support the hypothesis that "fast" and "slow" motoneurons are molecularly distinct before target innervation and that they use these differences to selectively fasciculate, pathfind to, and branch within the correct muscle fiber region from the outset of neuromuscular development.

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.

Determination of muscle contractile properties: the importance of the nerve

Contractile phenotype of muscle fibres is strongly influenced by hormones, stretch and influences from the motor neurones, although cell lineage probably also plays a role. Motor neurones can affect muscle fibres by releasing neurotrophic substances and by evoking electrical activity in the muscle. For regulating contractile properties such as speed, strength and endurance it has been demonstrated that electrical activity is crucial, while the role of putative neurotrophic substances remains unclear. The signal to change is coded in the pattern of electrical activity. Thus, high amounts of activity lead to slow shortening velocity and myosin heavy chains, while low amounts of activity lead to a fast phenotype. For regulation of twitch duration frequency also plays a role, and for preventing atrophy in denervated muscles high frequency seems to be beneficial, particularly in fast muscles. Little is known about the excitation-adaptation pathway linking action potentials to expression of genes that are relevant for contractile properties. Muscle specific transcription factors of the helix-loop-helix family such as myoD and myogenin could be important for regulating genes related to metabolic profile and fibre size/strength, while their role in determining myosin heavy chain expression and classical fibre type is more uncertain.

Both myoblast lineage and innervation determine fiber type and are required for expression of the slow myosin heavy chain 2 gene

Skeletal muscle fibers express members of the myosin heavy chain (MyHC) gene family in a fiber-type-specific manner. In avian skeletal muscle it is the expression of the slow MyHC isoforms that most clearly distinguishes slow- from fast-contracting fiber types. Two hypotheses have been proposed to explain fiber-type-specific expression of distinct MyHC genes during development-an intrinsic mechanism based on the formation of different myogenic lineage(s) and an extrinsic, innervation-dependent mechanism. We developed a cell culture model system in which both mechanisms were evaluated during fetal muscle development. Myoblasts isolated from prospective fast (pectoralis major) or slow (medial adductor) fetal chick muscles formed muscle fibers in cell culture, none of which expressed slow MyHC genes. By contrast, when muscle fibers formed from myoblasts derived from the slow muscle were cocultured with neural tube, the muscle fibers expressed a slow MyHC gene, while muscle fibers formed from myoblasts of fast muscle origin continued to express only fast MyHC. Motor endplates formed on the fibers derived from myoblasts of both fast and slow muscle origin in cocultures, and slow MyHC gene expression did not occur when neuromuscular transmission or depolarization was blocked. We have cloned the slow MyHC gene that is expressed in response to innervation and identified it as the slow MyHC 2 gene, the predominant adult slow isoform. cDNAs encoding portions of the three slow myosin heavy chain genes (MyHC1, slow MyHC 2, and slow MyHC 3) were isolated. Only slow MyHC 2 mRNA was demonstrated to be abundant in the cocultures of neural tube and muscle fibers derived from myoblasts of slow muscle origin. Thus, expression of the slow MyHC 2 gene in this in vitro system indicates that formation of slow muscle fiber types is dependent on both myoblast lineage (intrinsic mechanisms) and innervation (extrinsic mechanisms), and suggests neither mechanism alone is sufficient to explain formation of muscle fibers of different types during fetal development.

Selective innervation of fast and slow muscle regions during early chick neuromuscular development

The electrical properties of adult motoneurons are well matched to the contractile properties of the fast or slow muscle fibers that they innervate. How this precise matching occurs developmentally is not known. To investigate whether motoneurons exhibit selectivity in innervating discrete muscle regions, containing either fast or slow muscle fibers during early neuromuscular development, we caused embryonic chick hindlimb muscles to become innervated by segmentally inappropriate motoneurons. We used the in vitro spinal cord-hindlimb preparation to identify electrophysiologically the pools of foreign motoneurons innervating the posterior iliotibialis (pITIB), an all-fast muscle, and the iliofibularis (IFIB), a partitioned muscle containing discrete fast and slow regions. The results showed that the pITIB and the fast region of the IFIB were exclusively innervated by motoneurons that normally supply fast muscles. In contrast, the slow region of the IFIB was always innervated by motoneuron pools that normally supply slow muscles. Some experimental IFIB muscles lacked a fast region and were innervated solely by "slow" motoneurons. In addition, the intramuscular nerve branching patterns were always appropriate to the fast-slow nature of the muscle (region) innervated. The selective innervation was found early in the motoneuron death period, and we found no evidence that motoneurons grew into appropriate muscle regions, but failed to form functional contacts. Together, these results support the hypothesis that different classes of motoneurons exhibit molecular differences that allow them to project selectively to, and innervate, muscle fibers of the appropriate type during early neuromuscular development.

Innervation regulates myosin heavy chain isoform expression in developing skeletal muscle fibers

The influence of innervation on primary and secondary myogenesis and its relation to fiber type diversity were investigated in two specific wing muscles of quail embryo, the posterior (PLD) and anterior latissimus dorsi (ALD). In the adult, these muscles are composed almost exclusively of pure populations of fast and slow fibers, respectively. When slow ALD and fast PLD muscles developed in ovo in an aneurogenic environment induced after neural tube ablation, the cardiac ventricular myosin heavy chain (MHC) isoform was not expressed. The adult slow MHC isoform, SM2, appeared by embryonic day 7 (ED 7) in normal innervated slow ALD but was not expressed in denervated muscle. Analysis of in vitro differentiation of myoblasts from fast PLD and slow ALD muscles isolated from ED 7 control and neuralectomized quail embryos showed no fundamental differences in the pattern of MHC isoform expression. Newly differentiated fibers accumulated cardiac ventricular, embryonic fast, slow SM1 and SM3 MHC isoforms. Nevertheless, the expression of slow SM2 isoform in myotubes formed from slow ALD myoblasts only occurred when myoblasts were cultured in the presence of embryonic spinal cord. Our studies demonstrate that the neural tube influences primary as well as secondary myotube differentiation in avian forelimb and facilitates the expression of different MHC, particularly slow SM2 MHC gene expression in slow myoblasts.

Neural tube can induce fast myosin heavy chain isoform expression during embryonic development

We investigated the role of the neural tube in muscle cell differentiation in developing somitic myotome of chick embryo, particularly through fast myosin heavy chain (MHC) isoform expression. An embryonic fast MHC labeled with EB165 mAb was expressed in somitic cells from stage 15 of Hamburger and Hamilton (H.H.) (24 somites). Moreover, a distinct early embryonic fast MHC was expressed only from stage 15 of H.H. to stage 36 (E10). Like neonatal MHC, this isoform was labeled with 2E9 mAb but differed in its immunopeptide mapping. Expression of EB165-labeled embryonic fast MHC occurred in somitic myotomes deprived of neural tube influence by in ovo ablation as well as in somite explants cultured alone in vitro. Conversely, ablation of the neural tube prevented somitic expression of MHC labeled with 2E9 mAb. The neural tube induced in vitro expression of this MHC in explants of somites which failed to express it when cultured alone. These results indicate that signals emanating from the neural tube are required for the expression of early embryonic fast MHC isoform in developing somitic myotome.

Sixth Annual Stuart Reiner Memorial Lecture: embryonic development of nerve and muscle

This article provides a basic scheme of sequential anatomic and some physiologic events occurring during the course of embryonic development of motor neurons and muscles, leading to the establishment of mature nerve-muscle relationships. Motor neurons and muscles begin their development independently and during embryogenesis they become dependent on each other for further development and survival. Aspects of development which occur independently and those requiring mutual interactions are identified. The development of motor neurons is discussed with respect to their production, projection, neuromuscular transmission, myelination, sprouting, survival, and death. The development of muscles is discussed with respect to the origin, differentiation, and muscle fiber types. Discussion on the development of neuromuscular junction includes differentiation of presynaptic nerve terminal, postsynaptic components, and elimination of multiple axons.

Localisation of transforming growth factor beta 1 in developing muscles: implications for connective tissue and fiber type pattern formation

Skeletal muscles are highly ordered mixtures of cell types, with each muscle having its own characteristic pattern of fiber types, connective tissues, and vasculature. The precursors of the myogenic and connective elements of a muscle are initially intermixed and are proliferating and differentiating together in a manner that generates an ordered array of mature cells. The molecular basis of myogenesis is unknown, although in vitro studies have revealed numerous putative regulators. The results obtained from in vitro studies are not easily related to in vivo myogenesis because of a lack of information about the localisation of the putative regulators in developing muscles. The objective of this paper was therefore to describe the spatial and temporal distribution of transforming growth factor beta 1 (TGF-beta 1), a small peptide that affects cultured fibroblasts, myoblasts, and vascular endothelial cells. TGF-beta 1-immunoreactivity was associated with the epimysia, perimysia, and vasculature of the developing muscles. The expression of TGF-beta 1 within developing muscles had a distinct spatial and temporal pattern that correlated with the fate of adjacent myotubes. Myotubes which formed prior to the expression of TGF-beta 1 developed into slow fibers whereas those which formed adjacent to TGF-beta 1-containing connective tissue matured into fast fibers. The possibility that TGF-beta 1 is involved in the generation of the pattern of epi- and perimysia and/or fiber types is discussed.

Activity-dependent interactions between motoneurones and muscles: their role in the development of the motor unit

In this review article we have attempted to provide an overview of the various forms of activity-dependent interactions between motoneurones and muscles and its consequences for the development of the motor unit. During early development the components of the motor unit undergo profound changes. Initially the two cell types develop independently of each other. The mechanisms that regulate their characteristic properties and prepare them for their encounter are poorly understood. However, when motor axons reach their target muscles the interaction between these cells profoundly affects their survival and further development. The earliest interactions between motoneurones and muscle fibres generate a form of activity which is in many ways different from that seen at later stages. This difference may be due to the immature types of ion channels and neurotransmitter receptors present in the membranes of both motoneurones and muscle fibres. For example, spontaneous release of acetylcholine may influence the myotube even before any synaptic specialization appears. This initial form of activity-dependent interaction does not necessarily depend on the generation of action potentials in either the motoneurone or the muscle fibre. Nevertheless, the ionic fluxes and electric fields produced by such interactions are likely to activate second messenger systems and influence the cells. An important step for the development of the motor unit in its final form is the initial distribution of synaptic contacts to primary and secondary myotubes and their later reorganization. Mechanisms that determine these events are proposed. It is argued that the initial layout of the motor unit territory depends on the matching of immature muscle fibres (possibly secondary myotubes) to terminals with relatively weak synaptic strength. Such matching can be the consequence of the properties of the muscle fibre at a particular stage of maturation which will accept only nerve terminals that match their developmental stage. Refinements of the motor unit territory follows later. It is achieved by activity-dependent elimination of nerve terminals from endplates that are innervated by more than one motoneurone. In this way the territory of the motor unit is established, but not necessarily the homogeneity of the physiological and biochemical properties of its muscle fibres. These properties develop gradually, largely as a consequence of the activity pattern that is imposed upon the muscle fibres supplied by a given motoneurone. This occurs when the motor system in the CNS completes its development so that specialized activity patterns are transmitted by particular motoneurones to the muscle fibres they supply.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for myoblast-extrinsic regulation of slow myosin heavy chain expression during muscle fiber formation in embryonic development

Vertebrate muscles are composed of an array of diverse fast and slow fiber types with different contractile properties. Differences among fibers in fast and slow MyHC expression could be due to extrinsic factors that act on the differentiated myofibers. Alternatively, the mononucleate myoblasts that fuse to form multinucleated muscle fibers could differ intrinsically due to lineage. To distinguish between these possibilities, we determined whether the changes in proportion of slow fibers were attributable to inherent differences in myoblasts. The proportion of fibers expressing slow myosin heavy chain (MyHC) was found to change markedly with time during embryonic and fetal human limb development. During the first trimester, a maximum of 75% of fibers expressed slow MyHC. Thereafter, new fibers formed which did not express this MyHC, so that the proportion of fibers expressing slow MyHC dropped to approximately 3% of the total by midgestation. Several weeks later, a subset of the new fibers began to express slow MyHC and from week 30 of gestation through adulthood, approximately 50% of fibers were slow. However, each myoblast clone (n = 2,119) derived from muscle tissues at six stages of human development (weeks 7, 9, 16, and 22 of gestation, 2 mo after birth and adult) expressed slow MyHC upon differentiation. We conclude from these results that the control of slow MyHC expression in vivo during muscle fiber formation in embryonic development is largely extrinsic to the myoblast. By contrast, human myoblast clones from the same samples differed in their expression of embryonic and neonatal MyHCs, in agreement with studies in other species, and this difference was shown to be stably heritable. Even after 25 population doublings in tissue culture, embryonic stage myoblasts did not give rise to myoblasts capable of expressing MyHCs typical of neonatal stages, indicating that stage-specific differences are not under the control of a division dependent mechanism, or intrinsic "clock." Taken together, these results suggest that, unlike embryonic and neonatal MyHCs, the expression of slow MyHC in vivo at different developmental stages during gestation is not the result of commitment to a distinct myoblast lineage, but is largely determined by the environment.

The neural tube/notochord complex is necessary for vertebral but not limb and body wall striated muscle differentiation

The aim of this work was to investigate the role played by the axial organs, neural tube and notochord, on the differentiation of muscle cells from the somites in the avian embryo. Two of us have previously shown that neuralectomy and notochordectomy is followed by necrosis of the somites and consecutive absence of vertebrae and of most muscle cells derived from the myotomes while the limbs develop normally with muscles. Here we have focused our attention on muscle cell differentiation by using the 13F4 mAb that recognizes a cytoplasmic antigen specific of all types of muscle cells. We show that differentiation of muscle cells of myotomes can occur in the absence of notochord and neural tube provided that the somites from which they are derived have been in contact with the axial organs for a defined period of time, about 10 hours for the first somites formed at the cervical level, a duration that progressively reduces caudalward (i.e. for thoracic and lumbar somites). Either one or the other of the two axial organs, the neural tube or the notochord can prevent somitic cell death and fulfill the requirements for myotomal muscle cell differentiation. Separation of the neural tube/notochord complex from the somites by a surgical slit on one side of the embryo gave the same results as extirpation of these organs and provided a perfect control on the non-operated side. A striking finding was that limb and body wall muscles, although derived from the somites, differentiated in the absence of the axial organs. However, limb muscles that develop after excision of the neural tube started to degenerate from E10 onward due to lack of innervation. In vitro explantation of somites from different axial levels confirmed and defined precisely the chronology of muscle cell commitment in the myotomes as revealed by the in vivo experiments.

Ablation of the fetal mouse spinal cord: the effect on soleus muscle cytoarchitecture

A technique is described whereby it is possible to surgically ablate the lumbosacral spinal cord of a developing mouse fetus without interfering with fetal viability. The lumbosacral spinal cords of 14-day in utero, 129ReJ mice were ablated with a Cooper Nd-YAG laser, and the fetuses, enclosed in their membranes and attached to the uterus by their placentae, were allowed to develop in the abdominal cavity of the dam. The cytoarchitecture and the temporal pattern of organogenesis of aneural soleus muscles were studied in spaced, serial, transverse, ultrathin sections of muscles of 16- and 18-day gestation and newborn (20-day gestation) mice. At the time of surgery, the soleus muscle was a discrete mass consisting of primary myotubes and a pleomorphic population of mononucleated cells. Axon bundles and blood vessels were found at the muscle's periphery, but had not penetrated throughout the muscle mass. The organogenesis of the aneural muscle was remarkably similar to that of the innervated soleus muscle (Ontell et al., Am J Anat 181:267-278, 1988). In the aneural muscle, as in the innervated muscle, significant numbers of secondary myotubes formed all along the lengths of primary myotubes. Moreover, the time course of myotube formation, the dynamics of cluster formation and cluster dispersal, and the ultrastructural appearance of the myotubes mimicked that observed in innervated muscle. The frequency of necrotic myotubes was no greater in the aneural muscle than in the innervated soleus muscle. Myotube maturation was similar in aneural and innervated soleus muscles until 18 days gestation. However, at birth, aneural myotubes appeared to be slightly less mature than innervated myotubes. Thus, the major morphogenic phenomena that characterize the development of the soleus muscle appear to be independent of innervation.

Morphometric analysis of the developing, murine aneural soleus muscle

The pattern of organogenesis of the aneural soleus muscle of the 129ReJ mouse [rendered aneural by laser ablation of the lumbosacral spinal cord at 14 days in utero (during the period of primary myotube formation, but prior to the formation of secondary myotubes)] was evaluated quantitatively with spaced, serial ultrathin sections and computer-assisted morphometric analysis. Aneural muscles from 16- and 18-day gestation and newborn mice were analyzed to determine age-related changes in a number of parameters including: muscles' maximal girths, numbers of myotubes, myotube diameter distributions, and cluster frequency. Data were compared with a similar study of the organogenesis of the normal soleus muscle (Ontell et al: Am J Anat 181:279-288, 1988). Basic patterns of morphogenesis of the soleus muscle were unchanged by spinal cord ablation, and differences in development between the aneural and innervated muscles were quantitative rather than qualitative. At birth, the aneural muscle contained approximately 76% of the myotubes found in the innervated muscle (approximately 840 myotubes in the innervated muscle and approximately 640 in the aneural muscle). Evidence is presented consistent with the hypothesis that primary myotube formation is reduced by approximately 32% in the aneural muscles and that while extensive secondary myotube formation occurs (approximately 78% of the myotube present at birth in these muscles are secondary myotubes), there is a significant reduction in the number of secondary myotubes in aneural muscles. It is suggested that the reduced numbers of secondary myotubes may be related to the reduction in the number of primary myotubes, which are known to act as scaffolds for secondary myotube formation. The time course of secondary myotube formation and of cluster formation and cluster dispersal and the number of cells per cluster are similar in age-matched, innervated and aneural muscles. The absence of innervation has little effect on myotube growth until birth, when comparison of the myotube diameter distributions reveals a slight alteration in myotube diameter distributions of aneural as compared with innervated muscles.

Basal lamina and superfast myosin expression in regenerating cat jaw muscle

We investigated the possible role of extracellular matrix in specifying the expression of superfast myosin during cat jaw muscle regeneration. Equal proportions of muscle tissue from jaw and limb were minced together after killing cellular elements from one source. We allowed the mince to regenerate in the bed of a fast limb muscle. Regenerates were analyzed immunocytochemically at 71 to 294 days after operation. Fibers in control regenerates containing live cells from both sources expressed fast, superfast or slow myosins, or a mixture of these myosins. In regenerates containing only one type of live cells, we detected only myosins appropriate to the live cells. Our results suggest that during regeneration the original extracellular matrix of jaw-closing or limb muscle is unable to specify the expression of superfast or fast myosins, respectively; they point to the cellular elements, probably the satellite cells, as determinants of muscle specificity during regeneration.

Organization of motor units in the axolotl: a continuously growing animal

The characteristics of motor units in the iliotibialis posterior muscle of the axolotl hindlimb are described. Tension recording and intracellular electrophysiological methods demonstrate that the physiological properties of the population of motor units are continuously distributed rather than grouped into a series of discrete types. Overlap between motor units occurs and this is positively correlated with motor unit size but negatively correlated with differences in time to peak tension. Immunocytochemical staining with antimyosin antibodies combined with histochemical demonstration of actomyosin ATPase activity revealed at least four types of muscle fibre which were distributed asymmetrically within iliotibialis posterior. The results are discussed in terms of the continuous growth of the muscle and the interactions between muscle and nerve in the formation of the axolotl motor system.

Motoneuron and muscle fibre counts in normal and bilaterally innervated Xenopus hindlimbs

Motoneurons of the Lumbar Lateral Motor Column (LMC) and muscle fibres of gastrocnemius and tibialis anterior were counted in juvenile Xenopus frogs, including normal animals and those reared with a single bilaterally innervated hindlimb (monopodal frogs). In many monopodal frogs, the single hind limb becomes hyperinnervated by a large number of motoneurons on the contralateral side in addition to the normal ipsilateral number, even after the completion of cell death. In frogs with hyper-innervated limbs, muscle fibre number was evaluated beyond that expected for a normal population, but this increase was not commensurate with the quantity of extra innervation. When taken together with previous findings which showed that the supporting capacity of individual fibres was not elevated, the conclusion is that the ability of the single limb to support extra motoneurons cannot be completely explained by a commensurate increased proliferation of muscle fibres resulting from the operation or the bilateral innervation. The results give further evidence against the hypothesis that motoneuron numbers are controlled solely by peripheral competition. The study also provides evidence that muscle fibre numbers are regulated in part by the quantity of motor innervation received by the limb.

A reevaluation of the role of innervation in primary and secondary myogenesis in developing chick muscle

The neural dependence of primary and secondary myogenesis and its relation to fiber-type differentiation was immunocytochemically investigated in chicken limb muscles. In a previous study, we demonstrated that a novel combination of slow myosin and fast Ca2(+)-ATPase antibodies differentially stained mutually exclusive populations of myotubes, which in the slow region of the iliofibularis allowed us to visualize primary and secondary myotubes and to quantify their development. When these antibodies were used to stain myotubes in muscles that were either chronically paralyzed by d-tubocurarine or denervated, we were surprised to observe by both LM and EM analysis that secondary myotubes formed in both cases, in contrast to the widely held tenet that nerve activity is necessary for secondary myogenesis. Also, an unexpected decrease in the number of primary myotubes occurred before the onset of secondary myotube formation. Although the total quantity of myotubes formed was drastically reduced by curare treatment or denervation, the ratio of fast to slow myotubes increased normally between st 34 and 39 1/2. Paralysis by curare did produce a striking increase in the size of individual myotube clusters, indicating that blocking nerve activity either increases adhesion between myotubes or prevents a normal decrease in adhesion during development which may be necessary for myofiber separation from clusters. Our findings indicate that both slow primary and fast secondary myotube populations are composed of nerve-dependent and independent individuals and that the relative quantities of fast and slow myotubes are regulated independent of innervation.

Relationship of primary and secondary myogenesis to fiber type development in embryonic chick muscle

The formation of fast and slow myotubes was investigated in embryonic chick muscle during primary and secondary myogenesis by immunocytochemistry for myosin heavy chain and Ca2(+)-ATPase. When antibodies to fast or slow isoforms of these two molecules were used to visualize myotubes in the posterior iliotibialis and iliofibularis muscles, one of the isoforms was observed in all primary and secondary myotubes until very late in development. In the case of myosin, the fast antibody stained virtually all myotubes until after stage 40, when fast myosin expression was lost in the slow myotubes of the iliofibularis. In the case of Ca2(+)-ATPase, the slow antibody also stained all myotubes until after stage 40, when staining was lost in secondary myotubes and in the fast primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis. In contrast, the antibodies against slow muscle myosin heavy chain and fast muscle Ca2(+)-ATPase stained mutually exclusive populations of myotubes at all developmental stages investigated. During primary myogenesis, fast Ca2(+)-ATPase staining was restricted to the primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis, whereas slow myosin heavy chain staining was confined to all of the primary myotubes of the slow region of the iliofibularis. During secondary myogenesis, the fast Ca2(+)-ATPase antibody stained nearly all secondary myotubes, while primaries in the slow region of the iliofibularis remained negative. Thus, in the slow region of the iliofibularis muscle, these two antibodies could be used in combination to distinguish primary and secondary myotubes. EM analysis of staining with the fast Ca2(+)-ATPase antibody confirmed that it recognizes only secondary myotubes in this region. This study establishes that antibodies to slow myosin heavy chain and fast Ca2(+)-ATPase are suitable markers for selective labeling of primary and secondary myotubes in the iliofibularis; these markers are used in the following article to describe and quantify the effects that chronic blockade of neuromuscular activity or denervation has on these populations of myotubes.

Junctional acetylcholine receptor channel open time is not presynaptically regulated in developing muscle

The role of motor innervation in controlling the development of acetylcholine receptor (AChR) channel open time was tested by examining synaptic current durations in transplanted muscles of Xenopus tadpoles. The presumptive lower jaw region, which gives rise to the interhyoideus muscle, was transplanted to the tail, overlying the myotomal muscle cells. The transplanted muscles became innervated, presumably by spinal nerves which normally innervate myotomal muscle. Despite development in the presence of foreign innervation, synaptic currents in the transplanted interhyoideus were predominantly long in duration and resembled those in the normally innervated interhyoideus. They did not resemble those in the myotomal muscle, where synaptic currents are brief. The apparent lack of neural influence on development of AChR function in muscle contrasts with the evidence for presynaptic control of AChR open time in frog sympathetic ganglia. This may reflect a fundamental difference between nerve and muscle in the regulation of postsynaptic function.

Differentiation of fiber types in aneural musculature of the prenatal rat hindlimb

The presynaptic neurotoxin, beta-bungarotoxin, was injected into rat fetuses in utero to destroy the innervation of their hindlimb muscles. These injections were made prior to the invasion of motor axons into the muscles and, in some cases, prior to the cleavage of individual muscles. Examination of the lateral motor column of the spinal cord showed a dramatic reduction (greater than 95%) in the number of motoneuron cell bodies. Staining of sections of the hindlimb with silver and with antibodies to neurofilament proteins and to a synaptic vesicle protein indicated that the muscles were aneural. Anti-myosin antibodies applied to sections of the hindlimb revealed that these aneural muscles by the 20th day of gestation had the same types of fibers as were present in normal muscles of the same age. Moreover, fiber types in most muscles showed their characteristic intramuscular distributions. These findings suggest that fiber types can differentiate in the absence of the nervous system. However, some fibers achieved their ultimate fiber type fate without passing through the normal sequence of myosin expressions. Moreover, some slow fibers lost their slow expression, suggesting that the maintenance of the slow differentiation may require innervation. Muscle growth was dramatically affected by the absence of motoneurons; some muscles were decreased in size and others disappeared completely. In muscles which had not degenerated by the time secondary myogenesis normally begins, secondary muscle fibers were generated indicating that the genesis of these fibers is not strictly nerve dependent. Because fiber types differentiate independently of the nervous system, this study suggests that motoneurons selectively innervate fiber types during normal development.

Development and survival of thoracic motoneurons and hindlimb musculature following transplantation of the thoracic neural tube to the lumbar region in the chick embryo: anatomical aspects

Thoracic spinal cord transplanted to the lumbar region at the time of neural tube closure in the chick embryo survives and initially differentiates normally similar to in situ thoracic cord. Normal numbers of motoneurons are produced that innervate the host hindlimb musculature. In control thoracic cord approximately 70% of the motoneurons are lost by normal cell death between embryonic day (E) 6 and E11-E12. By contrast, the transplanted thoracic cord loses only about 30% of the motoneurons during this period. Transplantation of one hindlimb to the thoracic region also reduces the normal loss of in situ thoracic motoneurons. We conclude that some factor(s) associated with the increased target size provided by the hindlimbs promotes the survival of thoracic motoneurons. In contrast, by E16-E18 motoneuron numbers in the thoracic transplants decrease to below control levels. Dorsal root ganglion cells in the transplant were also initially increased (on E8) but later decreased to below control values. Hindlimb muscles innervated by thoracic motoneurons in the transplant also differentiated normally up to E10 to E12. Myotube size and numbers, muscle size and myotube types (fast versus slow) all developed normally in several thoracically-innervated hindlimb muscles. However, beginning on E14 myotube numbers and muscle size were markedly decreased resulting in muscle atrophy. Injections of horseradish peroxidase (HRP) into the thoracic transplants labelled neurons in the host spinal cord and brainstem rostral to the transplant thereby indicating an anatomical continuity between host and transplant neural tube. Injections of HRP into specific thoracically innervated hindlimb muscles on E8 labelled distinct pools of motoneurons in the transplants.(ABSTRACT TRUNCATED AT 250 WORDS)

A hierarchy of determining factors controls motoneuron innervation. Experimental studies on the development of the plantaris muscle (PL) in avian chimeras

Quail leg buds were grafted in place of chick leg buds or chick wing buds and vice versa at stages 18 to 21 after colonization by muscle precursor cells had been completed. Motor endplate pattern in the plantaris muscle of the grafts was analyzed before hatching by means of esterase and acetylcholinesterase staining techniques. Muscle fibre types were made visual using the myosin ATPase reaction. Investigations are based on the species-specific endplate pattern of the plantaris muscle: multiply innervated fibres in the chick and focally innervated fibres in the quail. Muscle pieces isolated from the adjacent medial gastrocnemius muscle of the grafted legs were histologically examined to judge their species-specific composition. Horseradish peroxidase was injected into the plantaris muscles of both the grafted and the opposite leg as well as in the plantaris muscle of normal quail embryos, in order to be sure that the plantaris muscle of the grafts is innervated by appropriate motoneurons. This procedural design offers for the first time a possibility to test experimentally the influences of motoneurons on endplate pattern formation under conditions corresponding to those in normal ontogenesis. It is shown that such appropriate motoneurons of one species which project to the plantaris muscle of the other species dictate the endplate pattern. When the plantaris muscle is innervated by inappropriate motoneurons, the endplate pattern inherent in the muscle primordium itself becomes realized. A sequence of hierarchically acting factors is proposed to bring different results in line. According to this, the neuronally set programme has priority compared with that set in the muscle. This is true for the normal development and might generate the high neuro-muscular specificity. If under experimental conditions the neuronal programme and the peripheral programme differ, the axons and muscle fibres selectively interact with respect to their inherent characteristics and the muscle-specific programme becomes expressed. If there is a lack of a certain axon type, muscle fibres might become innervated by non-corresponding motoneurons which alter the muscle fibre type.

Evidence for expression of a common myosin heavy chain phenotype in future fast and slow skeletal muscle during initial stages of avian embryogenesis

We have utilized a key biochemical determinant of muscle fiber type, myosin isoform expression, to investigate the initial developmental program of future fast and slow skeletal muscle fibers. We examined myosin heavy chain (HC) phenotype from the onset of myogenesis in the limb bud muscle masses of the chick embryo through the differentiation of individual fast and slow muscle masses, as well as in newly formed myotubes generated in adult muscle by weight overload. Myosin HC isoform expression was analyzed by immunofluorescence localization with a battery of anti-myosin antibodies and by electrophoretic separation with SDS-PAGE. Results showed that the initial myosin phenotype in all skeletal muscle cells formed during the embryonic period (until at least 8 days in ovo) consisted of expression of a myosin HC which shares antigenic and electrophoretic migratory properties with ventricular myosin and a distinct myosin HC which shares antigenic and electrophoretic migratory properties with fast skeletal isomyosin. Similar results were observed in newly formed myotubes in adult muscle. Future fast and slow muscle fibers could only be discriminated from each other in developing limb bud muscles by the onset of expression of slow skeletal myosin HC at 6 days in ovo. Slow skeletal myosin HC was expressed only in myotubes which became slow fibers. These findings suggest that the initial commitment of skeletal muscle progenitor cells is to a common skeletal muscle lineage and that commitment to a fiber-specific lineage may not occur until after localization of myogenic cells in appropriate premuscle masses. Thus, the process of localization, or events which occur soon thereafter, may be involved in determining fiber type.

The distribution of intracellular acetylcholine receptors and nuclei in developing avian fast-twitch muscle fibres during synapse elimination

The spatial distribution of intracellular acetylcholine receptors along the length of fibres from the avian posterior latissimus dorsi muscle has been investigated during embryonic development, when distributed synaptic sites are eliminated from the muscle fibres. Cell surface AChR were irreversibly blocked with unlabelled alpha-bungarotoxin (alpha-BGT). Muscles were then fixed and ultrasonically dissociated into fibre fragments, treated with 0.5% saponin and stained with 125I-alpha-BGT. This revealed an intracellular pool of curare sensitive binding sites equivalent to about 10% of total cell AChR. The spatial distribution of this pool was studied by autoradiography. Large (longer than 2 microns) AChR-clusters (AChR-C) characteristic of neuromuscular contacts were localized on the same fibres by immunofluorescence with an anti-AChR antibody. At E11, relatively high levels of intracellular AChR were observed throughout the length of fibres. Between E11 and E18 intracellular AChR declined (19 fold) in extrajunctional parts of fibres but remained high in segments of fibre corresponding to AChR-clusters. Treatment of E14 embryos with an inhibitor of protein synthesis (cycloheximide) reduced intracellular AChR to 22 +/- 6% (mean +/- SE) of control levels, suggesting that most of the intracellular binding represented newly-synthesized AChR. Between E11 and E18 cell nuclei were found to accumulate beneath AChR-C. The mean density of nuclei in segments of fibre corresponding to AChR-C increased 5 fold between E11 and E18, but remained unchanged in extrajunctional segments. It is suggested that the elimination of excess distributed AChR-C may be due to the preferential accumulation of nuclei at a single AChR-C on each fibre accompanied by the down regulation of AChR synthesis associated with nuclei at the remaining AChR-C.

Correlation of development stage and gap junction formation between chick embryo neurons and cloned skeletal muscle myoblasts

The frequency of gap junction formation between neurons and myoblasts of chick embryo leg skeletal muscle changes as a function of the developmental stage of the muscle. Cells from leg muscle of various ages and states of innervation were first cloned in vitro and then co-cultured with ciliary ganglion, spinal cord, or dorsal root ganglion neurons. The presence of gap junctions between cells was identified by the passage of fluorescent dyes or electric currents from one cell to another. The clones examined were fusing muscle clones (myoblasts) as well as nonfusing clones. Mononuclear cells of either kind of clone derived from legs younger than stage 24 (E4) dye-couple with other clone cells but do not dye-couple with neurons. Myoblasts of fusing muscle clones derived from stage 24 through stage 29 (E5) legs dye-couple with neurons at high frequency; nonfusing clones from these same embryos do not contain cells that dye-couple with neurons. Mononuclear cells of both fusing and nonfusing clones from normally innervated stage 30 (E6) through 38 (E12) legs do not dye-couple with neurons at significant frequencies. Additionally, aneural legs of denervated E10-E12 embryos yield muscle clones in which the myoblasts again dye-couple with neurons at high frequency. The ability to form communicating junctions between muscle cells and neurons in culture is restricted to myoblasts cloned from legs of those stages of development and conditions of innervation that define and limit neuron-dependent alteration of myoblast populations in vivo.

Lack of fiber type selectivity during reinnervation of neonatal rabbit soleus muscle

Fast and slow contracting fibers in neonatal mammalian skeletal muscle are each innervated in a highly specific manner by motor neurons of the corresponding type, even at an age when polyinnervation is widespread. Chemospecific recognition is a possible mechanism by which this pattern of innervation could be established. We have investigated this possibility by studying the degree of specificity during reinnervation of rabbit soleus muscle following nerve crush on Postnatal Day 1 or 4. We assayed fiber type composition by measuring the twitch rise times of motor units within 2 days of the onset of functional reinnervation (5-6 days after nerve crush). In contrast to the broad, bimodal distribution of single motor unit twitch rise times seen in normal muscles, motor units in reinnervated muscles yielded a narrower, unimodal distribution of rise times. Rise times of reinnervated units were intermediate to those of normal fast and slow units, suggesting that reinnervated units were composed of a mixture of fast and slow contracting fibers. An alternative possibility, that specific reinnervation was masked by contractile dedifferentiation of muscle fibers, was examined by maintaining a transmission blockade induced by botulinum toxin poisoning for an equivalent interval. Twitch rise times of treated motor units exhibited the distinctly bimodal distribution characteristic of normal muscles, suggesting that muscle fibers can retain contractile diversity during a transient period of denervation. We carried out computer simulations to estimate the amount of rise time diversity induced by varying degrees of specificity during reinnervation. Based on this analysis, we conclude that there is little if any selective reinnervation of muscle fiber types at the ages studied.

The development of topographical maps and fibre types in toad (Bufo marinus) glutaeus muscle during synapse elimination

1. The toad glutaeus muscle consists of two muscle compartments. A study has been made of the topographical distribution of motor units in these compartments, in relation to the fibre types which arise during different stages of development. 2. Monoclonal antibodies to myosin allowed the distribution of fibre types to be determined. In mature muscles (from toads of greater than 30 g body weight) clusters of type 5 (tonic) fibres were found exclusively at the dorsal surface of the muscle, surrounded by a layer of type 3 (slow-twitch) fibres. A homogeneous layer of type 2 (fast-twitch red) fibres was found beneath this dorsal rind of slow and tonic fibres. The rest of the muscle, including the ventral surface, consisted of a mosaic of type 1 (fast-twitch white) and type 2 fibres. 3. Glycogen-depletion methods, together with the myosin antibodies, allowed the distribution of single motor units and their fibre types to be determined. In mature muscles, axons originating from rostral spinal cord possessed muscle units located in a band extending from the ventral surface to beyond the middle of the muscle; these units consisted of 78% type 1 and 22% type 2 fibres found amongst the mosaic of type 1 and type 2 fibres. Intermediate axons possessed muscle units located primarily in the middle and dorsal half of the muscle. These units consisted mostly of type 2 fibres (29% type 1, 71% type 2) also found amongst the mosaic of type 1 and type 2 fibres. Thus rostral and intermediate units were of mixed fibre type, with type 1 fibres predominating in the former units and type 2 in the latter. Caudal axons possessed muscle units located mostly in the homogeneous layer of type 2 fibres, beneath the dorsal rind of tonic fibres; these units were almost always composed entirely of type 2 fibres. 4. The distribution of single motor units and their fibre types were determined for the caudal axons during development. In juvenile animals (toads of about 10 g body weight) the dorsal rind of tonic and slow fibres, together with the underlying homogeneous layer of type 2 fibres, were still present, but the rest of the muscle to the ventral surface consisted almost entirely of type 1 fibres. Caudal axons innervated the type 2 fibre layer at the dorsal surface as they do in mature animals. 5. The glutaeus in post-metamorphic toads (0.15 g body weight) had only a small number of tonic and slow-twitch fibres in the very dorsal layer of cells; the muscle was largely type 1.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of the troponin complex genes: transcriptional coactivation during myoblast differentiation and independent control in heart and skeletal muscles

We compared the developmental regulation of the three troponin genes that encode the proteins of the Ca2+ regulatory complex in striated muscles of the Japanese quail. Nuclear run-on transcription and RNA protection analyses showed that the fast skeletal troponin I, the fast skeletal troponin T, and the slow skeletal-cardiac troponin C genes were transcriptionally coactivated and that transcripts rapidly accumulated within 6 to 12 h after the initiation of myoblast differentiation. The fast-isoform mRNAs of troponin I and troponin T were coexpressed at similar levels in different skeletal muscles, whereas the slow-cardiac troponin C mRNA varied independently and was the only one of these genes expressed in embryonic and adult heart. We conclude that these troponin genes are transcriptionally coactivated during skeletal myoblast differentiation, indicating that their transcription is under precise temporal control. However, this troponin C gene is regulated independently is specialized striated muscles.

Changes in myosin and C-protein isoforms proceed independently of the conversion to singly innervated neuromuscular junctions in developing pectoral muscle

Changes in contractile protein expression during myogenesis are usually categorized as developmentally programmed or neuronally dependent. Studies on aneurogenic chick embryos indicated that the neuronally dependent phase begins at about Embryonic Day 15, immediately prior to the fetal transition in myosin and C-protein expression. The prime candidate for the neuronal event that induces the fetal transition is the conversion to the adult form of singly innervated neuromuscular junctions (NMJs), which occurs contemporaneously with the fetal transition. Using curare to inhibit the conversion to focal innervation, we find that the fetal transition proceeds unimpaired, demonstrating that there is no causal link between the fetal transition and the conversion to focal innervation. Furthermore, because the doses of curare used inhibit motor activity by more than 80%, the fetal transition can occur in the absence of normal levels of motor activity. These observations show that the fetal transition in ovo is not induced by either a specific change in innervation or use. Rather, the dependence on innervation seems to be a consequence of the need for muscle activity to prevent atrophy, and the fetal transition appears to have characteristics more like the preprogrammed contractile protein transitions that precede it.

Migration of Schwann cells and axons into developing chick forelimb muscles following removal of either the neural tube or the neural crest

A study has been made of the effects of neural crest and neural tube removal at the brachial level on the migration of Schwann cells and axons into the flexor digitorum profundus (fdp) and flexor carpi ulnaris (fcu) muscles of the avian forelimb. The identification of Schwann cells was based on the assumption that antibody HNK-1 uniquely labels these cells at the growing end of limb nerves. Myotubes and nerves were identified by using antibodies to myosin and to neurofilament protein, respectively. The removal of neural crest cells at stage 13 gave a complete Schwann cell-free embryo at the brachial level. Motor axons only grew to the base of the forelimb, forming a rudimentary plexus by stage 27, and failed to penetrate the limb. Removal of the neural tube at stage 13 did not prevent sensory axons from forming a plexus at the base of the limb; these axons subsequently developed into the brachialis longus inferior (bli n) and superior (bls n) nerves. By stage 27 the bli n had branched into the interosseus nerve (in n) and the medial-ulnar nerve (m-u n) trunks. However, unlike the result in control embryos, no nerves were detected amongst the developing fdp and fcu muscles, thus indicating that sensory axons do not grow into the muscles in the absence of motor axons. In contrast, Schwann cells were observed amongst the myotubes at the level of the in n and m-u nerve trunks. The present observations show that motor axons do not enter the limb bud and innervate limb muscles in the absence of Schwann cells. Furthermore, in the absence of motor axons (neural-tube-removed embryos) sensory axons still enter the limb (behind migrating Schwann cells) but fail to innervate developing muscles even though Schwann cells are present among the developing myotubes.

Expression of the troponin complex genes: transcriptional coactivation during myoblast differentiation and independent control in heart and skeletal muscles

We compared the developmental regulation of the three troponin genes that encode the proteins of the Ca2+ regulatory complex in striated muscles of the Japanese quail. Nuclear run-on transcription and RNA protection analyses showed that the fast skeletal troponin I, the fast skeletal troponin T, and the slow skeletal-cardiac troponin C genes were transcriptionally coactivated and that transcripts rapidly accumulated within 6 to 12 h after the initiation of myoblast differentiation. The fast-isoform mRNAs of troponin I and troponin T were coexpressed at similar levels in different skeletal muscles, whereas the slow-cardiac troponin C mRNA varied independently and was the only one of these genes expressed in embryonic and adult heart. We conclude that these troponin genes are transcriptionally coactivated during skeletal myoblast differentiation, indicating that their transcription is under precise temporal control. However, this troponin C gene is regulated independently is specialized striated muscles.