Differentiation of chick embryo myoblasts is transiently sensitive to functional denervation

Dynamics of nuclei of muscle fibers and connective tissue cells in normal and denervated rat muscles

The mitotic activity in muscles of growing rats and the effect of denervation were studied by means of continuous infusion of 5-bromo-2-deoxyuridine (BRDU). Denervated muscles after 10 weeks contained 20 to 60% fewer muscle nuclei than normal; BRDU labeled about 25% of the nuclei of normal soleus and extensor digitorum longus (EDL) and of denervated EDL muscles but only 5% in the denervated soleus muscle. Labeled nuclei persisted in denervated but not in normal muscles. After the main growth period, the turnover of myonuclei was at most 1 to 2% per week. The behavior of connective tissue nuclei was similar to that in muscle fibers. Infusion of BRDU had no effect on contractile properties. It is suggested that the exceptionally rapid atrophy of the denervated rat soleus associated with loss of satellite cells was due to loss of myonuclei and differentiation and fusion of satellite cells. The cause may possibly be that the phase of postdenervation fibrillation is shorter than in other muscles.

Satellite cells and myonuclei in long-term denervated rat muscles

BACKGROUND

The percentage of satellite cells rapidly decreases in aneurally regenerating soleus muscles of rat. Also denervation of intact muscles causes fiber loss and regeneration, but the fate of satellite cells is unknown; myonuclei have been suggested to undergo changes resembling those in apoptotic cells.

METHODS

Rat soleus and extensor digitorum longus (EDL) muscles were denervated at birth or at age 5 weeks and investigated after periods of up to 38 weeks. At least 400 myonuclei in each muscle were assessed by electron microscopy, and satellite cell nuclei were counted. In situ nick translation and tailing were performed after 30 weeks denervation in order to demonstrate DNA breaks associated with apoptosis.

RESULTS

Myotubes indicating regeneration were prominent in the adult denervated soleus and deep layers of EDL muscles after 7 weeks and in the superficial parts of EDL muscle after 16 weeks. The percentage of satellite cell nuclei slowly decreased to less than one fifth of normal after 20-30 weeks. Almost all satellite cells had vanished 10 weeks after neonatal denervation. Degenerating myonuclei in adult, but not in neonatally denervated muscles, remotely resembled apoptotic nuclei of lymphocytes, but no evidence of DNA breaks was found.

CONCLUSION

Denervation of rat skeletal muscles causes, in addition to fiber atrophy, loss of fibers with subsequent regeneration. Proliferation of satellite cells under aneural conditions may lead to exhaustion of the satellite cell pool. This process is more rapid in growing than in adult muscles. Myonuclei in denervated muscles do not show DNA breaks which can be demonstrated by in situ nick translation.

Myogenic cell lineages

For many years the mechanisms by which skeletal muscles in higher vertebrates come to be composed of diverse fiber types distributed in distinctive patterns has interested cell and developmental biologists. The fiber composition of skeletal muscles varies from class to class and from muscle to muscle within the vertebrates. The developmental basis for these events is the subject of this review. Because an individual multinucleate vertebrate skeletal muscle fiber is formed by the fusion of many individual myoblasts, more attention, in recent times, has been directed toward the origins and differences among myoblasts, and more emphasis has been placed on the lineal relationship of myoblasts to fibers. This is a review of studies related to the concepts of myogenic cell lineage in higher vertebrate development with emphases on some of the most challenging problems of myogenesis including the embryonic origins of myogenic precursor cells, the mechanisms of fiber type diversity and patterning, the distinctions among myoblasts during myogenesis, and the current hypotheses of how a variety of factors, intrinsic and extrinsic to the myoblast, determine the definitive phenotype of a muscle fiber.

In vitro development of precursor cells in the myogenic lineage

Expression of the muscle-specific integral membrane protein H36 and the intermediate filament protein desmin, detected by immunofluorescence, was used to identify cells at distinct stages in the skeletal myogenic lineage. These proteins were coordinately expressed in cultures of rat hindlimb myoblasts from 17- and 19-day fetuses and newborn pups, and in satellite cells from juveniles. Both H36+ and desmin+ cells were present in cultures from 13.5- and 15-day embryonic hindlimbs, but desmin expression was more prevalent: H36-/desmin+ myoblasts predominate during this early stage of development. H36 was not detected in Day 12 embryo hindlimb bud cells in vivo nor in cultures soon after plating. Initially, only 1% of the Day 12 limb bud cells expressed desmin. When these cells were serially passaged every 3-4 days, cells with all three possible myogenic phenotypes developed: that is, H36+/desmin-, H36+/desmin+, and H36-/desmin+ cells. There was a progressive increase in the frequency of H36+ cells, with 75% of cells positive by passage 6 (Day 27 in vitro). The maximum frequency of cells that expressed desmin occurred in passage 5 (Day 23 in vitro). These results demonstrate that precursors to the cells that express H36 and desmin are present in the 12-day embryo hindlimb bud and that the transition from H36-/desmin- precursors to cells with a myogenic phenotype can occur in vitro. MyoD1 and myogenin were not detected in these cells, suggesting that the initial expression of H36 and desmin in the myogenic lineage may precede and/or is independent of these regulatory proteins. The conversion of precursor cells in the 12-day limb bud to a more advanced stage of development serves to define additional cells in the myogenic lineage. The ability to monitor in vitro these stages of development affords the opportunity to study how they are regulated.

Paralysis and growth of the musculoskeletal system in the embryonic chick

Avian embryos can be completely paralyzed by injection of neuromuscular-blocking agents. We used a single injection of decamethonium iodide to paralyze embryos at 7, 8, or 10 days of incubation and analyzed the growth of individual bones (clavicle, mandible, ulna, femur, tibia, humerus) and of individual muscles that act upon some of those bones (clavicular and sternal heads of m. pectoralis, and mm. biceps brachii, depressor mandibulae, pseudotemporalis, and adductor externus). Growth of the bones is not equally affected by paralysis. Only 27% of clavicular growth (by mass) but 77% of mandibular growth occurred in paralyzed embryos, whereas the four long bones exhibited 52-63% of their normal growth. Analysis of muscle weight, fiber length and physiological cross-sectional area (weight/fiber length) indicate that there was greater reduction of the muscles acting on the limbs than of those acting on the mandible, i.e., diminished growth of the skeleton is correlated with reduced muscular activity. Specific retardation of clavicular growth is due to fusion of sternal rudiments and collapse of the thorax, as well as virtual absence of the musculature that normally attaches to the clavicle. We discuss these results in the light of intrinsic and extrinsic factors governing growth of the embryonic skeleton. Paralysis reduces skeletal growth by reducing both the movements taking place in ovo, and the loads imposed on the bones by muscle contraction, changes that represent alterations in the mechanical environment of the skeleton.

Growth and denervation response of skeletal muscle fibers of newborn rats

The cross-sectional area of the fibers of hindlimb muscles of rats increased 10-40 times during the first 6 weeks after birth. Denervation at birth stopped the growth of the muscle fibers. The number of satellite cells decreased, and eventually all fibers vanished. Reinnervation, if any, was poor. Partial denervation did not induce collateral reinnervation. Some denervated gastrocnemius muscles were reinnervated and after 8-12 months contained hypertrophic fibers and signs of necrosis and regeneration. When soleus muscles were completely denervated and cross-reinnervated after 4 weeks by the peroneal nerve, only half as many fibers became reinnervated after neonatal denervation as compared to muscles denervated at the age of 4 weeks. The experiments suggest that immature muscle fibers are less apt to become reinnervated than mature fibers. The few reinnervated fibers may be overloaded and therefore hypertrophy and eventually necrotize. Regeneration is abortive because satellite cells are scarce. These results may be relevant for the understanding of neuromuscular disorders with early (fetal) onset.

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.

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.

Molecular genetics in basic myology: a rapidly evolving perspective

Myology has greatly benefited from the recent unification of concepts in molecular, cellular, and developmental biology. The interplay between intrinsic and extrinsic factors in determining the physiologic characteristics of individual myofibers has emerged as an important theme. Of special note is the manner in which the study of contractile protein gene structure and expression has contributed to our understanding of the development and ultimate plasticity of the contractile apparatus. As mechanistic models of normal myogenesis achieve increasing sophistication, the opportunities for understanding the pathogenesis of progressive muscle disfunction improve. In this article we review recent progress in basic myology which will be of interest to clinicians studying the heritable neuromuscular disorders.

Gap junctions form in culture between chick embryo neurons and skeletal muscle myoblasts

Possible mechanisms by which neurons can affect the differentiated states of mononucleated cells in the early chick embryo leg skeletal myogenic lineage are being investigated. Under conditions of co-culture known to promote inductive interactions between embryonic neurons and leg cells, spinal cord neurons and mononucleated leg cells are coupled by gap junctions as shown by cell-to-cell passage of Lucifer yellow and hyperpolarizing or depolarizing currents. Co-cultures composed of ciliary ganglion neurons and clones of leg cells showed that the ability to form gap junctions with neurons is a differentiated function of myoblasts. Dye-coupling with neurons was found at high frequency only when the partner cell was a myoblast of a differentiating muscle clone. Myoblasts in undifferentiated clones formed gap junctions with each other but not with neurons. The nature of the junction-forming neuron is not restricted since neurons from spinal cord, ciliary ganglion and dorsal root ganglion all readily form junctions with myoblasts.

Primary, secondary and tertiary myotubes in developing skeletal muscle: a new approach to the analysis of human myogenesis

Monoclonal antibodies to myosins have been used to describe and define the appearance and maturation of 3 different classes of myotube in developing human quadriceps muscle. Five monoclonal antibodies were used: (i) MAb A against human slow myosin heavy chain; (ii) MAb B against a myosin heavy chain present in most adult Type 2 fibres; (iii) MAb C against myosin heavy chain present in all mature and immature Type 2 fibres; (iv) MAb D, with similar reactivity to MAb C; (v) MAb E against human embryonic myosin. The combined use of two of these antibodies (A and B) enables the confident early identification of each of 3 classes (primary, secondary, tertiary) of myotubes, which appear sequentially during myogenesis. Our results show that induction of slow myosin heavy chain synthesis is a biphasic phenomenon in developing human skeletal muscle. Slow myosin heavy chain was present in all the earliest (9 weeks gestation) primary myotubes, but was not detected in secondary or tertiary myotubes until about 29 weeks gestation. Each stage of fetal muscle development has a characteristic immunocytochemical pattern which reveals cellular heterogeneity not evident on myosin ATPase histochemistry. Myosin immunocytochemistry may usefully be applied to assess the gestational age of fetuses. A new interpretation of human skeletal muscle development is proposed, based on the separate programming of 3 different kinds of myotube. This may be important in the analysis of diseased muscle in which developmental abnormalities or regeneration are present.

Differential expression and modulation of Thy-1 on myoblast clones

In the preceding paper [J. S. Schweitzer, M. A. Dichter, and S. J. Kaufman, 1987, Exp. Cell Res. 172, 1] we demonstrated that expression of Thy-1 antigen by rat skeletal muscle myoblasts in culture is modulated by fibroblasts. Growth of myoblasts with myofibroblasts, or in medium in which fibroblasts have been grown, causes a sharp reduction in the percentage of myoblasts that express Thy-1 antigen on their membrane. In this paper we demonstrate that there are two populations of myoblasts that can be distinguished by their capacity to respond to the Thy-1-modulating factor (TMF). The majority of Thy-1-positive or -negative myoblasts grown at clonal densities are responsive to TMF. A second myoblast clonal type is Thy-1 positive and fuses to form contractile myotubes, but is insensitive to TMF and has a more fibroblast-like morphology. These clonal phenotypes are stable upon passage in vitro and may represent distinct subsets of the myogenic lineage.

Fibroblasts modulate expression of Thy-1 on the surface of skeletal myoblasts

Thy-1 antigen is a well-characterized cell-surface glycoprotein known to be variably expressed in many different tissues, including lymphocytes, brain, and muscle. Its function remains unknown. In skeletal muscle, both in vivo and in vitro, the antigen has been reported on immature but not on adult tissue, and its disappearance corresponds roughly to the time of myoblast fusion. Using monoclonal H36 antibody to identify myoblasts unambiguously, we demonstrate here that Thy-1 is expressed only on a small (less than 1%) fraction of rat skeletal muscle myoblasts in heterogeneous primary cultures, but the number of myoblasts that express Thy-1 rises to a steady level of about 70% when fibroblasts are removed from secondary cultures. Restitution of fibroblasts or growth of purified myoblasts in medium conditioned by fibroblasts greatly suppresses this increase in myoblast Thy-1 expression. Thus an interaction between fibroblasts and myoblasts, mediated by a soluble nondialyzable molecule, modulates expression of Thy-1 on the myoblast outer membrane.

Myoblast differentiation is induced by nerve transplanted to chick embryo legs

Chick embryos were denervated early in development in order to disrupt the normal inductive interactions between the nervous system and developing populations of mononucleated myoblasts and their precursors in the leg. Neural tissue, either spinal cord or ciliary ganglion, was transplanted to one leg of the denervated embryos; the other leg remained aneural. Clonal analysis of cell populations in the transplant-containing legs showed that ectopic nerve tissue can recapitulate some of the neuromuscular interactions that occur in normally developing embryos. Chief among these was the observation that transplantation induced the appearance of the CMR-III myoblast class in the leg muscle. Since the process by which CMR-III myoblasts are produced from a precursor is dependent on nerve both in vivo and in vitro (Bonner, P.H. and T.R. Adams, Dev. Biol., 90:175-184, 1982), it was concluded that transplanted nerve tissue can also induce myoblast differentiation.

Comparative anatomical analysis of human trisomies 13, 18, and 21: I. The forelimb

Human trisomies 13, 18, and 21 exhibit specific neuromuscular phenotypes (Pettersen and Bersu, '82) which include a high proportion of neuromuscular forelimb variations, many of which are atavistic in nature (de Beer, '58; Barash et al., '79; Aziz, '81a). In order to test the neuromuscular phenotype, examine the atavistic nature, and analyze the developmental delay of the trisomy forearm musculature, we dissected the forelimbs of five trisomy 13, ten trisomy 18, and two trisomy 21 cases. Our dissections compare favorably with the existing published trisomy cases (Opitz et al., '79; Pettersen and Bersu, '82). Additionally, we found significant differences in the stage at which developmental arrest occurred in trisomies 13 and 18 for the pectoral complex, extensor digitorum profundus, and intrinsic hand musculature. Some of these muscles, which occur normally in nonhuman primates (Cihak, '67, '69; Dunlap et al., '85), also appear briefly in normal human ontogeny (Cihak, '72), constituting further evidence for developmental delay in aneuploids. The disproportionately effected limb tissues also lend support to the evidence for some degree of autonomy in their development in normal individuals. Our observations are consistent with Shapiro's amplified developmental instability model ('83). Aneuploids may be viewed as genetic variants from which much may be learned about normal limb development, how aneuploidy affects dysmorphogenesis, and the kind of information which exists on the duplicated (or monosomic) chromosome.

Epigastrius with omphalocele--report of a case

An epigastrius (parasitic twinning in the epigastrium) was delivered via the vaginal route spontaneously and died 37 days later. The autosite had cardiac hypertrophy with VSD, PDA, and PFO and an omphalocele. The liver was partly conjoined. There was a small swelling at the lower part of the autosite's bifurcated sternum to which the parasitic left pelvis and lower extremity were attached. On the second day after birth, parasitectomy was performed. The amputated specimens consisted of a small left pelvic girdle with a free extremity, a scrotal mass, nipplelike structures and two small protuberances externally. The liver, intestines, two testes, one kidney, one ureter, and the bladder were contained within the omphalocele. Although all of the organs and external structures of the parasite had abnormal histopathological findings, differentiated muscle fibers and submucous and/or myenteric plexus were observed.

Temporal separation of the migration of distinct myogenic precursor populations into the developing chick wing bud

The developing chick limb bud contains several stage-specific populations of muscle colony-forming (MCF) cells. The cell lineage relationships of these populations are not completely understood, but previous studies have suggested that there may be at least two myogenic precursor populations present in the limb bud. To investigate the heterogeneity of the precursor population, the migration of myogenic precursor cells into the wing anlagen was stopped at various times by grafting stage 15-18 wing buds to the flank or chorioallantoic membrane regions of host embryos. The grafted wings were subsequently assayed for the various MCF cell populations. The data provide further support for the existence of two distinct myogenic precursor populations that migrate into the wing anlagen at different times; precursors to early MCF cells are present by stage 15, whereas precursors to late MCF cells are not present until the end of stage 16. The fact that wing buds which are grafted to a foreign location at stage 15 exhibit exclusive development of early MCF cells suggests that late MCF cells are not direct cell lineage descendants of either the early-type precursors or early MCF cells.

Spatial analysis of limb bud myogenesis: elaboration of the proximodistal gradient of myoblasts requires the continuing presence of apical ectodermal ridge

Myogenic tissue from embryonic chick wing and leg buds is composed of several subpopulations of myoblasts. These clonally distinct subpopulations first appear at different developmental stages, and are distributed differently along the proximo-distal axis of the buds, giving the appearance of a gradient of myoblast cell types. This myoblast distribution pattern has been utilized to investigate the dependence of muscle tissue outgrowth and development on the presence of the apical ectodermal ridge (AER). Wing buds which have had the AER removed at stages 17-18 (2 days) subsequently develop normal proximal regions, but fail to elaborate skeletal structures distal to the humerus. The myoblast pattern of operated buds is also normal proximally, but distal portions of the pattern are not observed. Removal of the AER at stage 20 (3 days) results in buds which develop slightly more distal skeletal structures and the coinciding portions of the myoblast pattern, but in which the more distal portions of the normal myoblast gradient are truncated. These data suggest that elaboration of the myogenic pattern in early limb buds is dependent on the continuing presence of the AER, and that early removal of the AER leads to the subsequent cessation of myoblast pattern specification.

Parasitic conjoined twins, two cases, and their relation to limb morphogenesis

Reports of parasitic conjoined twins have appeared rarely in the scientific literature and only one study has included a detailed description of the parasitic extremities. We report here two specimens consisting for the most part of normal hosts with ectopic legs attached to the thorax (case 1, thoracoparasitus) or the abdomen (case 2, gastroparasitus). These two specimens are in agreement with several others reported in the literature in having no sign of vertebral columns. These specimens also exhibit a marked decrease in innervation (case 1) or complete absence of somatic nerves (case 2). This finding is in complete contradiction with the idea of the dependence of limb skeletal structure upon peripheral nerves. The hypothesis is presented that this type of parasitic twin may result from duplication of the Wolffian ridge.

Myosin light chains and the developmental origin of fast muscle

Physiological characteristics of embryonic and fetal fast muscle function are similar to those of adult slow muscles, whereas most biochemical data suggest that embryonic and fetal fast muscles contain only fast muscle myosin. In the studies reported here, myofibrillar preparations from developing avian pectoral muscle (fast muscle) were isolated and analyzed for myosin light-chain type and synthesis. These analyses show that early in development avian fast muscle synthesizes and assembles myofibrils with light chains of both slow and fast myosins. Later in development, fast muscle no longer assembles myofibrils containing slow myosin light chains due to the cessation of synthesis of slow myosin light chains in mid-development. These in vivo studies indicate that the more developmentally primitive type of skeletal muscle is one that synthesizes both slow and fast myosin light chains independent of its anatomic location, and an event(s) late in fast muscle development results in the repression of synthesis of slow myosin light chains.