Clonal analysis of vertebrate myogenesis. IV. Medium-dependent classification of colony-forming cells

Cellular dynamics of regeneration reveals role of two distinct Pax7 stem cell populations in larval zebrafish muscle repair

Heterogeneity of stem cells or their niches is likely to influence tissue regeneration. Here we reveal stem/precursor cell diversity during wound repair in larval zebrafish somitic body muscle using time-lapse 3D confocal microscopy on reporter lines. Skeletal muscle with incision wounds rapidly regenerates both slow and fast muscle fibre types. A swift immune response is followed by an increase in cells at the wound site, many of which express the muscle stem cell marker Pax7. Pax7(+) cells proliferate and then undergo terminal differentiation involving Myogenin accumulation and subsequent loss of Pax7 followed by elongation and fusion to repair fast muscle fibres. Analysis of pax7a and pax7b transgenic reporter fish reveals that cells expressing each of the duplicated pax7 genes are distinctly localised in uninjured larvae. Cells marked by pax7a only or by both pax7a and pax7b enter the wound rapidly and contribute to muscle wound repair, but each behaves differently. Low numbers of pax7a-only cells form nascent fibres. Time-lapse microscopy revealed that the more numerous pax7b-marked cells frequently fuse to pre-existing fibres, contributing more strongly than pax7a-only cells to repair of damaged fibres. pax7b-marked cells are more often present in rows of aligned cells that are observed to fuse into a single fibre, but more rarely contribute to nascent regenerated fibres. Ablation of a substantial portion of nitroreductase-expressing pax7b cells with metronidazole prior to wounding triggered rapid pax7a-only cell accumulation, but this neither inhibited nor augmented pax7a-only cell-derived myogenesis and thus altered the cellular repair dynamics during wound healing. Moreover, pax7a-only cells did not regenerate pax7b cells, suggesting a lineage distinction. We propose a modified founder cell and fusion-competent cell model in which pax7a-only cells initiate fibre formation and pax7b cells contribute to fibre growth. This newly discovered cellular complexity in muscle wound repair raises the possibility that distinct populations of myogenic cells contribute differentially to repair in other vertebrates.

Satellite cell heterogeneity revealed by G-Tool, an open algorithm to quantify myogenesis through colony-forming assays

Background

Muscle growth and repair is accomplished by the satellite cell pool, a self-renewing population of myogenic progenitors. Functional heterogeneity within the satellite cell compartment and changes in potential with experimental intervention can be revealed by in vitro colony-forming cell (CFC) assays, however large numbers of colonies need to be assayed to give meaningful data, and manually quantifying nuclei and scoring markers of differentiation is experimentally limiting.

Methods

We present G-Tool, a multiplatform (Java) open-source algorithm that analyzes an ensemble of fluorescent micrographs of satellite cell-derived colonies to provide quantitative and statistically meaningful metrics of myogenic potential, including proliferation capacity and propensity to differentiate.

Results

We demonstrate the utility of G-Tool in two applications: first, we quantify the response of satellite cells to oxygen concentration. Compared to 3% oxygen which approximates tissue levels, we find that 21% oxygen, the ambient level, markedly limits the proliferative potential of transit amplifying progeny but at the same time inhibits the rate of terminal myogenic differentiation. We also test whether satellite cells from different muscles have intrinsic differences that can be read out in vitro. Compared to masseter, dorsi, forelimb and hindlimb muscles, we find that the diaphragm satellite cells have significantly increased proliferative potential and a reduced propensity to spontaneously differentiate. These features may be related to the unique always-active status of the diaphragm.

Conclusions

G-Tool facilitates consistent and reproducible CFC analysis between experiments and individuals. It is released under an open-source license that enables further development by interested members of the community.

The skeletal muscle satellite cell: still young and fascinating at 50

The skeletal muscle satellite cell was first described and named based on its anatomic location between the myofiber plasma and basement membranes. In 1961, two independent studies by Alexander Mauro and Bernard Katz provided the first electron microscopic descriptions of satellite cells in frog and rat muscles. These cells were soon detected in other vertebrates and acquired candidacy as the source of myogenic cells needed for myofiber growth and repair throughout life. Cultures of isolated myofibers and, subsequently, transplantation of single myofibers demonstrated that satellite cells were myogenic progenitors. More recently, satellite cells were redefined as myogenic stem cells given their ability to self-renew in addition to producing differentiated progeny. Identification of distinctively expressed molecular markers, in particular Pax7, has facilitated detection of satellite cells using light microscopy. Notwithstanding the remarkable progress made since the discovery of satellite cells, researchers have looked for alternative cells with myogenic capacity that can potentially be used for whole body cell-based therapy of skeletal muscle. Yet, new studies show that inducible ablation of satellite cells in adult muscle impairs myofiber regeneration. Thus, on the 50th anniversary since its discovery, the satellite cell's indispensable role in muscle repair has been reaffirmed.

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

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

Intrinsic phenotypic diversity of embryonic and fetal myoblasts is revealed by genome-wide gene expression analysis on purified cells

Skeletal muscle development occurs asynchronously and it has been proposed to be dependent upon the generation of temporally distinct populations of myogenic cells. This long-held hypothesis has not been tested directly due to the inability to isolate and analyze purified populations of myoblasts derived from specific stages of prenatal development. Using a mouse strain with the GFP reporter gene targeted into the Myf5 locus, a cell-sorting method was developed for isolating embryonic and fetal myoblasts. The two types of myoblasts show an intrinsic difference in fusion ability, proliferation, differentiation and response to TGFbeta, TPA and BMP-4 in vitro. Microarray and quantitative PCR were used to identify differentially expressed genes both before and after differentiation, thus allowing a precise phenotypic analysis of the two populations. Embryonic and fetal myoblasts differ in the expression of a number of transcription factors and surface molecules, which may control different developmental programs. For example, only embryonic myoblasts express a Hox code along the antero-posterior axis, indicating that they possess direct positional information. Taken together, the data presented here demonstrate that embryonic and fetal myoblasts represent intrinsically different myogenic lineages and provide important information for the understanding of the molecular mechanisms governing skeletal muscle development.

The mitotic clock in skeletal muscle regeneration, disease and cell mediated gene therapy

The regenerative capacity of skeletal muscle will depend on the number of available satellite cells and their proliferative capacity. We have measured both parameters in ageing, and have shown that although the proliferative capacity of satellite cells is decreasing during muscle growth, it then stabilizes in the adult, whereas the number of satellite cells decreases during ageing. We have also developed a model to evaluate the regenerative capacity of human satellite cells by implantation into regenerating muscles of immunodeficient mice. Using telomere measurements, we have shown that the proliferative capacity of satellite cells is dramatically decreased in muscle dystrophies, thus hampering the possibilities of autologous cell therapy. Immortalization by telomerase was unsuccessful, and we currently investigate the factors involved in cell cycle exits in human myoblasts. We have also observed that insulin-like growth factor-1 (IGF-1), a factor known to provoke hypertrophy, does not increase the proliferative potential of satellite cells, which suggests that hypertrophy is provoked by increasing the number of satellite cells engaged in differentiation, thus possibly decreasing the compartment of reserve cells. We conclude that autologous cell therapy can be applied to specific targets when there is a source of satellite cells which is not yet exhausted. This is the case of Oculo-Pharyngeal Muscular Dystrophy (OPMD), a late onset muscular dystrophy, and we participate to a clinical trial using autologous satellite cells isolated from muscles spared by the disease.

Embryonic and fetal rat myoblasts form different muscle fiber types in an ectopic in vivo environment

Limb muscle development is characterized by the migration of muscle precursor cells from the somite followed by myoblast differentiation and the maturation of myotubes into distinct muscle fiber types. Previous in vitro experiments have suggested that rat limb myoblasts are composed of at least two distinct myoblast subpopulations that appear in the developing hindlimb at different developmental stages. These embryonic and fetal myoblast subpopulations are believed to generate primary and secondary myotubes, respectively. To test this hypothesis, cells obtained from embryonic day 14 (ED 14) and ED 20 rat hindlimbs were analyzed for myosin heavy chain expression after long-term differentiation in adult rat brains. Fetal myoblasts from ED 20 hindlimbs produced muscle fibers with a phenotype similar to that seen in tissue culture--predominantly fast myosin with a small proportion also coexpressing slow myosin. However, injection sites populated by embryonic myoblasts from ED 14 hindlimbs produced a different phenotype from that previously reported in culture, with fibers expressing an entire array of myosin isoforms. In addition, a subpopulation of fibers expressing exclusively slow myosin was found only in the embryonic injection sites. Our results support the existence of at least three myogenic subpopulations in early rat limb buds with only one exhibiting the capability to differentiate in vitro. These findings are consistent with a model of muscle fiber type development in which the fiber type potential of myoblast populations is established before differentiation into myotubes. This process establishes myogenic subpopulations that have restricted adaptive ranges regulated by both intrinsic and extrinsic factors.

Seeking muscle stem cells

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.

The four populations of myoblasts involved in human limb muscle formation are present from the onset of primary myotube formation

To understand how and when myogenic precursor cells become committed to their particular developmental programs, we have analysed the different populations of myoblasts which grow out from explants of muscle tissue isolated from human limb buds from the beginning of primary fibre formation throughout subsequent development and post-natal growth. Four phenotypically distinct types of myoblasts were identified on the basis of their expression of desmin, myogenin and myosin heavy chain isoforms (MyHC), and after 5 and 20 divisions, cells were cloned. All four types of myoblasts were present at the beginning of primary myogenesis. Each respective phenotype was stably heritable through cloning and subsequent proliferation. The type 1 clones correspond to a novel class of myoblasts never described during human development, that biochemically differentiates, but does not fuse. Type 2 clones are composed of small myotubes expressing only embryonic MyHC. Type 3 clones are composed of thin and long myotubes expressing both embryonic and fetal MyHCs. The type 4 clones are composed of myotubes that have a phenotype very similar to human satellite cells. Contrasting with others species, no other population of myoblasts appear during fetal development and only the relative number of these four types changes.

Knowing chops from chuck: roasting myoD redundancy

The myf5 and myoD genes are implicated in the specification of vertebrate skeletal muscle. These genes have been thought to be functionally redundant because neonatal mice bearing homozygous null mutations in either gene show grossly normal muscle development. By analyzing the early embryonic development of the mutants, Michael Rudnicki and coworkers show that trunk muscle development is retarded in embryos bearing myf5 null mutations, while early limb and branchial arch muscle development is retarded by myoD null mutations. These results indicate that the myoD and myf5 genes are not redundant but that each controls the early specification of distinct muscle cell lineages.

Stability of chicken troponin T expression in cultured muscle cells

Cells prepared from chicken skeletal muscles of early developmental stages were cultured to study their troponin T isoform expression, using antisera specific to fast- and slow-muscle-type isoforms, and compared with the cells from later stages described in the previous study (Mashima at al., 1996). We found that cultured myogenic cells from chickens and chick embryos could be classified, as in the previous study, into two types, fast type and fast/slow type in which fast- and slow-muscle-type isoforms were coexpressed. Ratios of these two types of muscle cells varied depending on their origins and developmental stages, and fast/slow type cells were in the majority at early stages. Since two distinct populations of cells committed to myogenic cell lineages were supposed to give rise to the two types of myotubes, we investigated the intrinsic stability of troponin T expression of the cultured myogenic cells using the serial subcloning method. The results of clonal analysis suggested that the expression pattern of troponin T isoform in cultured muscle cells is stable and that myogenic cell lineages play an important role in giving rise to different muscle types.

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.

Myogenesis in paraxial mesoderm: preferential induction by dorsal neural tube and by cells expressing Wnt-1

Previous studies have demonstrated that the neural tube/notochord complex is required for skeletal muscle development within somites. In order to explore the localization of myogenic inducing signals within the neural tube, dorsal or ventral neural tube halves were cultured in contact with single somites or pieces of segmental plate mesoderm. Somites and segmental plates cultured with the dorsal half of the neural tube exhibited 70% and 85% myogenic response rates, as determined by immunostaining for myosin heavy chain. This response was slightly lower than the 100% response to whole neural tube/notochord, but was much greater than the 30% and 10% myogenic response to ventral neural tube with and without notochord. These results demonstrate that the dorsal neural tube emits a potent myogenic inducing signal which accounts for most of the inductive activity of whole neural tube/notochord. However, a role for ventral neural tube/notochord in somite myogenic induction was clearly evident from the larger number of myogenic cells induced when both dorsal neural tube and ventral neural tube/notochord were present. To address the role of a specific dorsal neural tube factor in somite myogenic induction, we tested the ability of Wnt-1-expressing fibroblasts to promote paraxial mesoderm myogenesis in vitro. We found that cells expressing Wnt-1 induced a small number of somite and segmental plate cells to undergo myogenesis. This finding is consistent with the localized dorsal neural tube inductive activity described above, but since the ventral neural tube/notochord also possesses myogenic inductive capacity yet does not express Wnt-1, additional inductive factors are likely involved.

Development and postnatal regulation of adult myoblasts

The myogenic precursor cells of postnatal and adult skeletal muscle are situated underneath the basement membrane of the myofibers. It is because of their unique positions that these precursor cells are often referred to as satellite cells. Such defined satellite cells can first be detected following the formation of a distinct basement membrane around the fiber, which takes place in late stages of embryogenesis. Like myoblasts found during development, satellite cells can proliferate, differentiate, and fuse into myofibers. However, in the normal, uninjured adult muscle, satellite cells are mitotically quiescent. In recent years several important questions concerning the biology of satellite cells have been asked. One aspect has been the relationship between satellite cells and myoblasts found in the developing muscle: are these myogenic populations identical or different? Another aspect has been the physiological cues that control the quiescent, proliferative, and differentiative states of these myogenic precursors: what are the growth regulators and how do they function? These issues are discussed, referring to previous work by others and further emphasizing our own studies on avian and rodent satellite cells. Collectively, the studies presented indicate that satellite cells represent a distinct myogenic population that becomes dominant in late stages of embryogenesis. Moreover, although satellite cells are already destined to be myogenic precursors, they do not express any of the four known myogenic regulatory genes unless their activation is induced in the animal or in culture. Furthermore, multiple growth factors are important regulators of satellite cell proliferation and differentiation. Our work on the role of one of these growth factors [platelet-derived growth factor (PDGF)] during proliferation of adult myoblasts is further discussed with greater detail and the possibility that PDGF is involved in the transition from fetal to adult myoblasts in late embryogenesis is brought forward.

Pax-3 expression in segmental mesoderm marks early stages in myogenic cell specification

Specification of the myogenic lineage begins prior to gastrulation and culminates in the emergence of determined myogenic precursor cells from the somites. The myoD family (MDF) of transcriptional activators controls late step(s) in myogenic specification that are closely followed by terminal muscle differentiation. Genes expressed in myogenic specification at stages earlier than MDFs are unknown. The Pax-3 gene is expressed in all the cells of the caudal segmental plate, the early mesoderm compartment that contains the precursors of skeletal muscle. As somites form from the segmental plate and mature, Pax-3 expression is progressively modulated. Beginning at the time of segmentation, Pax-3 becomes repressed in the ventral half of the somite, leaving Pax-3 expression only in the dermomyotome. Subsequently, differential modulation of Pax-3 expression levels delineates the medial and lateral halves of the dermomyotome, which contain precursors of axial (back) muscle and limb muscle, respectively. Pax-3 expression is then repressed as dermomyotome-derived cells activate MDFs. Quail-chick chimera and ablation experiments confirmed that the migratory precursors of limb muscle continue to express Pax-3 during migration. Since limb muscle precursors do not activate MDFs until 2 days after they leave the somite, Pax-3 represents the first molecular marker for this migratory cell population. A null mutation of the mouse Pax-3 gene, Splotch, produces major disruptions in early limb muscle development (Franz, T., Kothary, R., Surani, M. A. H., Halata, Z. and Grim, M. (1993) Anat. Embryol. 187, 153–160; Goulding, M., Lumsden, A. and Paquette, A. (1994) Development 120, 957–971). We conclude, therefore, that Pax-3 gene expression in the paraxial mesoderm marks earlier stages in myogenic specification than MDFs and plays a crucial role in the specification and/or migration of limb myogenic precursors.

Skeletal muscle satellite cells

Evidence now suggests that satellite cells constitute a class of myogenic cells that differ distinctly from other embryonic myoblasts. Satellite cells arise from somites and first appear as a distinct myoblast type well before birth. Satellite cells from different muscles cannot be functionally distinguished from one another and are able to provide nuclei to all fibers without regard to phenotype. Thus, it is difficult to ascribe any significant function to establishing or stabilizing fiber type, even during regeneration. Within a muscle, satellite cells exhibit marked heterogeneity with respect to their proliferative behavior. The satellite cell population on a fiber can be partitioned into those that function as stem cells and those which are readily available for fusion. Recent studies have shown that the cells are not simply spindle shaped, but are very diverse in their morphology and have multiple branches emanating from the poles of the cells. This finding is consistent with other studies indicating that the cells have the capacity for extensive migration within, and perhaps between, muscles. Complexity of cell shape usually reflects increased cytoplasmic volume and organelles including a well developed Golgi, and is usually associated with growing postnatal muscle or muscles undergoing some form of induced adaptive change or repair. The appearance of activated satellite cells suggests some function of the cells in the adaptive process through elaboration and secretion of a product. Significant advances have been made in determining the potential secretion products that satellite cells make. The manner in which satellite cell proliferative and fusion behavior is controlled has also been studied. There seems to be little doubt that cellcell coupling is not how satellite cells and myofibers communicate. Rather satellite cell regulation is through a number of potential growth factors that arise from a number of sources. Critical to the understanding of this form of control is to determine which of the many growth factors that can alter satellite cell behavior in vitro are at work in vivo. Little work has been done to determine what controls are at work after a regeneration response has been initiated. It seems likely that, after injury, growth factors are liberated through proteolytic activity and initiate an activation process whereby cells enter into a proliferative phase. After myofibers are formed, it also seems likely that satellite cell behavior is regulated through diffusible factors arising from the fibers rather than continuous control by circulating factors.(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.

Cellular and molecular diversity in skeletal muscle development: news from in vitro and in vivo

Skeletal muscle formation is studied in vitro with myogenic cell lines and primary muscle cell cultures, and in vivo with embryos of several species. We review several of the notable advances obtained from studies of cultured cells, including the recognition of myoblast diversity, isolation of the MyoD family of muscle regulatory factors, and identification of promoter elements required for muscle-specific gene expression. These studies have led to the ideas that myoblast diversity underlies the formation of the multiple types of fast and slow muscle fibers, and that myogenesis is controlled by a combination of ubiquitous and muscle-specific transcriptional regulators that may be different for each gene. We further review some unexpected results that have been obtained when ideas from work in culture have been tested in developing animals. The studies in vivo point to additional molecular and cellular mechanisms that regulate muscle formation in the animal.

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.

Temporal appearance of satellite cells during myogenesis

In this study, differences between fetal and adult myoblasts in clonal and high density culture have been used to determine when adult myoblasts can first be detected during avian development. The results indicate that avian adult myoblasts are apparent as a distinct population of myoblasts during the midfetal stage of development. Three different criteria were used to differentiate fetal and adult myoblasts and demonstrate when adult myoblasts become a major proportion of the myoblast population: (1) differences in slow myosin heavy chain 1 (MHC1) isoform expression, (2) initiation of DNA synthetic activity, and (3) average myoblast length. Fetal chicken (ED10-12) pectoralis muscle (PM) myoblasts form myotubes that express slow MHC1 after prolonged culture, while adult chicken PM myoblasts do not. Fetal avian myoblasts were active in DNA synthesis and large when first isolated, reaching peak rates of synthesis by 24 hr in culture, while adult myoblasts were inactive in DNA synthesis and small when first isolated, only reaching peak rates of DNA synthesis and size at 3 days of incubation. A dramatic decrease in the percentage of muscle colonies with fibers that expressed slow MHC1 was observed between the midfetal stage and hatching in the chicken, along with a corresponding decrease in myoblast DNA synthetic activity and average length during this same period in both the chicken and the quail. Myoblast activity and average length increased again 3-4 days posthatch and a small transient increase in the number of slow MHC1-expressing clones was also associated with the massive growth of muscle that occurs in the neonatal bird. We conclude that adult myoblasts are present as a distinct population of myoblasts at least as early as the midfetal stages of avian development.

Skeletal muscle satellite cells appear during late chicken embryogenesis

The emergence of avian satellite cells during development has been studied using markers that distinguish adult from fetal cells. Previous studies by us have shown that myogenic cultures from fetal (Embryonic Day 10) and adult 12-16 weeks) chicken pectoralis muscle (PM) each regulate expression of the embryonic isoform of fast myosin heavy chain (MHC) differently. In fetal cultures, embryonic MHC is coexpressed with a ventricular MHC in both myocytes (differentiated myoblasts) and myotubes. In contrast, myocytes and newly formed myotubes in adult cultures express ventricular but not embryonic MHC. In the current study, the appearance of myocytes and myotubes which express ventricular but not embryonic MHC was used to determine when adult myoblasts first emerge during avian development. By examining patterns of MHC expression in mass and clonal cultures prepared from embryonic and posthatch chicken skeletal muscle using double-label immunofluorescence with isoform-specific monoclonal antibodies, we show that a significant number of myocytes and myotubes which stain for ventricular but not embryonic MHC are first seen in cultures derived from PM during fetal development (Embryonic Day 18) and comprise the majority, if not all, of the myoblasts present at hatching and beyond. These results suggest that adult type myoblasts become dominant in late embryogenesis. We also show that satellite cell cultures derived from adult slow muscle give results similar to those of cultures derived from adult fast muscle. Cultures derived from Embryonic Day 10 hindlimb form myocytes and myotubes that coexpress ventricular and embryonic MHCs in a manner similar to cells of the Embryonic Day 10 PM. Thus, adult and fetal expression patterns of ventricular and embryonic MHCs are correlated with developmental age but not muscle fiber type.

Expression of MRF4, a myogenic helix-loop-helix protein, produces multiple changes in the myogenic program of BC3H-1 cells

Expression of MRF4, a myogenic regulatory factor of the basic helix-loop-helix type, produced multiple changes in the myogenic program of the BC3H-1 cell line. BC3H-1 cells that stably expressed exogenous MRF4 were prepared and termed BR cell lines. Upon differentiation, the BR cells were found to have three muscle-specific properties (endogenous MyoD expression, myoblast fusion, and fast myosin light-chain 1 expression) that the parent BC3H-1 cells did not have. Of the four known myogenic regulatory factors (MyoD, myogenin, Myf-5, and MRF4), only MRF4 was capable of activating expression of the endogenous BC3H-1 myoD gene. In addition, the pattern of Myf-5 expression in BR cells was the opposite of that in BC3H-1 cells. Myf-5 expression was low in BR myoblasts and showed a small increase upon myotube formation, whereas Myf-5 expression was high in BC3H-1 myoblasts and decreased upon differentiation. Though the MRF4-transfected BR cells fused to form large myotubes and expressed fast myosin light-chain 1, the pattern of myosin heavy-chain isoform expression was the same in the BR and the nonfusing parent BC3H-1 cells, suggesting that factors in addition to the MyoD family members regulate myosin heavy-chain isoform expression patterns in BC3H-1 cells. In contrast to the changes produced by MRF4 expression, overexpression of Myf-5 did not alter BC3H-1 myogenesis. The results suggest that differential expression of the myogenic regulatory factors of the MyoD family may be one mechanism for generating cells with diverse myogenic phenotypes.

Beta-enolase is a marker of human myoblast heterogeneity prior to differentiation

In this report, we define a muscle-specific marker, beta-enolase, that distinguishes proliferating myoblasts from different stages of development. Enolase exists as multiple isoforms and in the course of cardiac and skeletal muscle development the beta isoform progressively replaces the alpha isoform. In skeletal muscle, this change in gene expression, unlike most developmental changes in myogenic gene expression, is evident in undifferentiated myoblasts. Whereas myoblasts from fetal tissues express alpha-enolase mRNA, beta-enolase is the predominant mRNA expressed by myoblasts from postnatal tissues. Our results are consistent with the idea that distinct precursor myoblasts contribute to the diversity of fiber types characteristic of muscle tissue at different stages of development.

Expression of MRF4, a myogenic helix-loop-helix protein, produces multiple changes in the myogenic program of BC3H-1 cells

Expression of MRF4, a myogenic regulatory factor of the basic helix-loop-helix type, produced multiple changes in the myogenic program of the BC3H-1 cell line. BC3H-1 cells that stably expressed exogenous MRF4 were prepared and termed BR cell lines. Upon differentiation, the BR cells were found to have three muscle-specific properties (endogenous MyoD expression, myoblast fusion, and fast myosin light-chain 1 expression) that the parent BC3H-1 cells did not have. Of the four known myogenic regulatory factors (MyoD, myogenin, Myf-5, and MRF4), only MRF4 was capable of activating expression of the endogenous BC3H-1 myoD gene. In addition, the pattern of Myf-5 expression in BR cells was the opposite of that in BC3H-1 cells. Myf-5 expression was low in BR myoblasts and showed a small increase upon myotube formation, whereas Myf-5 expression was high in BC3H-1 myoblasts and decreased upon differentiation. Though the MRF4-transfected BR cells fused to form large myotubes and expressed fast myosin light-chain 1, the pattern of myosin heavy-chain isoform expression was the same in the BR and the nonfusing parent BC3H-1 cells, suggesting that factors in addition to the MyoD family members regulate myosin heavy-chain isoform expression patterns in BC3H-1 cells. In contrast to the changes produced by MRF4 expression, overexpression of Myf-5 did not alter BC3H-1 myogenesis. The results suggest that differential expression of the myogenic regulatory factors of the MyoD family may be one mechanism for generating cells with diverse myogenic phenotypes.

Myoblasts from fetal and adult skeletal muscle regulate myosin expression differently

We compared the expression of myosin heavy chains in myogenic cultures prepared from fetal (embryonic Day 10) and adult (12-16 weeks) chicken pectoralis muscle using immunofluorescence with isoform-specific monoclonal antibodies. We found that the majority of fetal myocytes (differentiated myoblasts) and myotubes coexpressed ventricular and embryonic myosin heavy chains in culture. Also, when fetal cells were plated at a clonal density most clones coexpressed both ventricular and embryonic isoforms. In contrast, all adult myocytes and newly formed adult myotubes expressed just ventricular myosin, whether plated at mass or clonal densities. Within 12-24 hr of the onset of fusion, adult myotubes began to express embryonic myosin as well. Eventually, the majority of adult myotubes coexpressed both ventricular and embryonic myosin. The delay of embryonic myosin expression until after fusion was also seen in passaged adult myoblasts and in myoblasts isolated from regenerating adult muscle. The expression of embryonic myosin can be abolished by inhibiting fusion with EGTA in adult but not in fetal cultures. We conclude that both fetal and adult myotubes express ventricular and embryonic myosins but only fetal myocytes express the embryonic isoform prior to fusion. This difference in the regulation of embryonic myosin expression between fetal and adult myoblasts supports the hypothesis that these cells may represent two distinct populations of myogenic precursors.

Immunocytochemical characterisation of two generations of fibers during the development of the human quadriceps muscle

We have carried out a comprehensive study of the formation of muscle fibers in the human quadriceps in a large series of well dated human foetuses and children. Our results demonstrate that a first generation of muscle fibers forms between 8-10 weeks. These fibers all express slow twitch myosin heavy chain (MHC) in addition to embryonic and foetal MHCs, vimentin and desmin. Between 10-11 weeks, a subpopulation of these fibers express slow tonic MHC, being the first primordia of muscle spindles. Extrafusal fibers of a second generation form progressively and asynchronously around the primary fibers between 10-18 weeks, giving the muscle a very heterogeneous aspect due to different degrees of organization of their proteins. By 20 weeks, these second generation fibers become homogeneous and thereafter undergo a process of maturation and differentiation when they eliminate vimentin, embryonic and foetal MHCs to express either slow twitch or fast MHC. The differentiation of these second generation fibers into slow and fast depends upon different factors, such as motor innervation or level of thyroid hormone. Around the intrafusal first generation fibers, additional subsequent generations of fibers are also progressively formed. Some differ from the extrafusal second generation fibers by expressing slow tonic MHC, others by continuous expression of foetal MHC. The differentiation of intrafusal fibers is probably under the influence of both sensory and motor innervation.

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.

Skeletal muscle satellite cell diversity: satellite cells form fibers of different types in cell culture

Following skeletal muscle injury, new fibers form from resident satellite cells which reestablish the fiber composition of the original muscle. We have used a cell culture system to analyze satellite cells isolated from adult chicken and quail pectoralis major (PM; a fast muscle) and anterior latissimus dorsi (ALD; a slow muscle) to determine if satellite cells isolated from fast or slow muscles produce one or several types of fibers when they form new fibers in vitro in the absence of innervation or a specific extracellular milieu. The types of fibers formed in satellite cell cultures were determined using immunoblotting and immunocytochemistry with monoclonal antibodies specific for avian fast and slow myosin heavy chain (MHC) isoforms. We found that satellite cells were of different types and that fast and slow muscles differed in the percentage of each type they contained. Primary satellite cells isolated from the PM formed only fast fibers, while up to 25% of those isolated from ALD formed fibers that were both fast and slow (fast/slow fibers), the remainder being fast only. Fast/slow fibers formed from chicken satellite cells expressed slow MHC1, while slow MHC2 predominated in fast/slow fibers formed from quail satellite cells. Prolonged primary culture did not alter the relative proportions of fast to fast/slow fibers in high density cultures of either chicken or quail satellite cells. No change in commitment was observed in fibers formed from chicken satellite cell progeny repeatedly subcultured at high density, while fibers formed from subcultured quail satellite cell progeny demonstrated increasing commitment to fast/slow fiber type formation. Quail satellite cells cloned from high density cultures formed colonies that demonstrated a similar change in commitment from fast to fast/slow, as did serially subcloned individual satellite cell progeny, indicating that the observed change from fast to fast/slow differentiation resulted from intrinsic changes within a satellite cell. Thus satellite cells freshly isolated from adult chicken and quail are committed to form fibers of at least two types, satellite cells of these two types are found in different proportions in fast and slow muscles, and repeated cell proliferation of quail satellite cell progeny may alter satellite cell progeny to increasingly form fibers of a single type.

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.

Myoblasts, myosins, MyoDs, and the diversification of muscle fibers

Distinct types of muscle fibers form and become innervated by appropriate motor neurons during development. Though the activity pattern of the innervating motor neuron affects fiber type in the adult, it is now clear that innervation is not required for the initial formation of fast and slow muscle fibers during embryonic and fetal development. In addition, multiple types of intrinsically different myoblasts are found at different stages of development and motor neurons may preferentially innervate specific types of muscle fibers at relatively early stages of myogenesis. Thus, at least some of the information required for the formation of specific motor units must be carried by muscle cells. Cellular and molecular analyses of the multiple types of myoblasts, myosin heavy chain isoforms, and myogenesis regulating proteins of the MyoD family are leading to a new understanding of the events that choreograph the formation of fast and slow motor units.

Accelerated age-related decline in replicative life-span of Duchenne muscular dystrophy myoblasts: implications for cell and gene therapy

An assessment of the replicative life-span of myoblasts is of fundamental importance in designing treatment strategies for Duchenne muscular dystrophy (DMD) based on cell or gene therapy. To ascertain myoblast life-span, or the total number of cell divisions of which a myoblast was capable, we serially passaged and counted the progeny of individual myoblasts until they senesced. We compared the life-span of myoblasts from eight DMD patients with controls: three individuals with no known neuromuscular disease, three DMD carriers, and three patients with other muscle degenerative diseases. A decline in replicative capacity was observed with increasing donor age, which was markedly accelerated for DMD relative to control myoblasts. The average myoblast from a 5-year-old control was capable of 56 doublings, or a potential yield of approximately 10(17) cells per cell. By contrast, at 2 years of age, the typical age at clinical onset, only 6% of DMD myoblasts had a life-span of 50 doublings in tissue culture, and by age 7 DMD myoblasts capable of 10 doublings were rare. Our results suggest that the myoblasts (satellite cells) of even the youngest DMD patients have undergone extensive division in an attempt to regenerate degenerating myofibers. These findings have implications for therapeutic intervention in DMD involving genetic engineering and myoblast implantation.

Myogenic programs of mouse muscle cell lines: expression of myosin heavy chain isoforms, MyoD1, and myogenin

Different mouse muscle cell lines were found to express distinct patterns of myosin heavy chain (MHC) isoforms, MyoD1, and myogenin, but there appeared to be no correlation between the pattern of MHC expression and the patterns of MyoD1 and myogenin expression. Myogenic cell lines were generated from unconverted C3H10T1/2 cells by 5-azacytidine treatment (Aza cell lines) and by stable transfection with MyoD1 (TD cell lines) or myogenin (TG cell lines). Myogenic differentiation of the newly generated cell lines was compared to that of the C2C12 and BC3H-1 cell lines. Immunoblot analysis showed that differentiated cells of each line expressed the embryonic and slow skeletal/beta-cardiac MHC isoforms though slow MHC was expressed at a much lower, barely detectable level in BC3H-1 cells. Differentiated cells of each line except BC3H-1 also expressed an additional MHC(s) that was probably the perinatal MHC isoform. Myogenin mRNA was expressed by every cell line, and, with the exception of BC3H-1 (cf., Davis, R. L., H. Weintraub, and A. B. Lassar. 1987. Cell. 51:987-1000), MyoD1 mRNA was expressed by every cell line. To determine if MyoD1 expression would alter the differentiation of BC3H-1 cells, cell lines (termed BD) were generated by transfecting BC3H-1 cells with MyoD1 under control of the beta-actin promoter. The MyoD1 protein expressed in BD cells was correctly localized in the nucleus, and, unlike the parental BC3H-1 cell line that formed differentiated MHC-expressing cells, which were predominantly mononucleated, BD cell lines formed long, multinucleated myotubes (cf., Brennan, T. J., D. G. Edmondson, and E. N. Olson. 1990. J. Cell. Biol. 110:929-938). Despite the differences in morphology and MyoD1 expression, BD myotubes and the parent BC3H-1 cells expressed the same pattern of sarcomeric MHCs.

Localized expression of a myogenic regulatory gene, qmf1, in the somite dermatome of avian embryos

qmf1 is a quail myogenic regulatory gene that is transcribed in skeletal myoblasts and differentiated muscle and shows sequence homology to MyoD1 and Myf5. We used the qmf1 transcript as an in situ hybridization marker for determined myogenic cells to study myogenic lineages in developing embryos. We present evidence for the temporal and spatial regulation of qmf1 mRNA expression and slow cardiac troponin C (TnC), fast skeletal troponin T (TnT), and alpha-cardiac actin contractile protein mRNA expression in the somite myotome and limb buds. Our results show that qmf1 is a marker for myogenic lineages during both somite formation and limb development and that qmf1 mRNAs, but not contractile protein mRNAs, localize in dorsal medial lip (DML) cells of the somite dermatome. We propose that the DML is a site of myogenic lineage determination.

Multiple cellular processes regulate expression of slow myosin heavy chain isoforms during avian myogenesis in vitro

We analyzed the expression of two slow myosin heavy chain isoforms (slow MHC1 and slow MHC2) in myotubes formed from embryonic and fetal chicken myoblasts in vitro and found that the myotubes formed from myoblasts of these two developmental periods had different patterns of expression of slow MHCs. In cultures of myogenic cells from the Embryonic Day 5 (E5) hindlimb, immunoblot analysis showed that two slow MHCs with the immunological and electrophoretic properties like those of slow MHC1 and slow MHC2 were expressed on Day 3 of culture and that both slow MHCs continued to be expressed through 10 days of culture. In cultures of myogenic cells from the fetal (E12) thigh, in contrast, slow MHC1 was not expressed on Day 3 of culture, but was expressed after Day 7; slow MHC2 was never expressed by myotubes formed from fetal myoblasts. Immunocytochemistry was used to further analyze slow MHC1 and slow MHC2 expression in individual myotubes formed from embryonic and fetal myoblasts in both clonal cultures and high density, cytosine arabinoside-treated cultures. These analyses showed that (i) E5 embryonic myoblasts were of two principal types, those that formed myotubes that expressed isoforms like slow MHC1 and MHC2 throughout the life of the myotube, and those that formed myotubes that did not express slow MHCs at any time; and (ii) E12 fetal myoblasts formed myotubes that at first expressed only fast MHC but expressed both fast MHC and slow MHC1--but not slow MHC2--as culture duration was lengthened. Thus, the expression patterns of slow MHC1 and slow MHC2-like isoforms appeared to be regulated by different cellular processes in myotubes formed from embryonic and fetal myoblasts.

Regulation of protein synthesis and degradation in L8 myotubes. Effects of serum, insulin and insulin-like growth factors

We have examined the regulation of protein turnover in rat skeletal myotubes from the L8 cell line. We measured protein synthesis by the rates of incorporation of radiolabelled tyrosine into protein in the presence of a flooding dose of non-radioactive tyrosine. We monitored degradation of proteins labelled with radioactive tyrosine by the release of acid-soluble radioactivity into medium containing excess nonradioactive tyrosine. Extracellular tyrosine pools and intracellular tyrosyl-tRNA equilibrate rapidly during measurements of protein synthesis, and very little reutilization of the radiolabelled tyrosine occurs during degradation measurements. Measured rates of protein synthesis and degradation are constant for several hours, and changes in myotube protein content can be accurately predicted by the measured rates of protein synthesis and degradation. Most of the myotube proteins labelled with radioactive tyrosine for 2 days are degraded, with half-lives (t1/2) of approx. 50 h. A small proportion (less than 2.5%) of the radiolabelled proteins are degraded more rapidly (t1/2 less than 10 h), and, at most, a small proportion (less than 15%) are degraded more slowly (t1/2 greater than 50 h). A variety of agents commonly added to primary muscle cell cultures or to myoblast cell lines (18% Medium 199, 1% chick-embryo extract, antibiotics and antifungal agents) had no effect on rates of protein synthesis or degradation. Horse serum, fetal bovine serum and insulin stimulate protein synthesis and inhibit the degradation of long-lived proteins without affecting the degradation of short-lived proteins. Insulin-like growth factors (IGF)-1 and -2 also stimulate protein synthesis and inhibit protein degradation. The stimulation of protein synthesis and the inhibition of protein degradation are of similar magnitude (a maximum of approx. 2-fold) and display similar sensitivities to a particular anabolic agent. Insulin stimulates protein synthesis and inhibits protein degradation only at supraphysiological doses, whereas IGF-1 and -2 are effective at physiological concentrations. These and other findings suggest that IGFs may be important regulators of skeletal muscle growth during the fetal and early neonatal periods.

Antagonistic effects of laminin and fibronectin on the expression of the myogenic phenotype

Skeletal myoblasts from fetal muscle respond adversely to fibronectin and laminin substrata: when primary mouse skeletal myoblasts are plated onto laminin, more myosin and desmin-positive myoblasts (myo+ cells) develop than on plates coated with fibronectin or collagen. In clonal cultures virtually all cells differentiate into postmitotic, fusion-capable myo + myoblasts on laminin after 3 days. In contrast, on fibronectin, the majority of the cells becomes myosin- and desmin-negative, partially due to proliferation of undifferentiated myoblast precursor cells, partially due to dedifferentiation or modulation of myoblasts into fibroblast-like myo- cells. Loss of the myogenic phenotype on fibronectin was also observed in cloned mouse myoblasts and in cultures of a differentiating mouse satellite cell line, MM14Dy, confirming that the appearance of desmin-negative cells is a result of myoblast modulation and not due simply to overgrowth by muscle fibroblasts. In the light of other effects of laminin on myoblasts, such as the stimulation of migration, differentiation and proliferation, our findings are consistent with the notion that laminin and fibronectin may be counteracting factors in the control of muscle differentiation.

Intracranial trigeminal nerve rhabdomyoma/choristoma in a child: a case report and discussion of possible histogenesis

Rhabdomyomas are rare tumors that usually arise within the heart, orocervical, or vulvovaginal regions. The cardiac tumors have a characteristic immature morphology, occur often in association with tuberous sclerosis, and are regarded as hamartomas rather than true neoplasms. The histogenesis of the extracardiac tumors and their true neoplastic nature are matters of controversy. We report the first case of a rhabdomyoma located inside the cranium. The intimate association with the mandibular division of the trigeminal nerve, the normal embryogenesis of the craniofacial muscles, and animal homograft and xenograft experiments provide a framework for considering this tumor, and possibly other rhabdomyomas, as a choristoma/hamartoma rather than a true neoplasm.

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.

The expression of slow myosin during mammalian somitogenesis and limb bud differentiation

The developmental pattern of slow myosin expression has been studied in mouse embryos from the somitic stage to the period of secondary fiber formation and in myogenic cells, cultured from the same developmental stages. The results obtained, using a combination of different polyclonal and monoclonal antibodies, indicate that slow myosin is coexpressed in virtually all the cells that express embryonic (fast) myosin in somites and limb buds in vivo as well as in culture. On the contrary fetal or late myoblasts (from 15-d-old embryos) express in culture only embryonic (fast) myosin. At this stage, muscle cells in vivo, as already shown (Crow, M.T., and F.A. Stockdale. 1986. Dev. Biol. 113:238-254; Dhoot, G.K. 1986. Muscle & Nerve. 9:155-164; Draeger, A., A.G. Weeds, and R.B. Fitzsimons. 1987. J. Neurol. Sci. 81:19-43; Miller, J.B., and F.A. Stockdale. 1986. J. Cell Biol. 103:2197-2208), consist of primary myotubes, which express both myosins, and secondary myotubes, which express preferentially embryonic (fast) myosin. Under no circumstance neonatal or adult fast myosins were detected. Western blot analysis confirmed the immunocytochemical data. These results suggest that embryonic myoblasts in mammals are all committed to the mixed embryonic-(fast) slow lineage and, accordingly, all primary fibers express both myosins, whereas fetal myoblasts mostly belong to the embryonic (fast) lineage and likely generate fibers containing only embryonic (fast) myosin. The relationship with current models of avian myogenesis are discussed.

Posttranscriptional control of embryonic rat skeletal muscle protein synthesis. Control at the level of translation by endogenous RNA

The onset of muscle cell differentiation is associated with increased transcription of muscle-specific mRNA. Studies from this laboratory using 19-d embryonic rat skeletal muscle, suggest that additional, posttranscriptional controls regulate maturation of muscle tissue via a quantitative effect upon translation, and that the regulatory component may reside within the poly A- RNA pool (Nathanson, M.A., E.W. Bush, and C. Vanderburg. 1986. J. Biol. Chem. 261:1477-1486). To further characterize muscle cell translational control, embryonic and adult total RNA were separated into oligo(dT)cellulose-bound (poly A+) and -unbound (poly A-) pools. Unbound material was subjected to agarose gel electrophoresis to resolve constituents of varying molecular size and mechanically cut into five fractions. Material of each fraction was electroeluted and recovered by precipitation. Equivalent loads of total RNA from 19-20-d embryonic rat skeletal muscle exhibited a 40% translational inhibition in comparison to its adult counterpart. Inhibition was not due to decreased message abundance because embryonic, as well as adult muscle, contained equivalent proportions of poly A+ mRNA. An inhibition assay, based upon the translatability of adult RNA and its inhibition by embryonic poly A- RNA, confirmed that inhibition was associated with a 160-2,000-nt poly A- fraction. Studies on the chemical composition of this fraction confirmed its RNA composition, the absence of ribonucleoprotein, and that its activity was absent from similarly fractionated adult RNA. Rescue of inhibition could be accomplished by addition of extra lysate or mRNA; however, smaller proportions of lysate were required, suggesting a strong interaction of inhibitor and components of the translational apparatus. Additional studies demonstrated that the inhibitor acted at the level of initiation, in a dose-dependent fashion. The present studies confirm the existence of translational control in skeletal muscle and suggest that it operates at the embryonic to adult transition. A model of muscle cell differentiation, based upon transcriptional control at the myoblast level, followed by translational regulation at the level of the postmitotic myoblast and/or myotube, is proposed.

Clonal analysis of vertebrate myogenesis. VIII. Fibroblasts growth factor (FGF)-dependent and FGF-independent muscle colony types during chick wing development

The effect of bovine fibroblast growth factor (FGF) on the in vitro differentiation of various stage-specific populations of skeletal muscle colony-forming (MCF) cells from the developing chick wing bud was examined. The results show that bovine FGF (3 ng/ml daily) delays the onset of differentiation of MCF cells obtained from Day 4-12 wing buds by about 1 day; but, in addition, the results demonstrate that a subset of colony-forming cells derived from stage 23-27 (Day 4-5) embryos require FGF for myogenic differentiation. The FGF-dependent MCF cells attach and grow in the absence of FGF, but do not differentiate unless given FGF within 1-3 days after inoculation. Thus, between stages 23 and 27 the myogenic population contains discrete subclasses that are FGF dependent and others that are FGF independent. Both subclasses are found within two of the previously classified MCF cell populations, the early and late MCF cells. FGF-dependent and independent early MCF cells are present within the wing bud until stage 25, after which only the FGF-independent early MCF subclass persists. Similarly, both FGF-dependent and -independent late MCF cells are present between stages 25 and 27, but only the FGF-independent late MCF subclass remains after stage 31. The mechanisms responsible for relative changes in the proportions of MCF cell subclasses and for the FGF requirements are not understood. In addition, while FGF is required, there is no evidence suggesting that FGF triggers skeletal muscle terminal differentiation within the FGF-dependent MCF cell subclasses.

Myogenicity in vitro and in vivo of mouse muscle cells separated on discontinuous Percoll gradients

Mouse muscle cells, obtained by enzymatically disaggregating newborn mouse muscle, were separated on a discontinuous Percoll gradient. The myogenicity in vitro of the resultant cell fractions was examined by counting the percentage of nuclei in myotubes. Myogenicity in vivo was assessed by implanting a cell suspension of one of the allotypes of glucose-6-phosphate isomerase (GPI) into a regenerating skeletal muscle graft of a second GPI allotype: the finding of hybrid GPI indicated that the implanted cells were myogenic. Separation of mouse muscle cells on a discontinuous Percoll gradient gave rise to two myogenic fractions, one of which was more myogenic in vitro than were the unseparated cells and one of which was less myogenic. Both of these fractions were myogenic in vivo. A cell fraction was also produced which was non-myogenic in vitro as well as in vivo. In vitro and in vivo measurements of myogenicity were therefore in broad agreement.

Control of myogenesis in the mouse myogenic C2 cell line by medium composition and by insulin: characterization of permissive and inducible C2 myoblasts

Using subcloning and manipulations of culture conditions we have isolated from the mouse myogenic cell line C2 a variant cell line that we named inducible. Unlike the progenitor cells that are referred to as permissive, inducible myoblasts differentiate poorly in Dulbecco modified Eagle medium plus fetal calf serum (FCS) and require the presence of insulin at a high concentration (1.6 10(-6) M) or insulin-like growth factor I (IGFI) at a lower concentration (2.5 10(-8) M) to differentiate. Permissive and inducible myoblasts fail to differentiate when grown in MCDB202 medium plus 20% FCS, even after a prolonged arrest in G1 phase. This shows that an arrest in G1 is in itself insufficient to trigger terminal differentiation. Both cell types also exhibit distinct patterns of accumulation of muscle mRNAs corresponding to sarcomeric actins and myosin light chain MLC1A. The possibility that these two cell lines might represent two different stages of the progression of myoblasts toward terminal differentiation is discussed.