Embryonic-like myosin heavy chains in regenerating chicken muscle

An antibody to chicken ventricular myosin was found to cross-react by enzyme immunoassay with myosin heavy chains from embryonic chicken pectorials, but not with adult skeletal myosins. This antibody, which was previously shown to label cultured muscle cells from embryonic pectoralis (Cantini et al., J cell biol 85 (1981) 903), was used to investigate by indirect immunofluorescence the reactivity of chicken skeletal muscle cells differentiating in vivo during embryonic development and muscle regeneration. Muscle fibers in 11-day old chick embryonic pectoralis and anterior latissimus dorsi muscles showed a differential reactivity with this antibody. Labelled fibers progressively decreasgd in number during subsequent stages and disappeared completely around hatching. Only rare small muscle fibers, some of which had the shape and location typical of satellite elements, were labelled in adult chicken muscle. A cold injury was produced with dry ice in the fast pectoralis and the slow anterior latissimys dorsi muscles of young chickens. Two days after injury a number of labelled cells was first seen in the intermediate region between the outer necrotic area and the underlying uninjured muscle. These muscle cells rapidly increased in number and size, thin myotubes were seen after 3 days and by 4–5 days a superficial layer of brightly stained newly formed muscle fibers was observed at the site of the injury. Between one and two weeks after the lesion the intensity of staining of regenerated fibers progressively decreased as their size further increased. These findings indicate that an embryonic type of myosin heavy chain is transitorily expressed during muscle regeneration.     

Regenerating adult chicken skeletal muscle and satellite cell cultures express embryonic patterns of myosin and tropomyosin isoforms

Regenerating areas of adult chicken fast muscle (pectoralis major) and slow muscle (anterior latissimus dorsi) were examined in order to determine synthesis patterns of myosin light chains, heavy chains and tropomyosin. In addition, these patterns were also examined in muscle cultures derived from satellite cells of adult fast and slow muscle. One week after cold-injury the regenerating fast muscle showed a pattern of synthesis that was predominately embryonic. These muscles synthesized the embryonic myosin heavy chain, beta-tropomyosin and reduced amounts of myosin fast light chain-3 which are characteristic of embryonic fast muscle but synthesized very little myosin slow light chains. The regenerating slow muscle, however, showed a nearly complete array of embryonic peptides including embryonic myosin heavy chain, fast and slow myosin light chains and both alpha-fast and slow tropomyosins. Peptide map analysis of the embryonic myosin heavy chains synthesized by regenerating fast and slow muscles showed them to be identical. Thus, in both muscles there is a return to embryonic patterns during regeneration but this return appears to be incomplete in the pectoralis major. By 4 weeks postinjury both regenerating fast and slow muscles had stopped synthesizing embryonic isoforms of myosin and tropomyosin and had returned to a normal adult pattern of synthesis. Adult fast and slow muscles yielded a satellite cell population that formed muscle fibers in culture. Fibers derived from either population synthesized the embryonic myosin heavy chain in addition to alpha-fast and beta-tropomyosin. Thus, muscle fibers derived in culture from satellite cells of fast and slow muscles synthesized a predominately embryonic pattern of myosin heavy chains and tropomyosin. In addition, however, the satellite cell-derived myotubes from fast muscle synthesized only fast myosin light chains while the myotubes derived from slow muscle satellite cells synthesized both fast and slow myosin light chains. Thus, while both kinds of satellite cells produced embryonic type myotubes in culture the overall patterns were not identical. Satellite cells of fast and slow muscle appear therefore to have diverged from each other in their commitment during maturation in vivo.     

Myosin types in cultured muscle cells

Fluorescent antibodies against fast skeletal, slow skeletal, and ventricular myosins were applied to muscle cultures from embryonic pectoralis and ventricular myocadium of the chicken. A number of spindle-shaped mononucleated cells, presumably myoblasts, and all myotubes present in skeletal muscle cultures were labeled by all three antimyosin antisera. In contrast, in cultures from ventricular myocardium all muscle cells were labeled by anti-ventricular myosin, whereas only part of them were stained by anti-slow skeletal myosin and rare cells reacted with anti-fast skeletal myosin. The findings indicate that myosin(s) present in cultured embryonic skeletal muscle cells contains antigenic determinants similar to those present in adult fast skeletal, slow skeletal, and ventricular myosins.     

Embryonic chicken skeletal, cardiac, and smooth muscles express a common embryo-specific myosin light chain

It has been demonstrated that embryonic chicken gizzard smooth muscle contains a unique embryonic myosin light chain of 23,000 mol wt, called L23 (Katoh, N., and S. Kubo, 1978, Biochem. Biophys. Acta, 535:401-411; Takano-Ohmuro, H., T. Obinata, T. Mikawa, and T. Masaki, 1983, J. Biochem. (Tokyo), 93:903-908). When we examined myosins in developing chicken ventricular and pectoralis muscles by two-dimensional gel electrophoresis, the myosin light chain (Le) that completely comigrates with L23 was detected in both striated muscles at early developmental stages. Two monoclonal antibodies, MT-53f and MT-185d, were applied to characterize the embryonic light chain Le of striated muscles. Both monoclonal antibodies were raised to fast skeletal muscle myosin light chains; the former antibody is specific to fast muscle myosin light chains 1 and 3, whereas the latter recognizes not only fast muscle myosin light chains but also the embryonic smooth muscle light chain L23. The immunoblots combined with both one- and two-dimensional gel electrophoresis showed that Le reacts with MT-185d but not with MT-53f. These results strongly indicate that Le is identical to L23 and that embryonic chicken skeletal, cardiac, and smooth muscles express a common embryo-specific myosin light chain.     

Changes in myosin isozymes during development of chicken breast muscle

The patterns of myosin isozymes in embryonic and adult chicken pectoralis muscle were examined by electrophoresis in a non-denaturing gel system (pyrophosphate acrylamide gel electrophoresis), and both light chains and heavy chains of embryonic and adult myosin isozymes were compared. In pyrophosphate acrylamide gel electrophoresis, the predominant isozyme component in embryonic pectoralis myosin could be clearly distinguished from adult myosin isozymes. SDS-polyacrylamide gel electrophoresis indicated that the light chain composition of embryonic myosin was also different from that of adult myosin. The pattern of peptide fragments produced by myosin digestion with a-chymotrypsin differed significantly between embryonic and adult skeletal myosin. These results suggest that myosin in the embryonic pectoralis muscle is different in both light and heavy chain composition from myosin in the same adult tissue.     

Myosin heavy chain in avian muscular dystrophy corresponds to the neonatal isozyme

We have previously demonstrated, based on comparison of homologous amino acid sequences and of two-dimensional CNBr peptide gel patterns, that the myosin heavy chain in pectoralis muscles of Storrs, Connecticut dystrophic chickens is different from that of their normal controls (Huszar, G., Vigue, L., De-Lucia, J. Elzinga, M., and Haines, J. (1985) J. Biol. Chem. 260, 7429-7434). Others have shown, however, that genomic banks and mRNA complements of the control and dystrophic birds are not different. In the present studies, we have examined the hypothesis that the "dystrophic" myosin heavy chain is not a novel gene product, but is a developmental isozyme which is expressed in pectoralis muscles of adult chickens due to the dystrophic process. Two-dimensional maps of myosin heavy chain CNBr peptides were prepared from breast muscles of 17-day in ovo (embryonic), 25-day posthatch (neonatal), and adult birds of the Storrs dystrophic and of two control strains. Also, myosin and actomyosin ATPase enzymatic activities of the various preparations were determined in the pH range of 5.5 to 9.0. Analysis of the peptide maps demonstrates that the embyronic, neonatal, and control adult myosin heavy chain isozymes are distinctly different gene products with only minute variations between the respective developmental isozymes in dystrophic and control muscles. However, the pectoralis myosin heavy chain of adult dystrophic birds, which is a homogeneous isozyme population by amino acid sequences and gel patterns, corresponds to that of the neonatal-type myosin heavy chain. The ATPase properties of the embryonic, neonatal, or adult pectoralis myosins and actomyosins were not different, whether the level of specific activity or the pattern of pH activation is considered. Since the mobility of neonatal chicks (primarily neonatal-type isozymes) is not restricted, the differences in myosin heavy chain structures are part of the syndrome, but not the cause of avian muscular dystrophy.     

Purification and properties of myosin from the "hatching muscle" (m. complexus) of geese

The present work is concerned with the study of myosin fractions prepared from the hatching muscle (m. complexus) and a control muscle (m. pectoralis) of the developing goose embryo. The m. complexus attained its maximum mass at hatching and in the 4-day-old bird the mass of this muscle was only one fourth of that recorded at hatching. The m. complexus was hypertrophied already on the 21st day. At days 21, 27 and 28 of incubation and at posthatching days myosin preparations were made from both muscles. Partial purification of myosins from both sources yielded a high molecular weight fraction characteristic of the adult bird and one other protein fraction with molecular mass half of myosin. Both preparations exhibited the characteristic properties of myosin. The lower molecular weight fraction was also shown to develop filamentous aggregates as did the higher molecular-weight, gel filtrated myosin. The phosphate content of the half molecular mass myosin fraction prepared from the embryonic m. complexus at days prior to hatching was considerably higher than that of the high molecular weight fraction and the predominant component was P-Arg. Since the embryonic myosin was still not available in the m. complexus of the 4-day-old birds and the hypertrophied muscle underwent regression after hatching it appears that this myosin fraction is actively involved in breaking through the shell during the hatching period in geese.     

Assembly of avian skeletal muscle myosins: evidence that homodimers of the heavy chain subunit are the thermodynamically stable form

Using a double antibody sandwich ELISA we examined the heavy chain isoform composition of myosin molecules isolated from chicken pectoralis major muscle during different stages of development. At 2- and 40-d posthatch, when multiple myosin heavy chain isoforms are being synthesized, we detected no heterodimeric myosins, suggesting that myosins are homodimers of the heavy chain subunit. Chymotryptic rod fragments of embryonic, neonatal, and adult myosins were prepared and equimolar mixtures of embryonic and neonatal rods and neonatal and adult rods were denatured in 8 M guanidine. The guanidine denatured myosin heavy chain fragments were either dialyzed or diluted into renaturation buffer and reformed dimers which were electrophoretically indistinguishable from native rods. Analysis of these renatured rods using double antibody sandwich ELISA showed them to be predominantly homodimers of each of the isoforms. Although hybrids between the different heavy chain fragments were not detected, exchange was possible under these conditions since mixture of biotinylated neonatal rods and fluoresceinated neonatal rods formed a heterodimeric biotinylated-fluoresceinated species upon renaturation. Therefore, we propose that homodimers are the thermodynamically stable form of skeletal muscle myosin isoforms and that there is no need to invoke compartmentalization or other cellular regulatory processes to explain the lack of heavy chain heterodimers in vivo.     

Regulation of protein synthesis and accumulation during oogenesis in Xenopus laevis

The tissue and developmental distribution of the various myosin subunits has been examined in bovine cardiac muscle. Electrophoretic analysis shows that a myosin light chain found in fetal but not in adult ventricular myosin is very similar and possibly identical to the light chain found in fetal or adult atrial and adult Purkinje fiber myosins. This light chain comigrates on two-dimensional gels with the bovine skeletal muscle embryonic light chain. Thus, this protein appears to be expressed only at early developmental stages in some tissues (cardiac ventricles, skeletal muscle) but at all stages in others (cardiac atria). The heavy chains of these myosins have been examined by one- and two-dimensional polypeptide mapping. The ventricular and Purkinje fiber heavy chains are indistinguishable. They are, however, different from the heavy chain found in cultured skeletal muscle myotubes, in contrast to the situation concerning the embryonic/atrial light chain.     

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.     

Structural and functional changes of myosin during development: comparison with adult fast, slow and cardiac myosin.

ATPase (Ca²⁺ and K⁺ activated) activity of myosin prepared from muscles of 3–4 week rabbit embryos (EM) is slighly lower than that of adult fast muscle myosin (FM), but in contrast to the less active adult slow muscle myosin (SM) is stable on exposure to pH 9.2. Studies of the time course, by means of Na dodecyl-SO₄ polyacrylamide gel electrophoresis, of changes in the pattern of polypeptides released by tryptic digestion show that in this regard EM is closest to SM. The light chain complement of EM appears identical with that of FM rather than of SM or cardiac myosin (CM) by the criteria of coelectrophoresis and removal by 5,5′-dithio-2-dinitrobenzoate treatment of LC₂ except that the relative amount of LC₃ is less in EM than in FM. The staining pattern of light meromyosin (EMM) paracrystals prepared from EM is distinct from either the FM, SM or CM LMM staining pattern. These studies suggest that different genes are involved in the coding for embryonic and adult heavy chains.     

Comparative analysis of chicken atrial and ventricular myosins.

1. Structural and enzymic properties of myosins from atrial and ventricular cardiac muscle of the chicken were investigated and compared with myosins from the fast skeletal pectoralis and the slow skeletal anterior latissimus dorsi muscle. 2. The Ca2+-ATPase activity, both in function of pH and [K+], of atrial myosin closely resembled that of the fast pectoralis myosin, whereas the enzymic properties of ventricular myosin were similar to those of slow skeletal myosin. 3. By sodium dodecyl sulphate polyacrylamide gel electrophoresis on gradient gel and two-dimensional electrophoresis, involving isoelectric focusing in the first dimension and SDS gel electrophoresis in the second dimension, no difference could be demonstrated in the light-chain pattern of atrial and ventricular myosin. Complete identity was also found between anterior latissimus dorsi and cardiac light chains. 4. Electrophoretic analysis of soluble peptides released by tryptic digestion of myosin and electron microscopic study of light meromyosin paracrystals showed significant differences between the heavy chains of atrial and ventricular myosins, as well as between the heavy chains of cardiac and skeletal myosins. 5. The results confirm previous immunochemical findings and provide direct biochemical evidence for the existence of a new, unique type of myosin in the chicken atrial tissue.     

Nascent muscle fiber appearance in overloaded chicken slow-tonic muscle

The application of a weight overload to the humerus of chickens induces a hypertrophy of anterior latissimus dorsi (ALD) muscle fibers. This growth is accompanied by a rapid and almost complete replacement of one slow-tonic myosin isoform, SM-1, by another slow-tonic isoform, SM-2. In addition, a population of small fibers appears mainly in extrafascicular spaces and, concurrently, three additional myosin bands are detected by gel electrophoresis. Five antibodies against myosin heavy chain (MHC) isoforms were selected as immunocytochemical probes to determine the cellular location and nature of these myosins. The antibodies react with ventricular, fast skeletal muscle and either SM-1 or SM-2, or both the slow-tonic MHCs. The antifast and antiventricular antibodies react with myosin present in the 10-day embryonic ALD muscle but do not react with myosin in posthatch ALD muscle. The small fibers in overloaded muscle contain a myosin isoform characteristically expressed during the embryonic stage of ALD muscle development and therefore are named nascent myofibers. Some of the nascent myofibers do not react with the antibody to both slow-tonic MHCs, indicating the lack of the normal adult slow-tonic myosins which are expressed in 10-day embryos. In order to explore the origin of the nascent fibers, an electron microscopic study was performed. Stereological analysis of the existing fibers shows a stimulation of numbers and sizes of satellite cells. In addition, the volume occupied by nonmuscle and undifferentiated cells increases dramatically. Myotube formation with incipient myofibrils is seen in extrafascicular spaces. These data suggest that new muscle fiber formation accompanies hypertrophy in overloaded chicken ALD muscle and the process may involve satellite cell migration.     

Amino-acid sequence of the hinge region in chicken myosin subfragment-2

The CNBr fragments of the hinge region in the carboxyterminal portion of long subfragment-2 derived from adult chicken pectoralis muscle myosin were isolated and sequenced by conventional methods. The alignment of these fragments was deduced from the homology of their sequences with those of other myosins, so that the sequence of the hinge region consisting of 127 amino-acid residues was determined. A comparison of this sequence with that of chicken embryonic skeletal muscle, chicken gizzard muscle and rabbit cardiac muscle (alpha-myosin) shows degrees of 95%, 36% and 82% sequence identities, respectively. Furthermore, the frequency with which hydrophobic residues are present at position "a" in seven-residues repeats of this region was significantly lower than the other portions of the rod.     

ATPase activity and the contractile capacity of the muscle tissue in chick embryos during development

The value of ATPase activity of the myofibril preparations and the value and duration of actomyosin superprecipitation were estimated for different muscles during the chick embryonic development. The ATPase level increases during embryogenesis 4.5-fold, in the leg muscle this change takes place distinctly earlier than in the leg muscle. The value and rate of actomyosin superprecipitation also markedly increase, to a lesser extent for m. soleus than for m. pectoralis. It is suggested that these changes and differences are mainly due to the delay in synthesis of certain types of the embryonic myosin light chains.     

Immunochemical analysis of C-protein isoform transitions during the development of chicken skeletal muscle

Isoforms of C-protein in adult chickens which differ in fast (pectoralis major, PM) and slow (anterior latissimus dorsi, ALD) skeletal muscles can be distinguished immunochemically with monoclonal antibodies (McAbs) specific for the respective fast (MF-1) and slow (ALD-66) protein variants (Reinach et al., 1982 and 1983). The expression of these C-proteins during chick muscle development in vivo has been analyzed by immunoblot and immunofluorescence procedures. Neither MF-1 nor ALD-66 reacted with whole-cell lysates or myofibrils from PM of 12-day-old embryos. However, both McAbs bound to peptides of 145 kDa in PM from late embryonic and young posthatched chickens. All of the myofibers in these muscles reacted with both antibodies, but the binding of the anti-slow McAb (ALD-66) diminished progressively with age and was completely negative with PM by 2 weeks after hatching. In contrast, the ALD muscle from 17 days in ovo thru adulthood only reacted with ALD-66; no binding of MF-1 could be detected at these stages. Since both fast and slow myosin light chains (LC) coexist within embryonic pectoralis and ALD muscles (e.g., G. F. Gauthier, S. Lowey, P. A. Benfield, and A. W. Hobbs, 1982, J. Cell Biol.92, 471–484) yet segregate to specific fast and slow muscle fibers at different stages of development, the temporal transitions of C-protein and myosin LC were compared during myogenesis. “Slow-type” C-protein appeared after the disappearance of slow myosin light chains, whereas the accumulation of the “fast-type” light chains occurred before the expression of “fast-type” C-protein. The pattern of isoform transitions appears to be far more complex than previously suspected.     

Cardiac ventricular myosin and slow skeletal myosin exhibit dissimilar chemomechanical properties despite bearing the same myosin heavy chain isoform

The myosin II motors are ATP-powered force-generating machines driving cardiac and muscle contraction. Myosin II heavy chain isoform-beta (β-MyHC) is primarily expressed in the ventricular myocardium and in slow-twitch muscle fibers, such as M. soleus. M. soleus–derived myosin II (SolM-II) is often used as an alternative to the ventricular β-cardiac myosin (βM-II); however, the direct assessment of biochemical and mechanical features of the native myosins is limited. By employing optical trapping, we examined the mechanochemical properties of native myosins isolated from the rabbit heart ventricle and soleus muscles at the single-molecule level. We found purified motors from the two tissue sources, despite expressing the same MyHC isoform, displayed distinct motile and ATPase kinetic properties. We demonstrate βM-II was approximately threefold faster in the actin filament–gliding assay than SolM-II. The maximum actomyosin (AM) detachment rate derived in single-molecule assays was also approximately threefold higher in βM-II, while the power stroke size and stiffness of the “AM rigor” crossbridge for both myosins were comparable. Our analysis revealed a higher AM detachment rate for βM-II, corresponding to the enhanced ADP release rates from the crossbridge, likely responsible for the observed differences in the motility driven by these myosins. Finally, we observed a distinct myosin light chain 1 isoform (MLC1sa) that associates with SolM-II, which might contribute to the observed kinetics differences between βM-II and SolM-II. These results have important implications for the choice of tissue sources and justify prerequisites for the correct myosin heavy and light chains to study cardiomyopathies.     

Developmental modulation of myosin expression by thyroid hormone in avian skeletal muscle.

It is well established that a rise in circulating thyroid hormone during the second half of chick embryo development significantly influences muscle weight gain and bone growth. We studied thyroid influence on differentiation in slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of embryos rendered hypothyroid by hypophysectomy or administration of an anti-thyroid drug. The expression of native myosins and myosin light chains (MLCs) was studied by electrophoretic analysis, and the myosin heavy chain (MHC) was characterized by immunohistochemistry. The first effects of hypothyroid status were observed at day 21 of embryonic development (stage 46 according to Hamburger and Hamilton). Analysis of myosin isoform expression in PLD muscles of hypothyroid embryos showed persistence of slow migrating native myosins and slow MLCs as well as inhibition of neonatal fast MHC expression, indicating retarded differentiation of this muscle. In ALD muscle, hypothyroidism maintained fast embryonic MHC and induced noticeable amounts of fast MLCs, thus delaying slow muscle differentiation. Our results suggest that thyroid hormones play a role in modulating the appearance of neonatal fast MHC and the disappearance of isomyosins transiently present during embryogenesis. However, T3 supplemental treatment would seem to compensate in part for the effects of hypothyroidism induced by hypophysectomy, suggesting that thyroid hormone might interfere with other factors also accounting for the observed effects.     

Regional differences in the expression of myosin light chains and tropomyosin subunits during development of chicken breast muscle

Types of myosin light chains and tropomyosins present in various regions and at different developmental stages of embryonic and posthatched chicken breast muscle (pectoralis major) have been characterized by two-dimensional gel electrophoresis. In the embryonic muscle all areas appear to accumulate both slow and fast forms of mysoin light chains in addition to α and β forms of tropomyosin. During development regional differences in myosin and tropomyosin expression become apparent. Slow myosin subunits become gradually restricted to areas of the anterior region of the muscle and finally become localized to a small red strip found on its anterior deep surface. This red region is characterized by the presence of slow and fast myosin light chains, α-fast, α-slow, and β-tropomyosin. In all other areas of the muscle examined only fast myosin light chains, β-tropomyosin and the α-fast form of tropomyosin, are found. In addition, β-tropomyosin also gradually becomes lost in the posterior regions of the developing breast muscle. In the adult, the red strip area represents less than 1% of the total pectoralis major mass and of the myosin extracted from this area approximately 15% was present as an isozyme that comigrated on nondenaturing gels with myosin from a slow muscle (anterior latissimus dorsi). The red region accumulates therefore fast as well as slow muscle myosin. Thus while the adult chicken pectoralis major is over 99% fast white muscle, the embryonic muscle displays a significant and changing capacity to accumulate both fast and slow muscle peptides.     

Changes in tropomyosin subunits and myosin light chains during development of chicken and rabbit striated muscles.

We have selected tropomyosin subunits and myosin light chains as representative markers of the myofibrillar proteins of the thin and thick filaments and have studied changes in the type of proteins present during development in chicken and rabbit striated muscles. The β subunit of tropomyosin is the major species found in all embryonic skeletal muscles studied. During development the proportion of the α subunit of tropomyosin gradually increases so that in adult skeletal muscles the α subunit is either the only or the major species present. In contrast, cardiac muscles of both chicken and rabbit contain only the α subunit which remains invariant with development. Two subspecies of the α subunit of tropomyosin which differ in charge only were found in adult and embryonic chicken skeletal muscles. Only one of these subspecies seems to be common to chicken cardiac tropomyosin. With respect to myosin light chains, embryonic skeletal fast muscle myosin of both species resembles the adult fast muscle myosin except that the LC₃ light chain characteristic of the adult skeletal fast muscle is present in smaller amounts. The significance of these isozymic changes in the two myofibrillar proteins is discussed in terms of a model of differential gene expression during development of chicken and rabbit skeletal muscles.