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The ability of muscle cells to respond to certain signaling molecules depends on the presence of specific proteins-receptors, ion channels and transporters, which create a system of metabolic inputs. All cells in the human body have a transmembrane signaling system that allows to get information from extracellular stimuli such as hormones, neurotransmitters, proteins, peptides, derivatives of amino acids and fat acids, sensor molecules. The results of researches published in the last two years, which expand knowledge of action of various factors on protein metabolism in skeletal muscles are presented in the review. The main attention is turned to the discovery and expansion of signaling pathways involved in the regulation of protein metabolism in skeletal muscle in different functional states of the person.

Scanning electron microscopy and histologic analysis were used in the comparative in vivo study of resorption of chitosan fibers implanted into endomysium and perimysium of a rat latissimus dorsi muscle. It was demonstrated that the mechanism and rate of chitosan fiber resorption depend on the position of fibers in muscular tissue. After implantation of chitosan fibers into endomysium (when chitosan was in direct contact with muscle fibers), the formation of cross-sectional cracks, fragmentation of implanted fibers and its partial resorption were observed in 14 days. Complete chitosan resorption in endomysium occurred after 30 days only. Chitosan fibers implanted into perimysium preserved integrity for 7 days, and fibrous tissue was formed around implants. After 45 days of exposure, no signs of chitosan fiber destruction were registered in this case. Biocompatibility of chitosan fibers proved by effective adhesion and proliferation of mesenchymal stem cells on their surface.

In this work, we have carried out a comparative analysis of the characteristics of mesenchymal stem cell lines isolated from different tissues of 5-6-weeks homan embryo: bone marrow (line FetMSC) and muscle of limb (line M-FetMSC). The basic characteristics of these lines were obtained at the 6th passage. Average population doubling time was 33.0 ± 1.4 h (FetMSC) and 25.0 ± 0.1 h (M-FetMSC). Growth curves also indicated active proliferation of cells of both lines. Numerical and structural karyotypic analysis showed that both lines have a normal karyotype: 46, XY. In order to determine the status of the lines, cell surface markers were analyzed by flow cytometry. The analysis revealed the presence of surface antigens specific for human MSCs, CD44, CD73, CD90, CD105, HLA-ABC, vimentin, and the lack of CD34 and HLA-DR, in both lines. The ability to differentiate into osteogenic, chondrogenic and adipogenic directions has been also shown for both lines. Im- munofluorescence and flow cytometry analysis has detected no expression of the surface antigen TRA-1-60 in both lines, but has revealed high expression of the surface antigen SSEA-4 and low expression of transcription factor Oct-4 characteristic of human embryonic stem cells. In these lines, immunofluorescence analysis has shown the presence of the markers of early differentiation in the derivates of three germ layers characteristic of human embryonic stem cells, which provides significant opportunities for MSC to be useful, in corresponding microenvironments, for repair of tissue injures. Dispite confirming MSC status for FetMSC and M-FetMSC lines, a number of interlinear differences related to growth characteristics and differentiation potential were revealed. Adipogenic differentiatiation potential of M-FetMSC line was reduced compared with FetMSC line. Immunofluorescence analysis showed that, in the process of skeletal-muscle differentiation, Z-disks were revealed only in sarcomeres of M-FetMSC line. These findings suggest the possible influence of different microenvironments in which the cells are in the body before their transfer in vitro.

The effect of mesenchymal stem cells (MSCs) on the growth of various tumors is ambiguous. MSCs derived from different tissues stimulate growth of some tumor types and exert antitumor effects on the other. Several recent reports have shown that crosstalk between tumor cells and MSCs contribute to these effects. The aim of this work was to study the effects of MSCs derived from fetal tissues on the proliferative activity of glioma cells in conditions of prolonged co-cultivation. We have analyzed the proliferative activity of glioma cells exposed to conditioned medium (CM) from MSCs derived from fetal bone marrow (FetMSC) and fetal muscle (M-FetMSC) as well as to CM from co-cultivation of the fetal MSCs with U251MG glioma cells. As a comparison, the influence of CM from adult dermal fibroblasts (DFs) was examined in identical experiments. Using MTT assay, we have found that CM from both the fetal MSCs and adult DFs (without their co-culturing with glioma cells) had no effect on U251MG and A172 glioma cell proliferation. However, CM from early co-cultures (3-9 days) of U251MG cells with FetMSC or M-FetMSC exerted stimulatory effect on U251MG cell proliferation up to 2.3-fold increase, while CM obtained later from the same co-cultures (15-21 days) had inhibitory effect on the proliferation up to arrest of cell division. Analogous experiments with adult DFs have revealed a persistent stimulation of U251MG cell proliferative activity for all 21 days of co-culturing. Immunofluorescence analysis revealed a reduction in the expression of cell cycle protein cyclin D1 in U251MG cells after their treatment with CM taken from 21-days co-cultures of U251MG cells with FetMSC or M-FetMSC. In contrast, CM from 21-days co-cultures of U251MG cells with DFs did not decrease the expression of cyclin D1. These results show that fetal MSCs have dual effect on glioma cell proliferation. In spite of the earlier stimulatory effect on the proliferative activity, prominent inhibition of glioma cell proliferation was observed after three week co-culturing of glioma cells with fetal MSCs. This is the first report demonstrating reversion of tumor cell proliferative program during co-culturing with MSCs for a long time. These data suggest that CM obtained at different time points of cell co-culturing can be used in the modeling of prolonged cellular crosstalk.

Enzyme AMPK is a part of the family of serine/threonine specific protein kinases. AMPK plays important role in the transfer extracellular signals through phosphorylation of multiple substrates in different metabolic reactions of skeletal muscles. AMPK is geterotrimetric complex, consisting of the catalytic subunit (AMPKalpha) and two regulatory subunits (AMPKbeta and AMPKgamma), which are encoded by seven different high-homologous genes (alpha1, alpha2, beta1, beta2, gamma1, gamma2, gamma3). AMPK regulates skeletal muscle metabolism through phosphorylation of various enzymes such as carbohydrate, lipid and protein metabolism, as well as factors of transcription and initiation. The AMPK expression occurs in response to a changing metabolic requests muscle cells and it leads to increased energy metabolism. The data of recent studies suggest the important role of AMPK in the regulation of intracellular metabolism and point to the need to study the molecular mechanisms involved in the regulation of gene expression in skeletal muscle.

In alcoholised rats, proliferation of satellite cells consistently decreased as well as the number of myonuclei, while phosphorylation of p90RSK became reduced. The mechano-growth factor abministration increased the proliferate activity of the myogenic precursors and restored the myonuclei pool. Phosphorylation of p90RSK increased too.

The investigation had the purpose to measure energy substrates in different fibers of 3- and 14-d suspended postural m. soleus and its main antagonist - anterior tibial muscle in rats. Hindlimb suspension served as a model of gravitational unloading. After suspension, left m. soleus or anterior tibial muscle was extracted. Serial cross-sections were subject to histochemical staining by Oil Red O (Sigma) for determination of intracellular triglycerides or by Schiff's reagent and iododic acid (Sigma-ALDRICH) for determination of glycogen and simultaneous immunohistochemical fibers typing by antibodies to heavy types I and II myosin chains (NCL-MHCs and NCL-MHCf, Novocastra). Analysis of intracellular energy substrates showed that both types of fibers in the anterior tibial muscle had accumulated triglycerides by week-2 of suspension. At the same time, glycogen level in equally slow and fast fibers of m. soleus and fast fibers anterior tibial muscle decreased substantially after 3 days of suspension but returned to baseline value by day 14. These changes are likely to be linked with altered activity of the muscles and hormonal factor.

Fetal liver stroma consists of different cell populations that are studied insufficiently. We have found skeletal muscle precursors that express MyoD in liver of 17 day and 20 day rat fetuses. Spontaneous myotube formation was observed in primary cultures of liver cells of 15, 17 and 20 day fetuses. Estimating antigenic profiles of these myogenic elements by immunocytochemistry and PCR methods unambiguously indicated their skeletal muscle nature. Comparative study of major myogenic gene expression demonstrated dependence of the myogenic potencies of liver cells on both the stage of fetal development and the duration of cell cultivation. It was shown that fetal liver MSCs were capable of myotube formation in the induction medium with 5-azacitidin. The results of our study, thereby, indicate that 15-20 day prenatal rat liver contains pre-existing skeletal muscle precursors expressing MyoD and, probably, inducible muscle precursors.

The aim of the present investigation was to detect the regularities of postnatal development of "motor end-plate-muscle fiber (MF)-vascular network" system in different calf muscles of intact albino rats. Gastrocnemius, plantaris and soleus muscles were studied in 72 albino rats aged from 14 to 180 days. Identification of MF type was performed on the basis of succinate dehydrogenase and NADH-diaphorase activity. Cholinesterase activity of the neuro-muscular synapse (NMS) and alkaline phosphatase activity in the vascular endothelium were demonstrated using a combined histochemical method. The diameter of vascular network and the number of enzyme-active zones (EAZ) per one MF were the earliest parameters to be stabilized (before day 30). Histochemical profile of skeletal muscle was stabilized by the end of day 60. Dynamics of MF diameter and EAZ in NMS, vessel diameter and numbers per one MF is characterized by the periods of active changes (days 14-30), decrease (days 30-60) and stabilization (after day 60) of variance rate. The association between the level of oxidative metabolism and MF diameter was demonstrated.

We suggested that satellite proliferation with subsequent incorporation of their nuclei in the fibres was essential for enhancement of the protein synthesis in the rat relieved m. soleus. In passive stretch of the muscle, the number of labelled cells exceeded 2.5-fold their number in hung out animals and 1.7-fold in the control animals. The expression level of the insulin-like growth factor I (IF-1) was determined in the m. soleus after hanging out with stretching, and no changes were revealed as compared with hanging out without stretching in the control group. We believe that the muscle IF-1 and satellite cell incorporation are not essential for preventing atrophy of relieved soleus muscle in stretching.

The effects of ultraviolet-A (UV-A, lambdam = 365 nm) and orange-red light (lambdam = 625 nm) on the attachment and proliferation of embryonic skin-muscle rat fibroblasts were investigated. It was found that orange-red light (ORL) produces both the stimulatory and inhibitory actions on the attachment and proliferation of fibroblasts, whereas UV-A radiation caused only the inhibition of the processes. Upon consecutive irradiation in both variants: ORL --> UV and UV --> ORL, the synergistic effect of the inhibitory action in both spectral regions was observed. Conversely, upon simultaneous irradiation with ORL (dose 3.6 J/cm2) and UV-A (dose 1.8 J/cm2), the inhibitory effect of ORL and UV-A weakened. Possible mechanisms of the effects are analyzed.

A cell culture consisting mainly of satellite cells and mononuclear myoblasts was derived from femoral muscles of infant (aged 3-7 days) and adult rats. Satellite cells identified by expression of the specific marker Pax7 accounted for approximately 80% of the isolated cell fraction. Mononuclear myoblasts represented by proliferating and postmitotic cell pools were identified immunocytochemically by the expression of markers Ki67 and desmin. Differentiation of satellite cells and myoblasts in the culture depended on the concentration of Ca2+ in the culture medium (F12 with different Ca2+ concentrations or DMEM). Differentiation of myogenic cells manifested in myoblasts fusion, formation of myotubes, and expression of myosin in myofibrils was observed only in the medium with a high Ca2+ concentration (2 mM). Satellite cells and myoblasts from the muscles of newborn and adult rats did not differ noticeably in their capacity for differentiation.

Dystrophin, subsarcolemmal protein communicating muscle fiber cytoskeleton to extracellular matrix, is believed to participate in mechanical signal transduction. Recent works testify possible signaling role of this protein to prevent development ofproteolytic processes accompanying muscle fiber atrophy and to stimulate the passive stretch anabolic effects. The experiment was carried out to assess the role of dystrophin in these processes. The study was performed on two months old C57 black and mdx (dystrophin-deficient) mice. Passive stretch resulted in attenuating atrophy development in two fiber types of both C57 black and mdx mice, at the same time fiber type slow-to-fast transformation did not occur in mdx soleus. We established ablatitious effect of chronic hindlimb unloading on SC proliferative activity in soleus muscle and drastic increase of proliferation under effect of passive stretch. We observed no relationship between altered dystrophin synthesis and satellite cell proliferation activity in soleus muscle under conditions of simulated microgravity and concurrent passive stretch. It is concluded that altered dystrophin synthesis partly retarded slow myofibers atrophy and had virtually no effect on passive stretch preventive action. Thus, the hypothesis about dystrophin key role in downregulation of atrophy signaling mechanisms has not found its confirmation concerning gravitational unloading atrophy.

Major aspects of the biology of muscle satellite cells are reviewed: the identification, origin in early development, mechanisms of self-renewal mediated by asymmetric divisions, content in different muscle types and in different ontogenetic stages, role of control genes of the Pax family (in particular, Pax7) and their products in proliferation control, and involvement of growth factors (HGF, FGF, IDF, and TGF-beta) in the activation of these cells after muscle damage. The characteristics of the early stages of myogenic differentiation of activated satellite cells along the pathway similar to muscle formation in embryonic development are discussed.

The development of contractile apparatus was subjected to comparative analysis during ontogenesis of the mussel Mytilus trossulus. Indirect immunofluorescence with the polyclonal antibody against mussel twitchin, a protein of thick filaments, and florescent phalloidin as a marker of filamentous cell actin were used to monitor changes in the developing muscle system at different larval stages. The first definitive muscle structures were found at the late trochophore stage (36 h after fertilization) and starting from the midveliger stage (96 h), striated muscles, which are never present in adult mussels, were distinctly seen. The striated muscle periodicity was 1.25 microm in both mussle larvae and adult scallop. The contractile activities of veliger and adult muscles were measured using an electronic signal-processing videosystem. This work is the first complex study of morphological, biochemical, and physiological characteristics of the muscle system in the larvae and adult mollusks.

N-acetylaspartylglutamate prevents the denervation-induced increase in the volume of muscle fibers in rat diaphragm, the phenomenon being more pronounced for the hydrolysable isomer. The effect of dipeptide manifested against the background of blockade of metabotropic glutamate receptors. It was hypothesized that N-acetylaspartylglutamate is involved in the regulation of the volume of skeletal muscle fibers via activation of ionotropic receptors by both dipeptide and glutamate molecules.

Effect of functional unloading on the course of degenerative process in muscles was studied in dystrophin-deficient mdx mice. Head down-hanging of animals led to a significant decrease in the cross-section area of muscle fibers, increase in the percentage of fibers with centrally located nuclei and of Evans blue-stained fibers. Gravitation unloading of 12-month-old animals with pronounced manifestations of muscular dystrophy did not inhibit this pathological process.

The effects of creatine oral supplementation combined with a 10-week resistive training of morphometric, contractile and molecular characteristics of human vast lateral muscle fibers were studied. 2 groups consisting of 9 young healthy men each were involved in resistive training of knee extensors for 10 weeks. Volunteers of the first group received per os 20 g of creatine for the 1st week of training and 5 g for the rest of the experimental training period. We found a significant increase of slow and fast-twitch fiber size in both trained groups and a significant increase of Ca-sensitivity of skinned single fiber contractility in creatine-supplemented group. The serum creatine phosphokinase activity in blood samples taken 24 hours after exercise session increased in all stages of the experimental training in both groups. At the same time, the adaptive decrease of the after-exercise CK concentration was observed in the placebo but not in the creatine-supplemented group. The altered integrity of the subsarcolemmal dystrophin layer was revealed in both groups after training.

The aim of study was to investigate the effect of oral creatine supplementation upon muscle performance and aerobic capacity of the organism. Knee extensor muscles of two groups with 9 subjects in each were subjected to strength training with and without creatine supplementation (Cre and Pla) for 10 weeks, three times a week with an effort of up to 85% of maximal voluntary contraction (MVC). The Cre group received 5 g of creatine monohydrate a day. After 10 weeks strength training, an increase of MVC by 29 and 40% in training (isotonic) regimen was recorded for the Pla and Cre groups respectively. The muscle isokinetic torque increments of 10-11% were obtained in the Pla group at angular velocities corresponding to training velocities, and in the Cre group increments of 11-17% were recorded at all angular velocities tested. No changes were found in the fatigue test by the Pla group, whereas Cre group showed a tendency for an increase. The aerobic and anaerobic capacities of the organism did not decrease in both groups. Thus the creatine supplementation during strength training potentates an increase of force-velocity characteristics of trained muscle group without impeding aerobic capacity of the organism.

Two sets of experiments were carried out. The first one involved chimeric mice, obtained by intravenously injections of bone marrow derived cells taken from transgenic C57BL/6 mice, expressing GFP, to 5 Gy X-ray irradiated mdx or C57BL/6 mice. In 2 months M. quadriceps femoris of chimeric mice were destroyed by surgical clamp. Following the next 4-5 weeks, the same muscles were studied for the presence of GFP-positive striated muscle fibres. In the case of chimeric C57BL/6 mice GFP-positive striated muscle fibres were observed in 0.3 +/- 0.5 and in 0.2 +/- 0.3 % of destroyed muscle, and in lateral (control) muscle, consequently. In the case of chimeric mdx mice, positive results were observed in 1.7 +/- 0.4 and in 0.5 +/- 0.3 % of destroyed and control muscles, respectively. In the second set of experiments, the GFP-positive bone marrow cells were used for multiple intramuscular injections to M. quadriceps femoris of C57BL/6 or mdx mice in a dose of 2 x 10(5)-5 x 10(5) cells per mouse. Before injection, GFP-positive bone marrow cells were fractionated in a 63 % Percoll solution and then were exhausted from differentiated cells by magnetic manner using CD4, CD8, CD38, CD45R, CD119, Ly-6G, and F4/80 antibodies. After 2-3 weeks, as many as 0.15 +/- 0.40 and 0.1 +/- 0.2 % of GFP-positive muscle fibres were found in injected and control muscles of C57BL/6 mice, respectively. In the case of mdx mice, the frequency of GFP-positive striated muscle fibres was 2.0 +/- 0.8 and 1.2 +/- 0.6 % for injected and control muscles, respectively. A conclusion is made that bone marrow stem cells can take part in differentiation of mdx mouse muscles after their delivery by needle injections.

We compared the effects of adenosine and cholinergic agonist carbachol on spontaneous secretion during local application of K+, ionomycin, and sucrose increasing Ca2+ concentration in the nerve terminal. Adenosine and carbachol had no effect on Ca2+ entry, but modulated later stages of exocytosis.

In a short review of literature and according to our own data the molecular mechanisms providing seasonal adjustments of the activity of membrane Ca-transporting systems (Ca-channels and Ca-pumps) in muscle tissues of animals which spent winter time in a deep torpor state (hibernation), were analyzed. Special attention was paid to changes of protein composition of sarcoplasmic reticulum membranes--the key intracellular structure responsible for excitation-contraction coupling in different types of muscles, as well as to participation of endogenous protein kinases in regulation of functional activity of plasma membrane Ca-channels and sarcoplasmic reticulum Ca-ATPase during hibernation.

The initial stages of myogenesis going in myoblasts include the stages of induction, determination, and differentiation. The induction and determination of cells in the myotomes are controlled by morphogenetic signals from neighboring tissues of the notochord and neural tube manifested as expression of genes of Shh and Wnt families, respectively. In fish (at the example of danio), this signal is passed to somite cells neighboring the notochord; later the cells migrate to the embryo surface and differentiate into slow muscle fibers. Synthesis of the main contractile proteins, primarily the components of myosin molecule--heavy chain (MHC) and individual isoforms of light chains (MLC1, MLC2, and MLC3)--are encoded by different genes during different ontogenetic stages. The peptide maps obtained after alpha-chymotrypsin digestion of MHCs from larvae, fast and slow skeletal muscle of loach are different, which points to differences in their primary structure. In addition, considerable differences were revealed in the structure of MLC isoforms at different ontogenetic stages. The definitive fast muscle contained three light chain types, MLC1, MLC2, and MLC3; slow muscle, MLC1 and MLC3; while the larval muscle fibers included a specific larval MLCL in addition to MLC3.

It was found that single total-body exposure to electromagnetic centimeter waves (8.15-18 GHz, 1 microW/cm2, 5 h) stimulated the proliferation of mouse T and B splenic lymphocytes. The same effects were observed upon in vivo treatment of rats for 5 h with millimeter waves (42.2 GHz, amplitude modulation 10 Hz, 1 microW/cm2). The whole-body irradiation with centimeter or millimeter waves did not cause any significant changes in natural activity of killer cells. The cellular responses induced by the irradiation of isolated animal cells in vitro did not coincide with those revealed after the total-body irradiation of animals. Thus, the in vitro irradiation of natural killer cells to millimeter waves for 1 h increased their cytotoxic activity whereas, after treatment to centimeter waves for the same time, the activity of killer cells did not change. On the contrary, irradiation of T and B lymphocytes with millimeter waves (42.2 GHz, amplitude modulation 10 Hz, 1 microW/cm2, 1 h) suppressed the blasttransformation of cells. The results show a higher immunostimulative potential of centimeter waves as compared to millimeter waves.

Are cited data about identification regulators of materials non-mediators of the nature executing direct and return (ortho- and retrograde) interplay of motoneurons and myocytes of a skeletal musculation. Neuro- and myotrophogenes are submitted by polypeptide materials dispossessed by specific specificity. The definite functional properties and endocellular processes in muscle cages and motoneurons are adjusted by miscellaneous kinds conforming neuro- and myotrophogenes.

Slovak experiment Quail SK-6 aboard orbital station Mir in the period of February 20-27, 1999 resulted in hatching out of viable chicks of the Japanese quail. The paper presents morphologic results of investigation of m. gastrocnemius of the chicks after 4-5-d life in space microgravity. On the whole, histologically the gastrocnemius of the space chicks was very much similar to that of the control group. Muscular fibers were, which is typical, striated; bundles of myofibrills and numerous nuclei were on the periphery of the fiber under the sarcolemma. However, despite the similarity of the muscular histology, the chicks borne and exposed in microgravity displayed some deviations in the gastrocnemius as compared to their controls. Thus, Z-lines in some of the muscular fibers had a zigzag form or were doubled. In places, mitochondria were increased in size and lined up in chains longer than a sarcomere; internal membranes of the mitochondria were thickened and vacuolized, there were expanded tubules in the sarcotubular apparatus, and muscular fibers with multiple fatty vacuoles.

A myogenic differentiation program can be realized during the cultivation of Mytilus trossulus cells derived from larvae in premyogenic developmental stages. About 10-15% of cells in such cultures showed that they are capable of contracting actively. The shape of such cells and the high concentration of actin microfilaments indicate a similarity with smooth muscle cells. However, the pattern of contractile activity and the protein composition of these cells differ significantly from the corresponding characteristics of differentiated smooth muscle cells. The proportion between the main proteins of the thick fiber, paramyosin, and myosin in cultivated cells is far lower than in the muscles of larvae or adult molluscs. We also found that substrates with different adhesional characteristics may determine cell development towards one or the other phenotype. Cells attached to the collagen substrate, but not spread on it, had high proliferative potential; the collagen substrate, however, inhibited myogenic differentiation.

Based on originally designed technique of myoblast cultivation and in accordance with the approved by the Russian Ministry of Health "one muscle treatment" protocol of myoblast transplantation to the Duchenne muscular dystrophy patients, the first in Russia clinical trial of this gene correction method was carried out. Immonologically related myoblast cultures (30 to 90 million cells per patient) were injected after all preliminary procedures into tibialis anterior muscles of four boys selected from a group of volunteer recipients (Duchenne muscular dystrophy patients) based on the analysis of a number of surface antigens in donor-recipient pairs. The condition of the patients remained satisfactory during the whole period of post-transplantation follow-up (from 6 months to 1.5 years). Six months after myoblast transplantation the presence of donor DNA or dystrophin synthesis was demonstrated in muscle biopsies of three out of four patients. This result confirms efficacy and safety of the procedure used.

Pharmacological and kinetic properties of the inward rectifier potassium current Iir the frog embryonic skeletal myocytes were found to be identical to those of adult frog skeletal muscle fibres. The data obtained suggest that the Iir plays the main role in maintaining the myocytes resting membrane potential (RMP) when chloride conductance is insignificant. Changes of the integral conductance Gir and the RMP values correlated with the T-system development. The inward rectifier K+ channels, from the early stages of the muscle seem to be located in the T-tubule membranes.

Molecular-biological and molecular-genetic mechanisms underlying regulation of muscle formation were analyzed from the earliest stages of commitment and determination of myogenic precursors to the terminal stages of muscle differentiation. It was shown how inductive influences of the neural tube and notochord on the somites leading to formation of myotomes are related to expression of genes of the Shh and Wnt families. Differentiation of proliferating myoblasts and myotubes is related to expression of the genes encoding synthesis of bHLH proteins: MyoD and Myf5 in the myoblasts and myogenin and MRF4 in the myotubes. The products of these genes are transcription factors and control the subsequent stages of myogenesis, specifically, myogenin controls expression of the genes of contractile proteins. Changes in the spectra of contractile proteins during ontogenesis and diversity of their isoforms are related both to the expression of various genes and to their alternative splicing. The mechanisms underlying the assembly of myofibrils in ontogenesis, which involve, in addition to myosin and actin, proteins of Z-bands and A-discs, were analyzed.

The development of membrane structures, providing E-C coupling, and the contractile apparatus organization were investigated in frog skeletal myocytes cultured for 1 to 10 days in conditions preventing both myocyte division and fusion. Ruthenium red was used to determine the membranous structures being in contact with the extracellular environment. The marked membrane structures (vesicles and short tubules) appeared to be near the cell membrane on the first days of culturing. The increase in the ratio of the surface area of all internal membranous structures, marked by Ruthenium red, to the external membrane area with aging was proven by morphometric calculations, that means a progressive development. Contractile filaments were found near the cell membrane on the first days of development. Bundles of filaments with initial signs of sarcomere organization were observed on the 3rd-4th days, and myofibrils with highly organized sarcomeres occupied the main part of the sarcoplasm on the 6th day of culturing. The triads appeared also on the sixth day, being regularly inserted into the sarcomere structure. Degenerative signs in the myocytes (sarcomere disorganization and T-tubule swelling) were observed on the 8-10th days, but the area occupied by contractile elements was increased. These results show that the myocyte fusion into myotubules is not a necessary condition for either sarcomere formation, or the formation of all membranous structures providing the E-C coupling.

Quantitative changes of sodium and potassium ionic currents were studied in the frog embryonic skeletal myocytes cultures under conditions preventing cell fusion and division. During 7 days of culturing the mean values of the sodium and potassium currents densities were 7- and 1.8-fold increased, resp. In currents clamp records the APs were observed only after 5 days of culturing, and in cells with proportion of sodium and potassium conductances above the critical level.