敲低RACK1基因抑制C2C12细胞中MyoG和MHC基因表达

本实验旨在研究RNA干扰(RNAi)RACK1基因对C2C12成肌细胞中肌分化标志基因MHC和MyoG表达的影响。将人工合成的靶向RAKC1基因的3条si RNA(1,2,3)转染C2C12细胞,用RT-qPCR方法检测并筛选干扰效率最高的1条siRNA。将筛选出的si RNA转染C2C12成肌细胞并诱导细胞分化,通过RT-qPCR、Western blot和免疫荧光方法在mRNA及蛋白水平检测RACK1基因沉默后对MHC和MyoG表达的影响。此外,Western blot方法检测RACK1基因沉默后对PI3K/Akt和Erk/MAPK通路激活的影响。结果表明:RACK1-si RNA-2干扰效率最高,转染48 h后抑制率可达70%。随后,C2C12细胞转染si RNA-2,诱导分化48h后RTqPCR表明,MHC和MyoG的m RNA表达均显著性下调(P<0.05);诱导分化72 h后,Western blot和免疫荧光结果表明MHC和MyoG的蛋白表达明显低于对照组,但AKT和Erk磷酸化水平未见明显变化。上述结果表明,干扰RACK1能显著抑制MHC和MyoG的表达,提示RACK1正向调控C2C12成肌细胞的分化。     

Decorin activates Akt downstream of IGF‐IR and promotes myoblast differentiation

Decorin, a small leucine‐rich proteoglycan, plays an important role in cellular activities through modification of growth factors. It also acts as a signaling molecule to non‐muscle cells through epidermal growth factor receptor or insulin‐like growth factor I receptor (IGF‐IR). However, it is unclear if decorin acts as a signaling molecule to myogenic cells. In this study, we investigated the effect of decorin on the differentiation of myoblasts and the signaling via IGF‐IR to myogenic cells. C2C12 myoblasts cultured in media containing decorin for 72 h showed more extensive formation of multinucleated myotubes than control cells cultured in the same media without decorin. The protein expressions of myogenin and myosin heavy chian were higher in decorn‐treated cells than in control cells. These results suggest that decorin enhances the differentiation of myoblasts. Western blot analysis and immunocytochemistry showed that IGF‐IR was expressed in myoblasts and myotubes. Furthermore, Akt, which is downstream of IGF‐IR, was more phosphorylated in myoblasts cultured in media containing decorin than those in media without decorin. These results suggest that decorin activates Akt downstream of IGF‐IR and enhances the differentiation of myogenic cells.     

绵羊骨骼肌卫星细胞分离培养、鉴定与成肌诱导分化

试验旨在分离绵羊骨骼肌卫星细胞（skeletal muscle satellite cells，SMSCs），建立绵羊SMSCs体外分离、培养及鉴定体系，为后续研究提供种子细胞。以新生健康绵羊为试验动物，采用胶原酶Ⅳ和胰酶两步酶消化法和差速贴壁法分离并纯化SMSCs。用RT-PCR和免疫荧光法鉴定SMSCs标记基因配对盒基因7（paired box 7，Pax7）、结蛋白（Desmin）和生肌调节因子1（myogenic regulatory factors 1，MyoD1）的表达情况；用血清撤离法诱导SMSCs向成肌细胞方向分化，成肌诱导后观察肌管的形成，免疫荧光法检测成肌分化特异性标志肌球蛋白重链（myosin heavy chain，MHC）的表达。RT-PCR结果显示，扩增条带与预期相符，所分离细胞表达SMSCs标记基因Pax7、Desmin和MyoD1；免疫荧光鉴定结果显示，所分离细胞表达SMSCs标记蛋白Pax7、Desmin和MyoD1；成肌诱导后镜下可见细胞相互融合形成多核的肌管，并表达成肌特异性标志MHC。本试验分离了绵羊SMSCs，建立了适用于绵羊SMSCs的体外培养体系，并成功进行了成肌诱导分化，为今后研究绵羊骨骼肌生长发育机制提供了试验材料和技术支撑。     

Growth factor and cytokine interactions in myogenesis. Part I. The effect of TNF-alpha and IFN-gamma on IGF-I-dependent differentiation in mouse C2C12 myogenic cells

The aim of this study was to examine the potential interactions of IGF-I with TNF-alpha and IFN-gamma with regard to regulation of the myogenesis and proliferative potential of mouse C2C12 myoblasts. The stimulation of myogenesis by IGF-I (30 nmol/l) was manifested by an enhanced myoblast fusion and expression of myosin heavy chain (MHC) during the first 3 days of differentiation. IGF-I-dependent fusion and MHC expression was reduced by TNF-alpha and IFN-gamma. Both cytokines prevented the stimulatory effect of IGF-I on MyoD expression with minor modification of the myogenin level. Both TNF-alpha and IFN-gamma activated the expression of cyclin A in myoblasts restimulated to proliferation; however, when used in combination with IGF-I these cytokines prevented the rise in cyclin A induced by growth factor. In conclusion: i) TNF-alpha and IFN-gamma reduce IGF-I-dependent myogenesis which was manifested by the reduction of myoblast fusion and MHC cellular levels, ii) Molecular mechanisms of inhibitory action of TNF-alpha and IFN-gamma on IGF-I-mediated differentiation involve a decrease in MyoD whereas myogenin level plays a minor role, iii) TNF-alpha and IFN-gamma increase the proliferative potential of myoblasts; however, they reduced the mitogenic effect of IGF-I, manifested by a decrease of IGF-I-stimulated cyclin A expression in myoblasts reinduced to proliferation. Interactions among IGF-I and proinflammatory cytokines are therefore important to establish a number of myoblasts and the onset of myogenesis during muscle regeneration.     

In ovo L‐arginine supplementation stimulates myoblast differentiation but negatively affects muscle development of broiler chicken after hatching

In this study, we tested the hypothesis that in ovo feeding (IOF) of L‐arginine (L‐Arg) enhances nitric oxide (NO) production, stimulates the process of myogenesis, and regulates post‐hatching muscle growth. Different doses of L‐Arg were injected into the amnion of chicken embryos at embryonic day (ED) 16. After hatching, the body weight of individual male chickens was recorded weekly for 3 weeks. During in vitro experiments, myoblasts of the pectoralis major (PM) were extracted at ED16 and were incubated in medium containing 0.01 mm L‐Arg, 0.05 mm L‐Arg, and (or) 0.05 mm L‐nitro‐arginine‐methyl‐ester (L‐NAME), an inhibitor of nitric oxide synthase (NOS). When 25 mg/kg L‐Arg/initial egg weight was injected, no difference was observed in body weight at hatch, but a significant decrease was found during the following 3 weeks compared to that of the non‐injected and saline‐injected control, and this also affected the growth of muscle mass. L‐NAME inhibited gene expression of myogenic differentiation antigen (MyoD), myogenin, NOS, and follistatin, decreased the cell viability, and increased myostatin (MSTN) gene expression. 0.05 mm L‐Arg stimulated myogenin gene expression but also depressed muscle cell viability. L‐NAME blocked the effect of 0.05 mm L‐Arg on myogenin mRNA levels when co‐incubated with 0.05 mm L‐Arg. L‐Arg treatments had no significant influence on NOS mRNA gene expression, but had inhibiting effect on follistatin gene expression, while L‐NAME treatments had effects on both. These results suggested that L‐Arg stimulated myoblast differentiation, but the limited number of myoblasts would form less myotubes and then less myofibers, while the latter limited the growth of muscle mass.     

干扰lnc4351对牛骨骼肌卫星细胞增殖与成肌分化的影响

为探究长链非编码RNA(lncRNA)对牛骨骼肌卫星细胞增殖及分化的影响,本研究以牛骨骼肌卫星细胞及已建立的体外成肌诱导分化模型为基础,以前期高通量测序获得的牛骨骼肌卫星细胞分化前后表达差异倍数较大的一个预测lncRNA为靶标,对其进行生物信息学分析及亚细胞定位,命名为lnc4351。设计合成lnc4351的siRNA,转染牛骨骼肌卫星细胞,采用EdU染色的方法检测干扰lnc4351对细胞增殖的影响;对转染siRNA的牛骨骼肌卫星细胞进行体外成肌诱导分化,观察肌管的形成状态,同时采用实时荧光定量PCR和Western blotting检测分化标志因子MyoG和MHC基因的mRNA及蛋白水平的表达变化,研究干扰lnc4351对细胞分化的影响。结果显示,lnc4351位于牛的14号染色体,不具有蛋白编码潜能,是一个未报道过的lncRNA,在牛骨骼肌卫星细胞的细胞质和细胞核内均有分布,主要存在于细胞核;干扰lnc4351表达后,EdU阳性细胞比率显著下降(P<0.05),说明下调lnc4351表达显著抑制了牛骨骼肌卫星细胞的增殖;下调lnc4351表达后肌卫星细胞经诱导分化产生的肌管量呈现增多趋势,分化标志因子MyoG和MHC的蛋白水平显著或极显著上调(P<0.05;P<0.01),说明干扰lnc4351能够促进牛骨骼肌卫星细胞的成肌分化过程。本研究结果表明,干扰lnc4351表达可以抑制牛骨骼肌卫星细胞的增殖并促进其成肌分化过程,为进一步开展lncRNA对牛骨骼肌发育的调控机制及肌肉发育相关研究提供参考。     

Ghrelin stimulates myogenic differentiation in a mouse muscle satellite cell line and in primary cultures of bovine myoblasts

Ghrelin is an acylated hormone that influences food intake, energy metabolism and reproduction, among others. Ghrelin may also stimulate proliferating myoblast cell differentiation and multinucleated myotube fusion. The aim of this work was to assess the effect of human ghrelin (hGHRL) and human ghrelin fragment 1-18 (hGHRL1-18) on myoblast differentiation by means of mRNA expression and protein level. Two types of cells were tested, the cell line i28 obtained from mouse skeletal muscle and primary cultures of bovine myoblasts. Both ghrelin and its N-terminal fragment hGHRL1-18 were used at concentrations of 0, 0.01, 0.1, 1, 10 and 100 nm. Treatments were applied to pre-confluent cultures and were maintained for 4 days. We determined that between 0.1 and 100 nm, hGHRL and hGRHL1-18 had similar effects on myogenic differentiation of i28 cells (p < 0.01). On the other hand, only the higher concentrations (10 and 100 nm) of hGHRL stimulated bovine myoblast differentiation. These results could be attributed to the presence, in both i28 cells and in bovine myoblasts, of the mRNA for GHS-R1a and CD36 receptors. The use of ghrelin in livestock production is still questionable because of the limited effects shown in this study, and additional research is needed in this field.     

UBE2L3对牛骨骼肌卫星细胞增殖分化的影响

将分离的牛骨骼肌卫星细胞(BSMSCs)进行体外培养,首先检测泛素结合酶UBE2L3在BSMSCs增殖分化过程中mRNA以及蛋白表达水平的变化.设计UBE2L3的3个干扰RNA(si-UBE2 L3-1、si-UBE2L3-2、si-UBE2L3-3),对干扰效果进行筛选.构建UBE2L3过表达质粒载体pcDNA3.1...     

牛骨骼肌卫星细胞的分离鉴定和诱导分化

本研究对牛骨骼肌卫星细胞进行体外分离培养、诱导分化和鉴定,采用胶原酶和胰酶联用的酶消化法分离肌卫星细胞,应用差速贴壁法进行纯化,观察卫星细胞及诱导分化后肌管的形态结构,并利用标志基因的反转录PCR（RT-PCR）和免疫荧光染色方法对分化前后细胞进行鉴定。结果显示,分离出的肌卫星细胞呈梭形生长,生长状态良好,RT-PCR和免疫荧光染色显示肌卫星细胞Pax7和MyoD呈阳性表达,纯化后的肌卫星细胞纯度大于93%;诱导分化后,卫星细胞融合生长,形成的肌管状态良好,分化标志基因MyoG和MHC呈阳性表达。本研究建立了一套从牛肌肉组织中分离和鉴定肌卫星细胞的方法,可以为肌肉的发育分化和肉牛肉质改良研究提供良好的细胞模型。     

多浪绵羊骨骼肌卫星细胞的分离培养、鉴定及成肌诱导分化

为了在体外细胞水平模拟多浪绵羊肌肉生长发育过程,本研究以多浪绵羊为试验动物,采用胶原酶和胰酶两步酶消化法分离多浪绵羊骨骼肌卫星细胞（satellite cells,SCs）,并利用差速贴壁的方法纯化分离得到的SCs。利用免疫荧光技术检测SCs标记基因Desmin、Pax7和MyoD1的表达情况,鉴定分离得到的SCs。采用血清撤离的方法诱导SCs向成肌方向分化。通过显微镜观察和成肌分化标记基因肌球蛋白重链（myosin heavy chain,MHC）的免疫荧光,检测肌管的形成情况。通过对SCs标记基因Desmin、Pax7和MyoD1的免疫荧光鉴定,确认本研究成功分离得到多浪绵羊SCs。采用血清撤离的方法诱导SCs成肌分化,显微镜观察和MHC免疫荧光可以明显观察和检测到肌管的形成。本研究对多浪绵羊SCs成功地进行了分离和鉴定,并建立了体外培养条件下多浪绵羊SCs的成肌诱导分化。     

METTL16在鸡不同类型肌肉中的表达规律及其对肌肉功能的调控作用

旨在了解m6A甲基转移酶样蛋白16(methyltransferase-like protein 16,METTL16)在鸡不同类型肌肉组织中的表达情况及其在鸡骨骼肌功能中的调控作用。本研究利用qRT-PCR技术检测METTL16基因在120日龄广西麻鸡母鸡不同类型肌肉组织中的表达,同时利用siRNA干扰鸡原代成肌细胞中METTL16基因的表达,分析其对成肌细胞增殖、分化和肌纤维形成的影响。结果显示,METTL16基因在鸡的不同类型肌肉组织(胸大肌、胸小肌、缝匠肌、耻坐骨肌内侧肌、耻坐骨肌外侧肌、髂胫外侧肌、腓肠肌内侧肌和背阔肌)中广泛表达,其中,在白肌纤维为主的胸大肌、胸小肌和髂胫外侧肌中表达量相对更高;细胞增殖检测结果显示,干扰METTL16基因表达后,鸡成肌细胞的增殖活力受到了抑制;在成肌细胞分化后干扰METTL16基因表达,鸡成肌细胞RNA m6A甲基化水平呈下降趋势,细胞分化关键基因MyoD表达显著下降(P<0.05),fast-MyHC基因表达显著上升(P<0.001),slow-MyHC基因表达呈下降趋势。综上,METTL16基因在不同类型肌肉中的表达与其肌纤...     

Isolation,Culture,Identification and Myogenic Differentiation of Skeletal Muscle Satellite Cells in Duolang Sheep

To mimic muscle development of Duolang sheep in vitro,we employed a two-step digestion method to separate satellite cells（SCs）and a differential adhesion method to purify the cells in Duolang sheep.Moreover,observation of microscopic images and immunofluorescence were used for identifying Duolang sheep SCs and its myogenic differentiation.Using immunofluorescence for Desmin,Pax7 and MyoD1 genes,we demonstrated that these marker genes all expressed in the SCs.The SCs formed significant myotubes when the serum was withdrawal from growth media,confirmed by the immunofluorescence for MHC and microscopic images.Taken together,we ssuccessfully isolated SCs and established the myogenic differentiation of SCs.     

Proliferation of myogenic cells in the diaphragm of the fetal rabbit.

Diaphragms obtained from 40 fetal rabbits at gestational ages of 20, 22, 25 and 30 days were examined by light and transmission electron microscopes. The percentage of myogenic cells undergoing mitosis was calculated by counting mitotic nuclei in 1 micron sections. At 20 and 22 days gestation, myogenic cells (myoblasts and satellite cells) were actively proliferating with 2.66 +/- 0.41% (n = 15) and 2.18 +/- 0.20% (n = 23), of the cells in division, respectively. The myogenic cells undergoing mitosis appeared to be of both myoblasts and satellite cells at these stages of the development. The myotubes on day 20 of gestation contained a large number of lipid droplets and an abundance of glycogen particles which were reduced by day 22 of gestation. The mitotic rate on day 25 of gestation was reduced to 1.16 +/- 0.11% (n = 28). The muscle fibers were well differentiated at this stage and the majority of the dividing cells were considered to be satellite cells. In diaphragms from the full term fetuses (day 30), the mitotic rate was reduced to 0.31 +/- 0.05% (n = 24). It was suggested that myoblasts of the fetal rabbit diaphragm proliferated for the myotube formation during the earliest stage of the development (day 20 of gestation) and then the number of satellite cells increased after day 22 of gestation. Growth of the organ after day 25 of gestation appeared to be mostly due primarily to the hypertrophy and differentiation of the muscle fibers rather than proliferation of myoblasts.     

Autonomous xenogenic cell fusion of murine and chick skeletal muscle myoblasts

Cell‐cell fusion has been a great technology to generate valuable hybrid cells and organisms such as hybridomas. In this study, skeletal muscle myoblasts were utilized to establish a novel method for autonomous xenogenic cell fusion. Myoblasts are mononuclear myogenic precursor cells and fuse mutually to form multinuclear myotubes. We generated murine myoblasts (mMBs) expressing green fluorescent protein (GFP) termed mMB‐GFP, and the chick myoblasts (chMBs) expressing Discosoma red fluorescent protein (DsRed) termed chMB‐DsRed. mMB‐GFP and chMB‐DsRed were cocultured and induced to differentiate. After 24 h, the multinuclear myotubes expressing both GFP and DsRed were observed, indicating that mMBs and chMBs interspecifically fuse. These GFP⁺/DsRed⁺ hybrid myotubes were able to survive and grew to hyper‐multinucleated mature form. We also found that undifferentiated mMB‐GFP efficiently fuse to the chMB‐DsRed‐derived myotubes. This is the first evidence for the autonomous xenogenic fusion of mammalian and avian cells. Myoblast‐based fusogenic technique will open up an alternative direction to create novel hybrid products.     

Myostatin down‐regulates the IGF‐2 expression via ALK‐Smad signaling during myogenesis in cattle

Myostatin (MSTN) is a negative regulator during muscle differentiation, whereas insulin‐like growth factors (IGFs) are essential for muscle development. MSTN and IGFs act oppositely during myogenesis, but there is little information on the mutual relationship of MSTN and IGFs. The present study was conducted to examine whether MSTN affects IGF expression during early myogenesis in cattle. IGF‐1 mRNA was similarly expressed in M. longissimus thoracis of double‐muscled (DM) and normal (NM) Japanese shorthorn cattle. IGF‐2 mRNA expression was consistently higher in the normal and regenerating muscle of DM cattle than those of NM cattle. When myoblasts were isolated from regenerating M. longissimus thoracis, IGF‐2 mRNA expression showed a significant increase in differentiating DM derived myoblasts (DM‐myoblasts) as compared with differentiating NM derived myoblasts (NM‐myoblasts). An addition of recombinant mouse myostatin (rMSTN) to myoblast cultures attenuated IGF‐2 mRNA expression and decreased myotube formation, but did not effect IGF‐1 mRNA expression. An activin‐like kinase (ALK) inhibitor, SB431542, mediates MSTN action, suppressed the translocation of Smad2/3 into the nucleus in DM‐myoblasts, and restored the attenuated IGF‐2 mRNA expression and the decreased myotube formation induced by rMSTN in myoblast cultures. The findings indicate that MSTN may negatively regulate myoblast differentiation by suppressing IGF‐2 expression via ALK‐Smad signaling.     

Exposure to TNF-alpha but not IL-1beta impairs insulin-dependent phosphorylation of protein kinase B and p70S6k in mouse C2C12 myogenic cells

Tumor necrosis factor (TNF)-alpha is a proinflammatory cytokine considered to play an important role in muscle catabolism, but little is known about the mechanisms of its action. The aim of the present study was therefore to examine the effect of TNF-alpha pretreatment on glucose uptake and protein synthesis as well as the cellular content and phosphorylation of protein kinase B (PKB), p70S6k, Mitogen Activated Protein (MAP) kinase and p90rsk in mouse C2C12 myotubes stimulated with insulin. To determine whether interleukin (IL)-1beta might be involved in the catabolic action of TNF-alpha, the effects of IL-1beta were also tested. Experiments were performed on mouse C2C12 myoblasts subjected to differentiation in the presence of increasing concentrations of TNF-alpha (0.1-100 ng/ml) or IL-1 (5-50 ng/ml) for 5 or 6 days. Insulin (100 nmol/l) markedly stimulated glucose uptake in C2C12 myotubes (202.6% of control). This effect was profoundly attenuated by pretreatment with TNF-alpha at a concentration of 1 ng/ml (122.2% of control) and completely abolished by higher cytokine concentrations. Pretreatment of cells with TNF-alpha at a concentration of 1 ng/ml was also effective in diminishing the effect of insulin on protein synthesis, whereas higher cytokine concentrations prevented hormonal stimulation of protein synthesis in C2C12 myotubes. Pretreatment with TNF-alpha caused a significant decrease in PKB protein content. Insulin-mediated activation of protein kinase B was significantly diminished in cells differentiated in the presence of TNF-alpha. Treatment of C2C12 cells with insulin led to the gel mobility retardation of p70S6k indicating its phosphorylation and activation. In cells differentiated in the presence of TNF-alpha an approximately 2-fold decrease of insulin-mediated p70S6k phosphorylation was noted. Six-day differentiation of myogenic cells in the presence of TNF-alpha did not affect the protein content of p42MAPK, p44MAPK, p90rsk and phosphorylation of p42MAPK. Neither glucose uptake nor protein synthesis stimulated by insulin were affected significantly by pretreatment with IL-beta. Preincubation of myogenic cells with IL-1beta did not modify either the protein content of PKB and p70S6k or the insulin-stimulated phosphorylation of these kinases. In conclusion: i) high concentrations of TNF-alpha, but not IL-beta, present in the extracellular environment during myoblast differentiation prevent the stimulatory action of insulin on glucose uptake and protein synthesis; ii) insulin resistance induced by TNF-alpha in C2C12 myogenic cells could be associated with the decreased insulin-mediated phosphorylation of PKB and p70s6k, but not with the basal phosphorylation of p42MAPK.     

共轭亚油酸对C2C12肌细胞生脂和生肌分化的影响

本研究分析了共轭亚油酸(CLA)对C2C12肌细胞生脂转分化和生肌分化的影响。分别培养并诱导C2C12鼠源肌细胞生脂转分化和正常的生肌分化,同时分别使用终浓度为50μmol/L的c9,t11-CLA和t10,c12-CLA处理细胞,并设对照组,取生脂转分化第10天和生肌分化第8天的细胞用于实时定量PCR检测,观察c9,t11-CLA和t10,c12-CLA对C2C12肌细胞不同分化的影响。结果表明:1)与对照组相比,c9,t11-CLA促进了C2C12肌细胞的生脂转分化,显著增加了细胞内甘油三酯(TG)含量(P0.05),显著上调了细胞内脂肪酸合成酶(FAS)、CCAAT增强子结合蛋白α(C/EBPα)、过氧化物酶体增殖剂激活受体γ(PPARγ)和脂肪酸结合蛋白4(FABP4)基因的表达水平(P0.05);与对照组相比,t10,c12-CLA则抑制了C2C12肌细胞的生脂转分化,显著减少了细胞内TG含量(P0.05),显著下调了细胞内C/EBPα、PPARγ和FA BP4基因的表达水平(P0.05)。免疫印迹杂交结果显示FAS和FABP4的蛋白质表达水平也发生了与基因表达相一致的变化。2)与对照组相比,t10,c12-CLA抑制了C2C12肌细胞的生肌分化,显著减少了细胞内肌管数/细胞数(P0.05),显著下调了细胞内肌细胞生成素(MYOG)和成肌分化抗原(MYOD)基因的表达水平(P0.05);与对照组相比,c9,t11-CLA则显著上调了细胞内MYOG基因的表达水平(P0.05),对C2C12肌细胞的生肌分化有一定程度的促进作用。免疫印迹杂交结果显示MYOG和MYOD的蛋白质表达水平也发生了与基因表达相一致的变化。以上结果表明,CLA对动物骨骼肌细胞的正常生肌分化和生脂转分化都具有重要的调节作用。     

Mixed mesodermal and mesenchymal origin of myotomal muscles in pike (Esox lucius: Teleostei)

During the myotomal myogenesis in pike (Esox lucius) two phases of muscle differentiation can be distinguished. In the first phase, the somite cells-derived stock, the primary myoblasts (of mesodermal origin), fuse to form multinucleate myotubes. Participation of myotomal cells of mesodermal origin is insufficient for further muscle development. In the second stage mesenchymal cells migrate, via myosepts, into the myotome between myotubes. Immunocytochemical detection of proliferating cell nuclear antigen (marker of S phase of cell cycle) showed their mitotic activity. Transmission electron microscope analysis revealed that the differentiation of these cells depends on their position. Cells remaining in the myosepts develop into fibroblasts and produce collagen fibres, while those that have migrated into the myotomes transform into secondary myoblasts. Mesenchymal cells in the studied species are believed to participate in hypertrophy and hyperplasy of muscle fibres. Thus the muscle fibres in pike (E. lucius) are of mesodermal-mesenchymal origin.