Dix D J, Eisenberg B R
Department of Physiology and Biophysics, University of Illinois School of Medicine, Chicago 60612.
J Cell Biol. 1990 Nov;111(5 Pt 1):1885-94. doi: 10.1083/jcb.111.5.1885.
Myofiber growth and myofibril assembly at the myotendinous junction (MTJ) of stretch-hypertrophied rabbit skeletal muscle was studied by in situ hybridization, immunofluorescence, and electron microscopy. In situ hybridization identified higher levels of myosin heavy chain (MHC) mRNA at the MTJ of fibers stretched for 4 d. Electron microscopy at the MTJ of these lengthening fibers revealed a large cytoplasmic space devoid of myofibrils, but containing polysomes, sarcoplasmic reticulum and T-membranes, mitochondria, Golgi complexes, and nascent filament assemblies. Tallies from electron micrographs indicate that myofibril assembly in stretched fibers followed a set sequence of events. (a) In stretched fiber ends almost the entire sarcolemmal membrane was electron dense but only a portion had attached myofibrils. Vinculin, detected by immunofluorescence, was greatly increased at the MTJ membrane of stretched muscles. (b) Thin filaments were anchored to the sarcolemma at the electron dense sites. (c) Thick filaments associated with these thin filaments in an unregistered manner. (d) Z-bodies splice into thin filaments and subsequently thin and thick filaments fall into sarcomeric register. Thus, the MTJ is a site of mRNA accumulation which sets up regional protein synthesis and myofibril assembly. Stretched muscles also lengthen by the addition of myotubes at their ends. After 6 d of stretch these myotubes make up the majority of fibers at the muscle ends. Essentially all these myotubes repeat the developmental program of primary myotubes and express slow MHC. MHC mRNA distribution in myotubes is disorganized as is the distribution of their myofibrils.
通过原位杂交、免疫荧光和电子显微镜技术，研究了拉伸肥大的兔骨骼肌肌腱结合处（MTJ）的肌纤维生长和肌原纤维组装情况。原位杂交显示，拉伸4天的纤维在MTJ处肌球蛋白重链（MHC）mRNA水平较高。对这些正在延长的纤维的MTJ进行电子显微镜观察发现，其细胞质空间较大，没有肌原纤维，但含有多核糖体、肌浆网和T膜、线粒体、高尔基体复合体以及新生的细丝组装物。电子显微镜照片计数表明，拉伸纤维中的肌原纤维组装遵循一系列特定的事件顺序。（a）在拉伸纤维的末端，几乎整个肌膜都呈电子致密状，但只有一部分附着有肌原纤维。通过免疫荧光检测发现，拉伸肌肉的MTJ膜处的纽蛋白大大增加。（b）细肌丝在电子致密部位锚定在肌膜上。（c）粗肌丝与这些细肌丝以未对齐的方式结合。（d）Z体拼接至细肌丝，随后细肌丝和粗肌丝进入肌节对齐状态。因此，MTJ是mRNA积累的部位，可启动局部蛋白质合成和肌原纤维组装。拉伸的肌肉还通过在其末端添加肌管而延长。拉伸6天后，这些肌管构成了肌肉末端大多数纤维。基本上所有这些肌管都重复初级肌管的发育程序并表达慢肌MHC。肌管中MHC mRNA的分布以及其肌原纤维的分布都是无序的。
