Bioimaging profiles in molecular and cellular adaptations of calcium release units (CRUs) in exercise trained rat skeletal muscle cells

横（t）-小管负责兴奋从肌膜快速向内扩散到肌纤维内部，并且t-小管的隔室在发育期间变得高度和有规律地组织。虽然已知骨骼肌纤维在发育过程中通过在肌腱连接处（MTJ）添加肌节而延长，但是对于在MTJ处组装新的t-小管结构不存在特定的模型。我们进行了电子显微镜检查的T-小管结构的组装在MTJ发育大鼠骨骼肌纤维。虽然纵向取向的t-小管元件只占成年肌纤维中总t-小管系统的一小部分，但在发育早期，它们在中间和MTJ区域的A-带和I-带区域都被观察到，并且在发育过程中逐渐在肌纤维的中间区域消失;然而，即使在成年肌纤维中，它们也保留在MTJ中。五分体和七分体（两个或三个T-小管元件与三个或四个末端池元件，与肌浆网的末端池紧密对齐）的频率在发育过程中下降，在中间区域的7至10周龄之间突然下降。有趣的是，虽然在早期（3至7周）发育中，中间区域的减少频率似乎高于MTJ区域，但这种模式发生了逆转，并且在后期发育（10周龄后）中MTJ的减少频率更高。在发育过程中，MTJ比纤维的中间区域更长时间地保持参与电-电耦合的未成熟膜系统的特征。因此，在出生后发育过程中，t-小管结构的组装在骨骼肌纤维的中间和MTJ区域遵循不同的过程。

The transverse (t) -tubule is responsible for the rapid inward spread of excitation from the sarcolemma to the inside of the muscle fiber, and the compartments of the t-tubule become highly and regularly organized during development. Although it is known that skeletal muscle fibers lengthen by adding sarcomeres at the myotendinous junction (MTJ) during development, no specific model exists for the assembly of new t-tubule architecture at the MTJ. We performed an electron-microscopic examination of the assembly of t-tubule architecture at the MTJ in developing rat skeletal muscle fibers. Although the longitudinally oriented t-tubule elements represent only a small fraction of the total t-tubule system in adult muscle fibers, they were observed at both A-band and I-band regions of middle and MTJ regions in early developmental stages, and gradually disappeared in the middle regions of muscle fibers during development ; however, they remained in the MTJ even in adult muscle fibers. The frequency of pentads and heptads (two or three t-tubule elements with three or four elements of terminal cisternae, closely aligned with terminal cisternae of the sarcoplasmic reticulum) decreased during development, with sudden decrease between 7 and 10 weeks of age in the middle regions. Interestingly, although the frequency of decrease appeared to be higher in the middle region than in the MTJ regions in early (3- to 7-week) development, this pattern reversed, and the frequency of decrease was higher in the MTJ in later development (after 10 weeks of age). The MTJ maintained the features of immature membrane systems involved in e-c coupling much longer than the middle region of the fiber during development. The assembly of t-tubule architecture during postnatal development thus follows different processes in the middle and MTJ regions of skeletal muscle fibers.