Flucher B E, Takekura H, Franzini-Armstrong C
Laboratory of Neurobiology, NINDS, National Institutes of Health, Bethesda, Maryland 20892.
Dev Biol. 1993 Nov;160(1):135-47. doi: 10.1006/dbio.1993.1292.
The formation and maintenance of the highly regular organization of membrane systems and proteins in striated muscle require specific membrane-membrane and membrane-cytoskeleton interactions. The development of T-tubules and sarcoplasmic reticulum (SR) was followed in gastrocnemius muscle fibers from chicken embryos between 12 days (E12) and 21 days (E21) of incubation, with particular attention to their relationship with one another and with the myofibrils. The fluorescent lipid analog DiIC16[3] was used to label either the external membranes (plasmalemma and transverse (T)-tubules) or the internal SR in living and fixed muscle. Short membrane invaginations can first be seen in fibers at E14, and at E15 longitudinal T-tubules appear in the periphery of the fibers. A complex network of T-tubules filling the whole fiber diameter develops suddenly at E16. In contrast, SR is abundant at the earliest observed stage (E12) and forms regularly spaced cross striations located at the I-Z-I bands. These correspond to a specific accumulation of smooth membranes around the Z-discs seen in electron micrographs. While SR is specifically associated with the newly formed myofibrils in the periphery of the fibers, the disposition of early T-tubules shows little specific relationship to either SR or the myofibrils. However, electron microscopy shows that junctions between T-tubules and SR are formed during this period (Takekura and Franzini-Armstrong, submitted for publication). Junctions do not acquire a specific relation to the myofibrils until around hatching when triads begin to reorganize into their mature location, the A-I junction. These findings indicate three key events in the organization of T-tubules and SR in the sarcomeres: (1) early SR/Z-line interactions independent of T-tubules; (2) SR/T-tubule interactions to form the triad junctions, independent from the myofibrils; and (3) the late association of the junctional complexes with the myofibrils at the A-I border.
横纹肌中膜系统和蛋白质高度规则组织的形成与维持需要特定的膜-膜以及膜-细胞骨架相互作用。对孵化12天（E12）至21天（E21）鸡胚腓肠肌纤维中T小管和肌浆网（SR）的发育进行了追踪，特别关注它们彼此之间以及与肌原纤维的关系。荧光脂质类似物DiIC16[3]用于标记活体和固定肌肉中的外部膜（质膜和横（T）小管）或内部SR。短膜内陷最早可在E14的纤维中看到，E15时纵向T小管出现在纤维周边。在E16时，一个充满整个纤维直径的复杂T小管网络突然形成。相比之下SR在最早观察阶段（E12）就很丰富，并形成位于I-Z-I带的规则间隔横纹。这些横纹对应于电子显微镜下在Z盘周围看到的平滑膜的特定聚集。虽然SR与纤维周边新形成的肌原纤维特异性相关，但早期T小管的分布与SR或肌原纤维几乎没有特定关系。然而，电子显微镜显示在此期间T小管和SR之间形成了连接（竹仓和弗兰齐尼-阿姆斯特朗，已提交发表）。直到孵化前后三联体开始重新组织到其成熟位置A-I连接处时，连接才与肌原纤维形成特定关系。这些发现表明了肌节中T小管和SR组织中的三个关键事件：（1）早期SR/Z线相互作用独立于T小管；（2）SR/T小管相互作用形成三联体连接，独立于肌原纤维；（3）连接复合体在A-I边界处与肌原纤维的后期关联。
