Reedy M C, Beall C
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710.
Dev Biol. 1993 Dec;160(2):443-65. doi: 10.1006/dbio.1993.1320.
In order to evaluate the effects of specific mutations on sarcomere assembly and function in vivo, we describe the course of normal development of Drosophila indirect flight muscle (IFM) in staged pupae using electron microscopy. We find that no contractile assemblies remain in larval muscle remnants invaded by imaginal myoblasts, establishing that myofibrils in IFM assemble de novo. Stress-fiber-like structures or other template structures are not prominent before or during sarcomere assembly. By 42 hr pupation (eclosion approximately 112 hr), thick and thin filaments have appeared simultaneously in slender, interdigitated arrays between regularly spaced Z-bodies. Each tiny, uniformly striated myofibril forms within a "sleeve" of microtubules, and both microtubules and myofibrils are attached to the cell membrane at each end of the fiber from the initial stages of assembly. Later in pupation, the microtubule "sleeves" disassemble. Sarcomere number appears to remain constant. We saw no evidence that terminal sarcomeres are sites for addition of new sarcomeres or that Z-lines split transversely, producing new, very short sarcomeres. Rather, initial thick and thin filaments and sarcomeres are much shorter than adult length. Sarcomere length increases smoothly and coordinately from approximately 1.7 to approximately 3.2 microns, reflecting increase in filament lengths and indicating that myosin and actin molecules must be incorporated into filaments after sarcomere formation. Myofilaments are not seen scattered in the cytoplasm at any time, nor do we detect filaments that could be in the process of being "trolleyed" along myofibrils into positions of lateral register. Myofibril diameter increases uniformly from approximately 4-thick filaments to approximately 36-thick filaments across, by peripheral addition of myofilaments. At each successive stage, all sarcomeres in a fiber attained similar length and diameter. Initial thick filaments are solid but within several hours these and all subsequently assembled thick filaments appear hollow. Initial Z-bodies do not show any internal lattice and are more irregularly shaped than adult Z-discs.
为了评估特定突变对体内肌节组装和功能的影响，我们利用电子显微镜描述了果蝇间接飞行肌（IFM）在不同发育阶段蛹期的正常发育过程。我们发现，在被成虫成肌细胞侵入的幼虫肌肉残体中没有收缩性组件残留，这表明IFM中的肌原纤维是重新组装的。在肌节组装之前或期间，应力纤维样结构或其他模板结构并不突出。到化蛹42小时（羽化约112小时）时，粗细肌丝同时出现在规则间隔的Z体之间细长的、相互交错的阵列中。每个微小的、均匀横纹的肌原纤维在微管的“套筒”内形成，并且从组装的初始阶段起，微管和肌原纤维在纤维的两端都附着在细胞膜上。在蛹期后期，微管“套筒”解体。肌节数量似乎保持不变。我们没有发现证据表明末端肌节是添加新肌节的部位，也没有发现Z线横向分裂产生新的、非常短的肌节。相反，最初的粗细肌丝和肌节比成虫的长度要短得多。肌节长度从约1.7微米平滑且协调地增加到约3.2微米，这反映了肌丝长度的增加，并表明肌球蛋白和肌动蛋白分子必须在肌节形成后整合到肌丝中。在任何时候都没有看到肌丝散在细胞质中，我们也没有检测到可能正在沿着肌原纤维“吊运”到横向对齐位置的肌丝。通过肌丝的外周添加，肌原纤维直径从约4根粗肌丝均匀增加到约36根粗肌丝。在每个连续阶段，纤维中的所有肌节都达到相似的长度和直径。最初的粗肌丝是实心的，但在几个小时内，这些以及所有随后组装的粗肌丝都呈现中空。最初的Z体没有显示任何内部晶格，并且形状比成虫的Z盘更不规则。
