Wilson S J, McEwan J C, Sheard P W, Harris A J
Department of Physiology, University of Otago Medical School, Dunedin, New Zealand.
J Muscle Res Cell Motil. 1992 Oct;13(5):534-50. doi: 10.1007/BF01737996.
The generation of myotubes was studied in the tibialis cranialis muscle in the sheep hindlimb from the earliest stage of primary myotube formation until a stage shortly before muscle fascicles began to segregate. Primary myotubes were first seen on embryonic day 32 (E32) and reached their maximum number by E38. Small numbers of secondary myotubes were first identified at E38, and secondary myotube numbers continued to increase during the period of study. The ratio of adult muscle fibre to primary myotube numbers was approximately 70:1, making it seem unlikely that every later generation myotube used a primary myotube as scaffold for its formation, as described in small mammals. By E62, some secondary myotubes were supporting the formation of a third generation of myotubes. Experiments with diffusible dye markers showed that primary myotubes extended from tendon to tendon of the muscle, whereas most adult fibres ran for only part of the muscle length, terminating with myo-myonal attachments to other muscle fibres in a series arrangement. Acetylcholinesterase (AChE) and acetylcholine receptor (AChR) aggregations appeared in multiple bands across the muscle shortly after formation of the primary generation of myotubes was complete. The number of bands and their pattern of distribution across the muscle as they were first formed was the same as in the adult. Primary myotubes teased from early muscles had multiple focal AChE and AChR deposits regularly spaced along their lengths. We suggest that the secondary generation of myotubes forms at endplate sites in a series arrangement along the length of single primary myotubes, and that tertiary and possibly later generations of myotubes in their turn use the earlier generation myofibres as a scaffold. Although the fundamental cellular mechanisms appear to be similar, the process of muscle fibre generation in large mammalian muscles is more complex than that described from previous studies in small laboratory rodents.
在绵羊后肢的胫骨前肌中，研究了从初级肌管形成的最早阶段到肌肉束开始分离前不久的肌管生成情况。初级肌管最早在胚胎第32天（E32）出现，并在E38时达到最大数量。少量次级肌管在E38时首次被识别，并且在研究期间次级肌管的数量持续增加。成年肌纤维与初级肌管数量的比例约为70:1，这使得像在小型哺乳动物中所描述的那样，每一代后期的肌管都以初级肌管作为其形成的支架这种情况似乎不太可能。到E62时，一些次级肌管开始支持第三代肌管的形成。用可扩散染料标记物进行的实验表明，初级肌管从肌肉的肌腱延伸到肌腱，而大多数成年纤维仅在肌肉长度的一部分内延伸，以肌-肌连接的形式与其他肌肉纤维呈串联排列终止。在第一代初级肌管形成完成后不久，乙酰胆碱酯酶（AChE）和乙酰胆碱受体（AChR）聚集物出现在整个肌肉的多条带中。最初形成时，这些带的数量及其在肌肉中的分布模式与成年时相同。从早期肌肉中分离出的初级肌管在其长度上有多个沿长度方向规则间隔的局灶性AChE和AChR沉积物。我们认为，次级肌管在单个初级肌管长度方向上以串联排列的终板部位形成，并且第三代以及可能更晚代的肌管依次以较早代的肌纤维作为支架。尽管基本的细胞机制似乎相似，但大型哺乳动物肌肉中的肌纤维生成过程比先前在小型实验啮齿动物中的研究描述的更为复杂。
