Tidball J G
Department of Physiological Science, University of California at Los Angeles 90024-1527.
Dev Biol. 1994 Jun;163(2):447-56. doi: 10.1006/dbio.1994.1161.
Myotendinous junctions (MTJs) are specialized sites at the surface of muscle fibers where force is transmitted between muscle and tendon. Morphogenesis of MTJs in the latter half of embryonic chick development is characterized by formation of basement membrane, appearance of subsarcolemmal densities, association of myofibrils with the membrane, and then membrane folding. In the present investigation, the time of appearance of proteins involved in force transmission at MTJs is compared to those previously described structural specializations occurring during MTJ development. In addition, P68, a recently discovered collagen-binding protein that is shown to be identical or closely related to laminin-binding lectin is also demonstrated at MTJs, and an improved technique for P68 purification is presented. P68 accumulates on the surface of collagen fibers that lie near the ends of myotubes in chick hindlimb muscle prior to the appearance of ultrastructurally discernible basement membrane. At Day 10, the stage when MTJ basement membrane is first discernible by electron microscopy, laminin and the beta 1 subunit of integrin are detectable at the MTJ. At this stage, laminin distribution coincides with P68 distribution and no laminin is detectable at sites other than the forming MTJs. Fibronectin is detectable at the MTJ at Day 12, but is not enriched at the MTJ relative to other sites on the myotubes. At Day 13, the stage at which subsarcolemmal densities first appear, talin is demonstrable at the MTJ and fibronectin concentration at the MTJ is enriched. Collagen type IV cannot be discerned at the MTJ until Day 15. Thus, the earliest demonstrable specializations at sites of MTJ formation are: (1) the appearance of P68 on collagen fibers that lie at the ends of myotubes, which occurs prior to ultrastructurally discernible specializations at the MTJ and (2) the accumulation of laminin at the MTJ with a distribution identical to that of P68 at the stage of MTJ formation in which basement membrane first appears. These findings indicate that P68 and laminin may mediate early events in MTJ assembly.
肌腱连接点（MTJ）是肌纤维表面的特殊部位，负责在肌肉和肌腱之间传递力量。胚胎期鸡发育后半段MTJ的形态发生特征为基底膜的形成、肌膜下致密物的出现、肌原纤维与膜的结合，随后是膜折叠。在本研究中，将参与MTJ力量传递的蛋白质出现时间与先前描述的MTJ发育过程中发生的结构特化时间进行了比较。此外，还在MTJ处证实了P68，一种最近发现的胶原结合蛋白，它被证明与层粘连蛋白结合凝集素相同或密切相关，并提出了一种改进的P68纯化技术。在超微结构上可辨别的基底膜出现之前，P68积聚在鸡后肢肌肉肌管末端附近的胶原纤维表面。在第10天，即MTJ基底膜首次通过电子显微镜可辨别的阶段，在MTJ处可检测到层粘连蛋白和整合素的β1亚基。在此阶段，层粘连蛋白的分布与P68的分布一致，并且在形成中的MTJ以外的部位未检测到层粘连蛋白。在第12天，在MTJ处可检测到纤连蛋白，但相对于肌管上的其他部位，MTJ处的纤连蛋白并未富集。在第13天，即肌膜下致密物首次出现的阶段，在MTJ处可证明有踝蛋白，并且MTJ处的纤连蛋白浓度增加。直到第15天才能在MTJ处辨别出IV型胶原。因此，MTJ形成部位最早可证明的特化是：（1）P68出现在肌管末端的胶原纤维上，这发生在MTJ超微结构可辨别特化之前；（2）在MTJ形成阶段，基底膜首次出现时，层粘连蛋白在MTJ处积聚，其分布与P68相同。这些发现表明，P68和层粘连蛋白可能介导MTJ组装的早期事件。
