Kahane N, Cinnamon Y, Kalcheim C
Department of Anatomy and Cell Biology, Hebrew University-Hadassah Medical School, Jerusalem 91120, PO Box 12272, Israel.
Development. 1998 Nov;125(21):4259-71. doi: 10.1242/dev.125.21.4259.
We have shown that a subset of early postmitotic progenitors that originates along the medial part of the epithelial somite gives rise to the primary myotome (Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998). Mech. Dev. 74, 59-73). Because of its postmitotic nature, further myotome expansion must be achieved by cell addition from extrinsic sources. Here we investigate the mechanism whereby the dermomyotome contributes to this process. Using several different methods we found that cell addition occurs from both rostral and caudal edges of the dermomyotome, but not directly from its dorsomedial lip (DML). First, labeling of quail embryos with [3H]thymidine revealed a time-dependent entry of radiolabeled nuclei into the myotome from the entire rostral and caudal lips of the dermomyotome, but not from the DML. Second, fluorescent vital dyes were injected at specific sites in the dermomyotome lips and the fate of dye-labeled cells followed by confocal microscopy. Consistent with the nucleotide labeling experiments, dye-labeled myofibers directly emerged from injected epithelial cells from either rostral or caudal lips. In contrast, injected cells from the DML first translocated along the medial boundary, reached the rostral or caudal dermomyotome lips and only then elongated into the myotome. These growing myofibers had always one end attached to either lip from which they elongated in the opposite direction. Third, following establishment of the primary myotome, cells along the extreme dermomyotome edges, but not the DML, expressed QmyoD, supporting the notion that rostral and caudal boundaries generate myofibers. Fourth, ablation of the DML had only a limited effect on myotomal cell number. Thus, cells deriving from the extreme dermomyotome lips contribute to uniform myotome growth in the dorsoventral extent of the myotome. They also account for its expansion in the transverse plane and this is achieved by myoblast addition in a lateral to medial direction (from the dermal to the sclerotomal sides), restricting the pioneer myofibers to the dermal side of the myotome. Taken together, the data suggest that myotome formation is a multistage process. A first wave of pioneers establishes the primary structure. A second wave generated from specific dermomyotome lips contributes to its expansion. Because dermomyotome lip progenitors are mitotically active within the epithelia of origin but exit the cell cycle upon myotome colonization, they can only provide for limited myotome growth and subsequent waves must take over to ensure further muscle development.
我们已经表明，一部分起源于上皮体节内侧部分的有丝分裂后早期祖细胞会产生初级肌节（Kahane, N., Cinnamon, Y. 和 Kalcheim, C. (1998). Mech. Dev. 74, 59 - 73）。由于其有丝分裂后的性质，肌节的进一步扩展必须通过从外部来源添加细胞来实现。在这里，我们研究了生皮肌节对这一过程的贡献机制。使用几种不同的方法，我们发现细胞添加发生在生皮肌节的头端和尾端边缘，但不是直接来自其背内侧唇（DML）。首先，用[3H]胸腺嘧啶核苷标记鹌鹑胚胎，发现放射性标记的细胞核从生皮肌节的整个头端和尾端边缘进入肌节，而不是从背内侧唇进入。其次，将荧光活性染料注射到生皮肌节边缘的特定部位，并通过共聚焦显微镜追踪染料标记细胞的命运。与核苷酸标记实验一致，染料标记的肌纤维直接从注射的头端或尾端唇的上皮细胞中出现。相比之下，从背内侧唇注射的细胞首先沿着内侧边界移位，到达头端或尾端生皮肌节边缘，并仅在那时延伸到肌节中。这些生长中的肌纤维总是一端附着在它们向相反方向延伸的边缘上。第三，在初级肌节形成后，沿着生皮肌节极端边缘（但不是背内侧唇）的细胞表达QmyoD，支持头端和尾端边界产生肌纤维的观点。第四，切除背内侧唇对肌节细胞数量的影响有限。因此，源自生皮肌节极端边缘的细胞有助于肌节在背腹范围内均匀生长。它们还负责其在横平面上的扩展，这是通过成肌细胞从外侧向内侧方向（从真皮侧到硬骨膜侧）添加来实现的，将先驱肌纤维限制在肌节的真皮侧。综上所述，数据表明肌节形成是一个多阶段过程。第一波先驱者建立了初级结构。从特定生皮肌节边缘产生的第二波细胞有助于其扩展。由于生皮肌节边缘祖细胞在起源上皮内有丝分裂活跃，但在肌节定殖时退出细胞周期，它们只能提供有限的肌节生长，后续的波必须接管以确保进一步的肌肉发育。
