Kusakabe Rie, Kuratani Shigeru
Laboratory for Evolutionary Morphology, Center for Developmental Biology, RIKEN, Kobe, Japan.
Dev Dyn. 2005 Dec;234(4):824-34. doi: 10.1002/dvdy.20587.
The myotome in gnathostome vertebrates, which gives rise to the trunk skeletal muscles, consists of epaxial (dorsal) and hypaxial (ventral) portions, separated by the horizontal myoseptum. The hypaxial portion contains some highly derived musculature that is functionally as well as morphologically well differentiated in all the gnathostome species. In contrast, the trunk muscles of agnathan lampreys lack these distinctions and any semblance of limb muscles. Therefore, the lamprey myotomes probably represent a primitive condition compared with gnathostomes. In this review, we compare the patterns of expression of some muscle-specific genes between the lamprey and gnathostomes. Although the cellular and tissue morphology of lamprey myotomes seems uniform and undifferentiated, some of the muscle-specific genes are expressed in a spatially restricted manner. The lamprey Pax3/7 gene, a cognate of gnathostome Pax3, is expressed only at the lateral edge of the myotomes and in the hypobranchial muscle, which we presume is homologous to the gnathostome hypobranchial muscle. Thus, the emergence of some part of a hypaxial-specific gene regulatory cascade might have evolved before the agnathan/gnathostome divergence, or before the evolutionary separation of epaxial and hypaxial muscles.
颌口类脊椎动物中产生躯干骨骼肌的肌节由轴上（背侧）和轴下（腹侧）部分组成，二者由水平肌隔分隔。轴下部分包含一些高度特化的肌肉组织，在所有颌口类物种中，其在功能和形态上都有很好的分化。相比之下，无颌七鳃鳗的躯干肌肉缺乏这些区分以及任何类似肢体肌肉的结构。因此，与颌口类相比，七鳃鳗的肌节可能代表一种原始状态。在这篇综述中，我们比较了七鳃鳗和颌口类之间一些肌肉特异性基因的表达模式。尽管七鳃鳗肌节的细胞和组织形态似乎是均匀且未分化的，但一些肌肉特异性基因以空间受限的方式表达。七鳃鳗的Pax3/7基因是颌口类Pax3的同源基因，仅在肌节的外侧边缘和鳃下肌中表达，我们推测鳃下肌与颌口类的鳃下肌同源。因此，轴下特异性基因调控级联的某些部分可能在无颌类/颌口类分化之前，或者在轴上肌和轴下肌进化分离之前就已经进化出来了。
