Department of Zoology, University of Oxford, Oxford, UK.
Dev Biol. 2012 Apr 15;364(2):214-23. doi: 10.1016/j.ydbio.2012.02.002. Epub 2012 Feb 10.
In the early mouse embryo monocilia on the ventral node rotate to generate a leftward flow of fluid. This nodal flow is essential for the left-sided expression of nodal and pitx2, and for subsequent asymmetric organ patterning. Equivalent left fluid flow has been identified in other vertebrates, including Xenopus and zebrafish, indicating it is an ancient vertebrate mechanism. Asymmetric nodal and Pitx expression have also been identified in several invertebrates, including the vertebrates' nearest relatives, the urochordates. However whether cilia regulate this asymmetric gene expression remains unknown, and previous studies in urochordates have not identified any cilia prior to the larval stage, when asymmetry is already long established. Here we use Scanning and Transmission Electron Microscopy and immunofluorescence to investigate cilia in the urochordate Ciona intestinalis. We show that single cilia are transiently present on each ectoderm cell of the late neurula/early tailbud stage embryo, a time point just before onset of asymmetric nodal expression. Mapping the position of each cilium on these cells shows they are posteriorly positioned, something also described for mouse node cilia. The C. intestinalis cilia have a 9+0 ring ultrastructure, however we find no evidence of structures associated with motility such as dynein arms, radial spokes or nexin. Furthermore the 9+0 ring structure becomes disorganised immediately after the cilia have exited the cell, indicative of cilia which are not capable of motility. Our results indicate that although cilia are present prior to molecular asymmetries, they are not motile and hence cannot be operating in the same way as the flow-generating cilia of the vertebrate node. We conclude that the cilia may have a role in the development of C. intestinalis left-right asymmetry but that this would have to be in a sensory capacity, perhaps as mechanosensors as hypothesised in two-cilia physical models of vertebrate cilia-driven asymmetry.
在早期的小鼠胚胎中，腹节的纤毛向左侧旋转，产生了一个向左的液体流动。这个节点流动对于节点和 pitx2 的左侧表达以及随后的不对称器官模式形成是必不可少的。在其他脊椎动物中，包括 Xenopus 和斑马鱼，已经确定了等效的左侧流体流动，这表明它是一种古老的脊椎动物机制。在包括脊椎动物最近的亲属尾索动物在内的几种无脊椎动物中，也已经确定了不对称的节点和 Pitx 表达。然而，纤毛是否调节这种不对称的基因表达仍然未知，而且在尾索动物中的先前研究在不对称性已经长期建立之前，尚未在幼虫阶段之前鉴定出任何纤毛。在这里，我们使用扫描和透射电子显微镜和免疫荧光法研究了尾索动物 Ciona intestinalis 的纤毛。我们表明，在晚期神经胚/早期尾芽胚胎的每个外胚层细胞中都存在短暂的单个纤毛，这是开始不对称节点表达之前的时间点。将每个纤毛在这些细胞上的位置映射表明它们位于后部，这与小鼠节点纤毛的描述相同。C. intestinalis 的纤毛具有 9+0 环超微结构，但我们没有发现与运动相关的结构的证据，例如动力蛋白臂、辐条或连接蛋白。此外，纤毛退出细胞后，9+0 环结构立即变得紊乱，表明纤毛不能运动。我们的结果表明，尽管纤毛在分子不对称性之前存在，但它们不能运动，因此不能以与脊椎动物节点产生流动的纤毛相同的方式发挥作用。我们得出的结论是，纤毛可能在 C. intestinalis 左右不对称的发育中起作用，但这必须是在感觉能力方面，也许正如在脊椎动物纤毛驱动的不对称性的双纤毛物理模型中假设的那样，作为机械感受器。
