The formation and positioning of cilia in Ciona intestinalis embryos in relation to the generation and evolution of chordate left-right asymmetry.
The formation and positioning of cilia in Ciona intestinalis embryos in relation to the generation and evolution of chordate left-right asymmetry.
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DOI：
10.1016/j.ydbio.2012.02.002
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发表时间：
2012-04
影响因子：
2.7
通讯作者：
Helen Thompson;M. Shaw;H. Dawe;S. Shimeld
中科院分区：
文献类型：
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作者：
Helen Thompson;M. Shaw;H. Dawe;S. Shimeld
在早期小鼠胚胎中，腹侧淋巴结上的单纤毛旋转产生向左流动的液体。这种节流对于左侧表达nodal和pitx2以及随后的不对称器官模式是必不可少的。在其他脊椎动物（包括爪蟾和斑马鱼）中也发现了等效的左流体流动，表明这是一种古老的脊椎动物机制。在一些无脊椎动物中也发现了不对称的节点和Pitx表达，包括脊椎动物的近亲——尾脊索动物。然而，纤毛是否调节这种不对称基因表达仍不清楚，先前对尾脊索动物的研究没有发现在幼虫期之前的任何纤毛，而幼虫期的不对称已经建立很久了。在这里，我们使用扫描和透射电子显微镜和免疫荧光研究了尿脊索动物的纤毛。我们发现单个纤毛在神经末梢/尾芽早期胚胎的每个外胚层细胞上短暂存在，这是在不对称淋巴结表达开始之前的一个时间点。在这些细胞上绘制每根纤毛的位置显示它们位于后位，这也描述了小鼠淋巴结纤毛的位置。大肠杆菌纤毛具有9+0环状超微结构，但我们没有发现与运动相关的结构，如动力蛋白臂、径向辐条或连接蛋白。此外，纤毛离开细胞后，9+0环结构立即紊乱，表明纤毛不具备运动能力。我们的研究结果表明，尽管纤毛在分子不对称之前就存在，但它们不具有运动性，因此不能像脊椎动物淋巴结中产生血流的纤毛那样运作。我们得出结论，纤毛可能在C.肠子左右不对称的发展中起作用，但这必须是一种感觉能力，可能像脊椎动物纤毛驱动不对称的双纤毛物理模型中假设的机械传感器一样。
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.