Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Republic of Singapore.
Open Biol. 2013 May 29;3(5):130052. doi: 10.1098/rsob.130052.
Cilia are microtubule-based hair-like organelles that project from the surface of most eukaryotic cells. They play critical roles in cellular motility, fluid transport and a variety of signal transduction pathways. While we have a good appreciation of the mechanisms of ciliary biogenesis and the details of their structure, many of their functions demand a more lucid understanding. One such function, which remains as intriguing as the time when it was first discovered, is how beating cilia in the node drive the establishment of left-right asymmetry in the vertebrate embryo. The bone of contention has been the two schools of thought that have been put forth to explain this phenomenon. While the 'morphogen hypothesis' believes that ciliary motility is responsible for the transport of a morphogen preferentially to the left side, the 'two-cilia model' posits that the motile cilia generate a leftward-directed fluid flow that is somehow sensed by the immotile sensory cilia on the periphery of the node. Recent studies with the mouse embryo argue in favour of the latter scenario. Yet this principle may not be generally conserved in other vertebrates that use nodal flow to specify their left-right axis. Work with the teleost fish medaka raises the tantalizing possibility that motility as well as sensory functions of the nodal cilia could be residing within the same organelle. In the end, how ciliary signalling is transmitted to institute asymmetric gene expression that ultimately induces asymmetric organogenesis remains unresolved.
纤毛是一种微管为基础的毛发状细胞器，从大多数真核细胞表面伸出。它们在细胞运动、液体运输和各种信号转导途径中发挥着关键作用。虽然我们对纤毛发生的机制和其结构的细节有了很好的了解，但它们的许多功能需要更清晰的理解。其中一个功能仍然像它首次被发现时一样令人着迷，那就是节点中的摆动纤毛如何驱动脊椎动物胚胎左右不对称的建立。争论的焦点是为了解释这一现象而提出的两种思想流派。虽然“形态发生素假说”认为纤毛运动负责将形态发生素优先运输到左侧，但“双纤毛模型”则假设运动纤毛产生的向左的流体流动被节点周围的不动感觉纤毛以某种方式感知。最近对小鼠胚胎的研究支持后一种情况。然而，这一原则在其他使用节点流来指定左右轴的脊椎动物中可能并不普遍保守。与硬骨鱼斑马鱼的研究提出了一个诱人的可能性，即节点纤毛的运动和感觉功能可能存在于同一个细胞器中。最终，纤毛信号如何传递，从而建立最终诱导不对称器官发生的不对称基因表达，仍然没有解决。
