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Abstract：
Asymmetric development of the vertebrate embryo has fascinated embryologists for over a century. Much has been learned since the asymmetric Nodal signaling cascade in the left lateral plate mesoderm was detected, and began to be unraveled over the past decade or two. When and how symmetry is initially broken, however, has remained a matter of debate. Two essentially mutually exclusive models prevail. Cilia-driven leftward flow of extracellular fluids occurs in mammalian, fish and amphibian embryos. A great deal of experimental evidence indicates that this flow is indeed required for symmetry breaking. An alternative model has argued, however, that flow simply acts as an amplification step for early asymmetric cues generated by ion flux during the first cleavage divisions. In this review we critically evaluate the experimental basis of both models. Although a number of open questions persist, the available evidence is best compatible with flow-based symmetry breakage as the archetypical mode of symmetry breakage.
摘要：
脊椎动物胚胎的不对称发育使胚胎学家着迷了一个多世纪。自从检测到左外侧板中胚层中的不对称Nodal信号级联以来，已经学到了很多东西，并在过去的十年或二十年中开始瓦解。对称最初何时以及如何被打破，然而,仍然是一个争论的问题。两种基本上相互排斥的模式占上风。纤毛驱动的细胞外液向左流动发生在哺乳动物中，鱼类和两栖动物的胚胎。大量实验证据表明，这种流动确实是对称破坏所必需的。另一种模式认为，然而,该流动仅充当在第一次裂解分裂期间由离子通量产生的早期不对称线索的放大步骤。在这篇综述中，我们批判性地评估了这两种模型的实验基础。尽管一些悬而未决的问题仍然存在，现有证据与基于流的对称破坏作为对称破坏的典型模式最佳兼容。
