Fig. 2. Left-right organizers and the flow model of symmetry breakage. (A) Left-right organizers (LROs) come in different forms (Blum et al., 2007). In zebrafish, the LRO is known as Kupffer's vesicle and is a closed sphere. In Xenopus, the gastrocoel roof plate (GRP) acts as the LRO and is a flat triangular to diamond-shaped epithelium. In mouse, the LRO (the posterior notochord/‘node’) is an indentation at the distal tip of the egg cylinder. In all cases the LRO is positioned at the posterior pole of the notochord (gray). Axes are indicated: a, anterior; p, posterior; l, left; r, right. (B) Depiction of leftward flow at the ciliated epithelium of an LRO. Motile and polarized cilia (positioned at the posterior pole of cells) rotate in a clockwise fashion to produce a leftward fluid flow in the extracellular space. Flow is sensed by unpolarized cilia on cells bordering the LRO. In mouse and Xenopus these cilia have been described as being immotile (Boskovski et al., 2013; McGrath et al., 2003). These cells express both Nodal and the Nodal inhibitor Coco. As a result of flow, Coco becomes downregulated on the left side (Hojo et al., 2007; Nakamura et al., 2012; Schweickert et al., 2010), thereby derepressing and liberating Nodal protein. Also shown is the transfer of an unidentified asymmetric signal (likely to be Nodal protein; blue octagon labeled with question mark) to the left lateral plate mesoderm (LPM), where the Nodal cascade is induced. Nodal transfers across the somites and intermediate mesoderm (not shown) to the LPM, where it induces its own transcription and that of its feedback inhibitor Lefty as well as expression of Pitx2.
Image published in: Blum M et al. (2014)
Copyright © 2014. Image reproduced with permission of the publisher and the copyright holder. This is an Open Access article distributed under the terms of the Creative Commons Attribution License.
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