Xenbase Image ID: 180083

Figure 1. Outline of the basic sequence of symmetry breaking events in the frog Xenopus. (1) Foxj1 expression (green) in the early gastrula embryo marks the LRO precursor (superficial mesoderm), from which (2) the LRO derives when the SM involutes to line the roof of the primitive gut. (3) Central LRO cells harbor motile cilia which polarize to the posterior cell poles and rotate in a clockwise manner to generate a leftward flow of extracellular fluids. Lateral LRO cells possess immotile cilia and co-express nodal1 (blue) and the Nodal inhibitor dand5 (orange). Flow sensing at the left LRO margin results in dand5 down-regulation, which allows Nodal to signal and (4) transfer to the LPM. In the left LPM, (5) Nodal signaling induces the three target genes of the nodal cascade, nodal1 itself, the feedback inhibitor lefty1 and the homeobox transcription factor pitx2. Spreading of left-asymmetric signaling is prevented by lefty1 expression at the midline (notochord and floor plate). (6) The Nodal cascade in due course drives asymmetric organ morphogenesis. Timeline on the left indicates developmental stages. An animated version of this scheme can be found on the author’s home page (https://zoologie.uni-hohenheim.de/en/agembryology_eng). Image published in: Blum M and Ott T (2019) Copyright © 2019. This image is 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. Larger Image Printer Friendly View

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