Xenbase Image ID: 153581

Figure 2. Breaking Symmetry and Establishing Asymmetric Gene Expression in and around the Node In the mouse embryo (A), between E7.5 and E8.5, Shh is expressed in the axial mesendoderm (future notochord, in dark blue), while Fgf-8 is expressed in the primitive streak (in yellow). Shh, Fgf-8, Activin βB, and Activin receptor IIA (ActRIIA) are all expressed in the mouse node without any obvious asymmetry, and it is postulated that a leftward nodal flow (indicated by the green arrow, see main text for details) activates left-specific gene expression, represented by asymmetric expression of Nodal in and around the node. In the mouse, Shh activity is required for lefty-1 expression in the left side of the node and midline (specifically, in the left prospective floor plate; PFP). In the chick embryo (B), between stages 4 and 6, the initially symmetric expression of Shh in the node is restricted to the left side (in light blue) by an inferred Activin-like signal (possibly Activin βB), which also causes upregulation of ActRIIA on the right side (in light yellow). Shh is also expressed in the head process or nascent notochord (in dark blue). Shh acts upstream of lefty-1 expression in the left node and left side of the midline, and also acts upstream of Nodal in a domain adjacent to the left side of the node. The inferred Activin-like signal simultaneously activates Fgf-8 in the right side of the node (in light yellow); Fgf-8 is also expressed in the primitive streak (in yellow). The red squares indicate the regions of the embryos shown in detail to the right of the panels. Image published in: Capdevila J et al. (2000) Copyright © 2000. Image reproduced with permission of the Publisher, Elsevier B. V. Larger Image Printer Friendly View

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