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Figure 2. Direction vs. position on the Left-Right axis. (A) A tethered chiral structure can give information as to which direction is Right or Left. However, gastrulating embryos exhibit paired cell fields on either side of the midline, only one of which expresses certain genes. (B) For example, Nodal, a TGF-ß family member, is expressed in a small and large domain (green arrows) on only one side of the midline. This requires that cells know their position with respect to the midline; this information is different from direction, and requires coordination with respect to the whole embryo (in contrast to the localized directional information generated by a chiral molecule).
