Figure 3. A model of the LR pathway based on cytoplasmic motor protein movement. This highly schematized diagram draws mainly on Xenopus embryogenesis and attempts to follow known timing data for each step. (A) In the unfertilized egg (which is thought to possess radial symmetry about the animal-vegetal axis), maternal mRNAs for key ion transporters are evenly distributed. (B) Cytoskeletal re-arrangements following fertilization set up microfilaments or microtubules which are oriented along the newly established LR axis. (C) Motor proteins (such as dynein [LRD] and kinesin [KIF-3B]) translocate along these tracks and result in an asymmetric localization of certain mRNAs. (D) These mRNAs are translated, the resulting proteins perhaps targeted to correct regions and held in place by ankyrin proteins such as inv, and thus initiate ion flux. (E) The differential ion flux results in LR-asymmetric gradients of pH and voltage. In particular, cells across the ventral midline possess significantly different membrane potential levels. (F) The system of gap-junctional communication is set up, featuring junctional isolation across the ventral midline and a path of GJC circumferentially around it. (G) The voltage gradient between the L and R sides imposes a unidirectional net movement of as-yet-uncharacterized small signaling molecules. This results in accumulation on one side of the midline from an initially random (homogenous) distribution. (H) The accumulation of these small molecule morphogens on one side induces gene expression in conventional ways. (I) This initiates the known cascade of asymmetrically expressed signaling factors which form the middle of the LR pathway which dictates the situs of asymmetric organs.