Fig. 5. Axonal growth pattern is dependent on H⁺ fluxes. Green arrows indicate normal axon patterning; black arrows indicate abnormal axon number and/or location. (A) The normal pattern of regenerating axons is parallel to the main body axis, extending to the tip of the bud at 72 hpa. This is altered in V-ATPase-inhibited tails, where axons are absent from the central core of the new tissue of partial regenerates at 72 hpa (B), or are present in a tangled mass and not aligned parallel to the tail's primary axis (B'). The expression of the concanamycin-insensitive yeast PMA H⁺ pump in the tail can rescue the normal axonal patterning (C). (D) In refractory-stage larvae, axons terminate in a loop at a considerable distance from the edge, and perpendicular to the main tail axis. Expression of PMA rescues the neuronal phenotype, resulting in axon growth to the very edge of the wound in non-regenerating tails (E), and parallel outgrowth of axons into the new tissue of regenerates (E'). The normal pattern of axons, reaching to the end of the tissue at 7 dpa (F), is not affected by irradiation (G), which abolishes cell proliferation and regeneration; axons reach to the distal edge of the irradiated tissue. Tails shown in A-E are 72 hpa; those in F-G are 7 dpa.