Fig. 3. The dynamics of axon retraction and regeneration. Each row represents the results for a temperature shift initiated at the time indicated. Each column represents the structure at a different time with respect to the temperature shift. The left column shows examples immediately before the temperature shift, the middle column immediately after the temperature shift and the right column illustrates the adult CNS after the temperature shift. (A) Temperature shift initiated at the beginning of pupariation (P0). (A1) Schematic version of the GF at P0. The GF has not reached the target area in the early phase of growth and the axons are very thin, making it difficult to stain and visualize with our methods. The schematic illustrates the approximate location of the axons at this time as well as showing the portion of the CNS represented in all the panels (dashed box). (A2) The axons at the end of a temperature shift that began at pupariation. Both axons have retracted into the anterior end of the thorax and each exhibits a retraction bulb (arrow) near the terminal and a thin retraction tail extending toward the target. (A3) Overgrowth of the axon in the adult after regeneration at the permissive temperature. (B) Temperature shift initiated at 33% of pupal development (Phase II). (B1) The structure of the GF at 33% of development. Most specimens exhibited the laterally projecting terminal illustrated by the left GF, a minority of GFs have just begun to make the bend as seen for the right GF. (B2) Dissection at the end of the temperature shift (50%) shows that both axon terminals have retracted into the first thoracic neuromere and terminate in retraction bulbs (arrow). In this specimen the dendrites of the TTMn were also visible (arrowheads) indicating the extent of the GF retraction. The dendrites of the TTMn appeared normal in this specimen. (B3) After regeneration, both GFs tapered to an end in the target area and neither showed the laterally projecting terminal. (C) Temperature shift initiated at 50% of pupal development. (C1) Both GFs exhibited the normal lateral extension of the presynaptic terminal at 50% of pupal development. (C2) Retraction of the presynaptic terminal immediately after a temperature shift. Note that the presynaptic terminal has withdrawn but the axon did not retract away from the target area. (C3) The adult regenerated axon. In this case a large swollen region was present on the right GF just anterior to the presynaptic terminal. (D) Temperature shift initiated at 66% of pupal development. (D1) The GF structure at 66% of pupal development exhibited normal lateral extensions. (D2) Immediately after a temperature shift, there was no obvious defect. (D3) An adult specimen indistinguishable from controls. Genotype of all specimens; UAS-lacZ/+;A307/+;UAS-shi^ts/+. Scale bar: 20 µm.

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