Organelle flux in intact and transected crayfish giant axons

The flux of organelles moving by fast axonal transport in distal segments of severed crayfish medial giant axons (MGAs) and lateral giant axons (LGAs) was measured for survival times of up to 35 days (MGAs) or 60 days (LGAs). The response to transection occurred in 4 phases: (1) Organelle fluxes remained nearly normal for the first 24 h. (2) Fluxes then declined continuously until day 6 or 7. (3) A rebound toward normal levels lasted until day 21 (MGAs) or longer (LGAs). (4) During the final phase, fluxes declined either to zero (MGAs) or plateaued at a level which was a significant percentage of normal flux (LGAs). Changes in anterograde and retrograde flux were identical. The distribution of various size classes of translocating vesicles in distal segments of these axons was normal until day 4, with small and medium size, rapidly moving vesicles predominating. Afterwards, larger, slower vesicles predominated. During long-term survival, the axons remained physiologically intact, and cytoskeletons appeared to be normal, retaining intact microtubules which remained normally oriented with positive ends pointing distally. The evidence suggests that the two initial phases of the response to transection represent clearance from distal segments of organelle traffic which normally moves between axon and cell body. The rebound phase may be trauma induced, possibly a transient phase of cytoplasmic degeneration resulting from the loss of trophic support from the cell body. Differences between LGAs and MGAs with respect to organelle flux during prolonged survival, i.e. during the 4th phase of the response to transection, are consistent with different mechanisms of long-term survival which have been proposed for these axons.