Retrograde responses of developing lateral motor column neurons.

Removing the right hind limb of Rana pipiens larvae at different stages during development initiates diverse responses within lateral motor column (LMC) neurons whose axons have entered the limb and are forming neuromuscular junctions; Severing the hind limb at stage V inhibits the future differentiation of LMC neurons and, ultimately, 95% of the presumably injured cells degenerate; Ultrastructural observations indicate that the affected neurons become pyknotic, but display little evidence of lysosomal involvement. However, a quite different reaction is encountered when limb ablation is delayed to stage X. At this stage, LMC neurons rapidly disclose an intense retrograde response, which includes the genesis of numerous secondary lysosomes in degenerating cells; If, however, limb removal is postponed until stage XV or XX then a clasic chromatolysis can be recognized in axotomized neurons. The intensity of the chromatolysis appears to be inversely related to the age of the neurons. This investigation tends to support the premise that differentiating LMC neurons pass through three crucial stages in their development which can be identified following axotomy.