Axonal atrophy: the retraction reaction.

Recent studies indicate that morphological alterations of axon branches that are removed during normal development are similar to those that occur following ablation of postsynaptic cells in adult animals. In both situations, axons retract (rather than degenerate), the calibers of withdrawing axon branches are markedly reduced, and spherical swellings near (or at) the axon terminations appear. The similarity between naturally occurring and target-deprived axon withdrawal suggests that both developing and adult axons withdraw from target cells that no longer provide support.