Pruning an Axon Piece by Piece Samir Koirala, Chien-Ping Ko  Neuron  Volume 44, Issue 4, Pages 578-580 (November 2004) DOI: 10.1016/j.neuron.2004.11.006

Figure 1 Three Models of Axon Loss during Synapse Elimination at the Developing Mammalian Neuromuscular Junction (A) Two axon branches (blue and red) surrounded by Schwann cells (green) innervate the same synaptic site on a muscle fiber. Proposed mechanisms for removal of one input (red) include (B) Wallerian-type degeneration, characterized by degenerative changes (gray shading) including high cytoplasmic density, vesicle clumps, disrupted mitochondria, and whole-sale fragmentation of the axon branch; and (C) retraction, in which the entire branch is absorbed proximally into the parent axon (red arrows in [C]–[E] indicate the direction of axon withdrawal). Bishop et al. now describe a novel mechanism, axosome shedding (D–F), in which axons, as they withdraw, shed fragments rich in normal synaptic organelles (the boxed area in [E] is enlarged in [F]). These axosomes are engulfed by neighboring Schwann cells and may be assimilated into the Schwann cell cytoplasm. (G) Removal of the competing input leads to innervation by a single nerve terminal, which typically expands to cover the full synaptic site. (Figure prepared by Zhihua Feng and adapted from Bishop et al., 2004.) Neuron 2004 44, 578-580DOI: (10.1016/j.neuron.2004.11.006)