Neuron
Volume 44, Issue 4, 18 November 2004, Pages 651-661
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Article
Axon Branch Removal at Developing Synapses by Axosome Shedding

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Abstract

In many parts of the developing nervous system, the number of axonal inputs to each postsynaptic cell is dramatically reduced. This synapse elimination has been extensively studied at the neuromuscular junction, but how axons are lost is unknown. Here, we combine time-lapse imaging of fluorescently labeled axons and serial electron microscopy to show that axons at neuromuscular junctions are removed by an unusual cellular mechanism. As axons disappear, they shed numerous membrane bound remnants. These “axosomes” contain a high density of synaptic organelles and are formed by engulfment of axon tips by Schwann cells. After this engulfment, the axosome's contents mix with the cytoplasm of the glial cell. Axosome shedding might underlie other forms of axon loss and may provide a pathway for interactions between axons and glia.

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3

These authors contributed equally to this work.

4

Present address: E.F. Ball Medical Education Center, Ball State University, Muncie, Indiana 47306.

5

Present address: Wilmer Ophthalmological Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287.

6

Present address: Molecular Neurobiology Program, Skirball Institute, Department of Physiology and Neuroscience, New York University School of Medicine, New York, New York 10016.