Three-dimensional organisation of mitochondrial clusters in regenerating dorsal root ganglion (DRG) neurons from neonatal rats: evidence for mobile mitochondrial pools

We report for the first time the rearrangement of mitochondrial arrays in developing dorsal root ganglion (DRG) neurons isolated from neonatal rats in culture. Neurons were loaded with the mitochondria-specific fluorescent dye JC-1, and three-dimensional (3D) reconstruction of mitochondrial fluorescence was performed by confocal laser sectioning in fresh neurons and neurons kept in culture up to a week. We found that after 24 hours the mitochondria become reorganised to form clusters in the axonal hillocks. Axonal extension and neuronal network formation coincided with a redistribution of the mitochondrial clusters. In the extended axons the mitochondria become spaced along the axonal length; however,they formed clusters in the branch points and growth cones. We conclude that the initial clusters of mitochondria may be storage pools of mobile mitochondria able to be mobilised to provide energy for axonal transport during neuronal regeneration and neuronal outgrowth. These findings may have relevance to the rate of axonal regeneration and axonal transport in adult DRG neurons, and neuronal polarisation and axonal outgrowth regulation in developing DRG neurons.