Axonal regeneration and neural network reconstruction in mammalian CNS

Following injury to the white matter of the adult mammalian central nervous system (CNS), severed axons fail to regenerate beyond the lesion site. Recent studies have revealed that the CNS white matter contains numerous axon growth inhibitors. These findings can easily lead to the concept that regenerating axons cannot grow in the CNS white matter because of the growth inhibition by these inhibitory molecules. This “misconception” appears to be generally accepted. However, it is erroneous because axons can grow along the CNS white matter very rapidly. Neurons cultured on a slice of adult rat brain can extend their neurites along the white matter tract, while axons of neurons transplanted into the adult rat spinal cord white matter can grow along the CNS white matter very rapidly, at more than 1 mm/day. Not only artificially transplanted neurons, but also in situ CNS neurons can elongate axons linearly within the CNS white matter at this rate. The idea that a CNS neuron can regenerate a severed axon along the CNS white matter has great significance when thinking about reconstruction of original neural networks after focal destruction due to CNS injury.