Morphological responses to local CNS trauma: sprouting and synaptogenesis within membranes implanted into mature...

The purpose of this study was to document in vivo neuronal sprouting in response to local trauma of the mature cerebral neocortex. The experimental approach enabled direct and unequivocal ultrastructural identification of newly sprouted neuronal processes and their synaptic relationships. Morphologically detectable factors that influence neuronal sprouting were also assessed. Tiny fragments of porous, synthetic membrane were implanted into the mature neocortex of the rat. After one week, axons, dendrites, growth cone-like structures, synapses, and processes from astrocytes and microglia were present deep within the interstices of the implant. Morphometric analysis of synapses within the implant, support the qualitative impression of active neuronal ingrowth and de novo synaptogenesis by these sprouted elements. Astrocytic processes appear to comprise the major barrier to neuronal invasion of the implant. Several observations suggested that glia sometimes may facilitate neuronal ingrowth. These findings provide direct morphological documentation of axonal and dendritic plasticity in the mature rat neocortex. The work demonstrates a strategy that could be useful for further evaluating the cellular mechanism of in vivo neuronal regeneration.