Alterations in neurofilaments associated with reactive brain changes and axonal sprouting following acute physical injury to the rat neocortex

In order to study the changes in axons related to acute localized physical trauma, a 25 gauge needle was inserted into the somatosensory cortex of anaesthetized adult rats. Animals were examined over 11 time points, from 30 min to 14 days postinjury. Initially, the central needle tract was surrounded by ‘reactive’ abnormal axons characterized by their bulb‐ or ring‐like immunoreactivity for neurofila ments. Quantification demonstrated that these structures reached a peak density at 24 h postinjury, followed by a gradual decrease over 2 weeks. By 5 days postinjury, long axons showing high levels of neurofilament labelling were localized to the lesion area, either aligned parallel to the tract edges or extending into the bridge of tissue forming between the tract edges. Double‐labelling demonstrated a close association between sprouting axons and ferritin‐labelled microglia. Immunolabelling for GAP43 also demonstrated the presence of sprouting axons within this tissue bridge. Ultrastuctural examination showed that sprouting axons contained a high density of neurofilaments, with a leading edge lacking these filaments. Injury to the adult neocortex is associated with reactive and sprouting changes within axons, coordinated with the proliferation of microglia and wound healing. These data also support a role for neurofilaments in axonal sprouting following brain injury.