Detecting structural and inflammatory response after in vivo stretch injury in the rat median nerve via second harmonic generation

Background: Second Harmonic Generation (SHG) microscopy is a promising method for visualizing the collagenous structure of peripheral nerves. Assessing collagen continuity and damage after a stretch injury provides inferential insight into the level of axonal damage present. New methods: This study utilizes SHG microscopy after a calibrated in vivo stretch injury of rat median nerves to evaluate collagen continuity at several time points throughout the recovery process. Endoneurial collagen was qualitatively assessed in nerves that were subjected to low strain (LS) and high strain (HS) injuries using SHG microscopy, conventional histology, and immunohistochemistry. Results: Following an in vivo stretch injury, both LS and HS damaged nerves exhibit signs of structural collagen damage in comparison with sham control nerves (SC). Furthermore, LS nerves exhibit signs of full regeneration while HS nerves exhibited signs of only partial regeneration with lasting damage and intra-neural scar formation. Comparison with existing methods: SHG observations of structural changes and inflammatory response due to stretch injury were validated upon comparison with conventional histological methods Conclusions: We propose that SHG microscopy can be utilized to visualize significant structural artifacts in sectioned median nerves following in vivo stretch injury. Based on the findings in this study, we believe that the in vivo application of SHG microscopy should be further investigated as a means for real-time, intra-operative, quantitative assessment of nerve damage.