Identification of Regenerative Tissue Cables Using In Vivo MRI after Spinal Cord Hemisection and Schwann Cell Bridging Transplantation

Abstract

The purpose of this study was to examine the feasibility of a non-invasive in vivo magnetic resonance imaging (MRI) procedure, performed at 1.5 T, to detect regenerative tissue cables in a rat spinal cord hemisection and Schwann cell (SC) bridging transplantation paradigm. Two months after implantation of a SC-seeded guidance channel (1.25 mm in diameter and 3.0 mm in length) into a T8 spinal cord hemisection-gap lesion, axial fast-spin echo (FSE) T2-weighted MR imaging (T2WI) was performed. Axial T2WI through the graft identified a circular area of low intensity surrounded by high-intensity signal within the guidance channel lumen. Correlative histological assessments of Toluidine blue-stained sections confirmed that the low-intensity signal represented a tissue cable, which, in most cases, contained a substantial number of myelinated axons oriented along the rostro-caudal axis of the spinal cord. The percentage of guidance channel cross-sectional area occupied by the tissue cable, expressed as the tissue cable index (TCI), was also determined from histological sections. Linear regression analysis of the TCI plotted relative to the number of myelinated axons revealed a strong positive correlation (r ² = 0.85) between these two outcome measures. In addition, the sensitivity of MRI to detect regenerative tissue cables within guidance channels was 86%. These results demonstrate that (1) 1.5 T MR imaging performed 2 months after spinal cord hemisection and SC bridging transplantation is sensitive in detecting low-intensity regenerative tissue cables, and (2) the TCI strongly correlates with the extent of axonal regeneration into implanted SC-seeded guidance channels.