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, 33 (18), 1685-95

Relating Histopathology and Mechanical Strain in Experimental Contusion Spinal Cord Injury in a Rat Model

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Relating Histopathology and Mechanical Strain in Experimental Contusion Spinal Cord Injury in a Rat Model

Tim Bhatnagar et al. J Neurotrauma.

Abstract

During traumatic spinal cord injury (SCI), the spinal cord is subject to external displacements that result in damage of neural tissues. These displacements produce complex internal deformations, or strains, of the spinal cord parenchyma. The aim of this study is to determine a relationship between these internal strains during SCI and primary damage to spinal cord gray matter (GM) in an in vivo rat contusion model. Using magnetic resonance imaging and novel image registration methods, we measured three-dimensional (3D) mechanical strain in in vivo rat cervical spinal cord (n = 12) during an imposed contusion injury. We then assessed expression of the neuronal transcription factor, neuronal nuclei (NeuN), in ventral horns of GM (at the epicenter of injury as well as at intervals cranially and caudally), immediately post-injury. We found that minimum principal strain was most strongly correlated with loss of NeuN stain across all animals (R(2) = 0.19), but varied in strength between individual animals (R(2) = 0.06-0.52). Craniocaudal distribution of anatomical damage was similar to measured strain distribution. A Monte Carlo simulation was used to assess strain field error, and minimum principal strain (which ranged from 8% to 36% in GM ventral horns) exhibited a standard deviation of 2.6% attributed to the simulated error. This study is the first to measure 3D deformation of the spinal cord and relate it to patterns of ensuing tissue damage in an in vivo model. It provides a platform on which to build future studies addressing the tolerance of spinal cord tissue to mechanical deformation.

Keywords: MRI; biomechanics; contusion; histology; image registration; in vivo; spinal cord injury.

Conflict of interest statement

Author Disclosure Statement No competing financial interests exist.

Figures

<b>FIG. 1.</b>
FIG. 1.
Labeling ventral horns of gray matter. Left (A) and right (B) ventral horns were identified on the pre-injury transverse images of the spinal cord.
<b>FIG. 2.</b>
FIG. 2.
Strain-type visualizations. Left: In the transverse plane, the three fundamental strain types are the normal strains in the ‘X’ and ‘Y’ directions (eXX and eYY, respectively) and the shear strains (eXY and eYX, where |eXY| = |eYX|). Right: By rotating the coordinate system by an angle, θ, all strains can be represented by two normal strains, emax and emin, acting along the new ‘1’ and ‘2’ axes, respectively. These two normal strains, emax and emin, are referred to as principal strains.
<b>FIG. 3.</b>
FIG. 3.
Sample histological data for NeuN-positive quantification. (A) Acquired images at 20× magnification. (B) Manually identified regions of interest of ventral horns. (C) Sample NeuN-positive neurons with nucleoli presenting as a darkened center of the cell nucleus. NeuN, neuronal nuclei.
<b>FIG. 4.</b>
FIG. 4.
Craniocaudal distribution of each transverse-plane strain type. Sample data (IV 1) show strain magnitude in transverse slices of the spinal cord. Magnetic resonance transverse slice image illustrates general anatomical location of ventral horns of gray matter. X, Y, and Z indicate lateral, dorsal, and cranial directions, respectively. Row 1: mediolateral (X-dir) normal strain; row 2: dorsolateral (Y-dir) normal strain; row 3: transverse-plane (X-Y) shear strain; row 4: maximum principal strain; row 5: minimum principal strain.
<b>FIG. 5.</b>
FIG. 5.
Linear regression of NeuN-positive density against transverse-plane strain types for pooled data. Scatter plots with trendlines and calculated R2 values are shown for each transverse-plane strain: lateral normal strain (eXX-blue); transverse-plane shear strain (eXY-green); dorsoventral normal strain (eYY-red); minimum principal strain (emin-black); and maximum principal strain (emax-purple). Asterisk (*) indicates a significant relationship at α = 0.05. NeuN, neuronal nuclei.
<b>FIG. 6A.</b>
FIG. 6A.
Linear regression of NeuN-positive density against transverse-plane strain types for individual animal data (IV 1–4). Scatter plots with trendlines and calculated R2 values are shown for each transverse-plane strain: lateral normal strain (eXX-blue); transverse-plane shear strain (eXY-green); dorsoventral normal strain (eYY-red); minimum principal strain (emin-black); and maximum principal strain (emax-purple). Asterisk (*) indicates a significant relationship at α = 0.05. NeuN, neuronal nuclei.
<b>FIG. 6B.</b>
FIG. 6B.
Linear regression of NeuN-positive density against transverse-plane strain types for individual animal data (IV 9–12). Scatter plots with trendlines and calculated R2 values are shown for each transverse-plane strain: lateral normal strain (eXX-blue); transverse-plane shear strain (eXY-green); dorsoventral normal strain (eYY-red); minimum principal strain (emin-black); and maximum principal strain (emax-purple). Asterisk (*) indicates a significant relationship at α = 0.05. NeuN, neuronal nuclei.
<b>FIG. 7.</b>
FIG. 7.
Strain and histology data for each animal. NeuN-positive density data (red, left axis) and interpolated minimum principal strain (emin-blue, right axis) are plotted over the craniocaudal region of interest for left (dark) and right (light) ventral horns of gray matter. The noninjury NeuN-positive density threshold (mean with standard deviation error bars, black) from the control animal observations is included in all plots. NeuN, neuronal nuclei.

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