Mechanical characterisation of rodent spinal cord tissue in health, trauma and disease
Spinal cord tissue is amongst the softest tissues in the body. Studying its mechanical properties is important as many cell types in the spinal cord react to their mechanical environment. Injury and disease can change spinal cord tissue stiffness, and conversely, altered tissue stiffness can affect healing and regeneration. Improving our understanding of spinal cord tissue mechanics might reveal new therapeutic targets. Here, I used atomic force microscopy (AFM) to measure the stiffness of healthy, damaged and diseased spinal cord tissue with high accuracy and high spatial resolution.
First, I investigated how the force and speed of an AFM measurement affected the measured stiffness of healthy grey and white matter in the rat. I found that even though increasing these parameters generally led to an increase in the measured stiffness, grey and white matter were affected to a different degree. This led to a decrease in the ratio of grey to white matter stiffness. For very low forces and speeds relevant for cellular mechanosensing, I found grey matter to be stiffer than white matter in all three anatomical planes. Next, I assessed how spinal cord tissue stiffness measured with such low forces and speeds changes with the age of the rats. There were no significant differences between the sexes, albeit between Lister Hooded and Sprague Dawley rats. While grey matter stiffness was relatively stable throughout life, white matter stiffness sharply decreased in the first four weeks of life, before remaining constant until old age. The decrease of white matter stiffness correlated well with an increase in the amount of white matter, which suggests that myelination might play a key role in white matter softening.
To elucidate how injury affects spinal cord tissue mechanics, I used a clinically relevant cervical contusion model in the rat and studied spinal cord tissue stiffness in subacute and chronic spinal cord injury. At the subacute stage, spinal cord stiffness was overall significantly decreased. However, different tissue areas were differentially affected. While the central grey matter of the ventral horns had become significantly softer, the dorsal white matter had stiffened. At the chronic stage, grey matter stiffness was still reduced within several hundred micrometres of the lesion border, but white matter stiffness appeared normal. This shows that stiffness alterations following injury can be spatially different and dynamic over time. Lastly, I examined the spinal cord tissue stiffness of wobbler mice which suffer from a neurodegenerative condition resembling human amyotrophic lateral sclerosis. I found the white matter of these mice to be significantly stiffer compared to healthy controls, whereas grey matter stiffness was normal, indicating a so far understudied change in white matter tissue architecture.
My data show that spinal cord tissue stiffness is spatially heterogeneous and temporally dynamic. The mechanical properties of the spinal cord change throughout life and during pathological processes, which may have important consequences for cellular function and regeneration.