Seeing is Believing—Spinal Chamber for Spinal Cord Injury Research

Abstract

Spinal cord injury remains a vexing problem that has evaded the development of effective treatments despite modern scientific advances. The lack of effective injured axon regeneration, despite many years of intense research among scientists and surgeons, contributes to the lack of clinical effective therapy to spinal cord injury. One of the technical difficulties in spinal cord injury research is the absence of direct and longitudinal observation of the interaction of axons, inflammatory cells, myelin, and glial cells after an induced injury in animal models. Various strategies to promote axon regeneration have been proposed with variable histological success in regenerating injured motor axons pass the lesion site. However, there has been little direct evidence that regenerated spinal motor axons innervate the lower motor neurons in the spinal cord. In a recent issue of Nature Methods, Farrar et al provided spinal cord injury researchers a very useful tool to study axon regeneration.1 By designing a spinal chamber that can be implanted onto the experimental animals for long-term experiments, an actual glass window provides direct visualization of the exposed spinal cord long term. Through this window, two-photon excited fluorescence (2PEF) microscopy can be performed to visualize axons, blood vessels, inflammatory cells and microglia in longitudinal fashion in vivo and in real time (Figure). The observations can be performed as early as minutes after the experimental spinal cord injury or up to 5 months in some animals.Figure: A, photograph of the imaging chamber. B, schema showing the implantation of the imaging chamber in mice at the T11– T12 vertebra, just below the dorsal fat pad (taupe). C, photograph showing the spinal cord imaged through the implanted chamber 144 d after the surgery. D, photograph of a mouse with an implanted chamber (same mouse as in C). Reprinted by permission from Macmillan Publishers Ltd: Nature Methods: Vol 9(3):297-302, copyright 2012.The failure of functional gain after severe spinal cord injury is hallmarked by the aborted axon regeneration attempts at the injury site. With the spinal chamber, the authors were able to observe the interactions of the injured axons, the infiltration of the inflammatory cells, and the accumulation of microglia scars at the injury site. More importantly, the authors observed a heterogeneous response of the axons to the injury. Certain groups of axons underwent very rapid Wallerian degeneration after injury. However, there were other groups of axons that demonstrated slow dieback or nascence around the lesion sites with clear evidence of regeneration attempts. It is our own experience that even in the best experimental condition with genetically modified neurons, there are only a certain percentage of axons that are able to successfully regenerate through the lesion sites.2 With this technique, evaluating changes in spinal cord injury lesions may reveal how axons, inflammatory cells and microglia interact on an in vivo scale that may explain how axons can or cannot regenerate. This new tool can potentially help to answer many questions that have been an obstacle for experimental spinal cord injury researchers for decades. It also represents a creative and novel way of approaching how to visualize neural injury in vivo, bringing us one step closer to observing what actually happens in damaged spinal cord essentially as it happens.