Three-dimensional reconstruction of corticospinal tract using one-photon confocal microscopy acquisition allows detection of axonal disruption in spinal cord injury

Abstract

The principal motor tract involved in mammalian locomotor activities is known as the corticospinal tract (CST), which starts in the brain motor cortex (upper motor neuron), extends its axons across the brain to brainstem and finally reaches different regions of spinal cord, contacting the lower motor neurons. Visualization of the CST is essential to carry out studies in different kinds of pathologies such as spinal cord injury or multiple sclerosis. At present, most studies of axon structure and/or integrity that involve histological tissue sectioning present the problem of finding the region where the CST is predominant. To solve this problem, one could use a novel technique to make the tissues transparent and observe them directly without histological sectioning. However, the disadvantage of this procedure is the need of costly and non-conventional equipment, such as two-photon fluorescence microscopy or ultramicroscopy to perform the image acquisition. Here, we show that labeling the CST with FluoroRuby in the motor cortex and then performing the clearing technique, the z-acquisition of the entire CST in unsectioned tissue followed by three-dimensional reconstruction can be carried out by standard one-photon confocal microscopy, with yields similar to those obtained by two-photon microscopy. In addition, we present an example of the application of this method in a spinal cord injury model, where the disruption of CST is shown at the lesion site.