Morphology Analysis of Descending Tracts in the Mouse Spinal Cord Using Tissue Clearing, Tissue Expansion and Tiling Light Sheet Microscopy Techniques

Abstract

Abstract Descending tracts are the pathways to send motor signals from the brain to spinal cord playing an important role in movement coordination. However, there are rare reports to show the full view of the tracts with a long span projection and variety axon morphologies from 3D perspective. Herein, five less-studied descending tracts in the mouse spinal cord were analyzed that originate from midbrain, cerebellum or brainstem individually by the tissue clearing method in combination with tiling light sheet microscopy. By tracing dozens of axons in spinal cord, we reported several novel findings of the different tracts: the collateral “sister” branches occurred only in the axons originating from the cerebellospinal tracts among these five tracts; the axons from superior colliculus terminated only at upper cervical but with abundant branches in the brainstem; the SP5 (spinal trigeminal nucleus) axons crossed the midline of spinal cord to contralateral while the MdV (medullary reticular formation, ventral part) axons had plenty of branches in both cervical and lumbar segments. The characteristics of these branch features in different tracts hinted the function of specific descending pathway. Furthermore, we preliminarily investigated the synaptic connection between the tracts and motor neurons in the spinal cord through hydrogel-based tissue expansion and found several synaptic connections between MdV axons and motor neurons. The present study, for the first time, described the full-view of the projection patterns and axon morphologies of five descending tracts in the mouse spinal cord under 3D view, which was essential for better understanding the function of these descending tracts. Meanwhile, we developed a new method for future study of descending tracts by three-dimensional imaging.