Temporal dynamics of spinal cord repair in juvenile and adult zebrafish

Understanding the process of successful spinal cord repair in the zebrafish holds significant potential for improving patient health following spinal cord injury (SCI). Presently, beyond early larval stages, we have only limited understanding of the temporal cascade of events facilitating functional recovery, in particular the relationship between the immune response and ependymoglia activity. Here, we investigated this question by comparing the timeline of cellular activity and swimming recovery in a novel juvenile model of SCI, alongside the commonly studied adult model. We show for the first time that similar to larval SCI, neutrophils are the first responders to injury with peak numbers tightly associated with heightened pro-inflammatory cytokines il-1β and il-8. In both juveniles and adults, maximal microglial recruitment was observed by 3-dpi and sustained onwards, overlapping with peak ependymoglia proliferation. Juveniles reached peak proliferative activity by 3-dpi compared to 7-dpi in adults. Importantly, we found increased canal diameter closely associated with peak ependymoglia proliferation, with a greater proportion of cycling cells adjacent the canal. Proliferating ependymoglia produced newborn neurons, including a small number of motor neurons, though output was higher in juveniles. Lastly, we show that while juvenile and adult animals were characterized by different timelines of functional recovery over the 4-wpi, at 4 weeks post-SCI juveniles had attained a greater percent swimming recovery compared to adults. Our results map the temporal relationship of critical cellular events leading to functional recovery in post-larval models of SCI, identifying key times during the regenerative process to study the regulatory mechanisms orchestrating the repair process.