Fig. 1: Primary MNs, the largest spinal cord neurons, survive after injury.

a Experimental approach for retrograde labeling of spinal motoneurons (MNs) by injecting the Dextran tracer. Lateral view of whole-mount confocal fluorescence image showing traced MNs (magenta) along with all spinal neurons (HuC/D⁺, green). b Quantification of the soma size of motoneuron types (pMNs and sMNs) and other spinal neurons (HuC/D⁺ and NeuN⁺, excluding the dextran traced pMNs). c Reconstructed neurobiotin-filled pMN showing the extended dendritic arborizations to adjacent spinal cord segments. The dashed black lines define the home segment (shaded area). Illustration of the pMN soma (solid circle) and the rostrocaudal distribution of the dendrites obtained from the lateral view reconstructions. The lines represent the maximum and minimum rostrocaudal dendritic position. d Experimental design and representative whole-mount microphotographs show retrogradely traced motoneurons (magenta) in control, after injury, and during regeneration. Yellow dashed lines indicate the spinal segments. e Analysis of the proportion of the retrogradely traced pMNs number per hemi-segment located caudally to the injured site. Quantification of the pMN soma size in all experimental conditions. f Inverted confocal images from whole-mount adult zebrafish spinal cord showing the PI⁺ cells after injury. Spatial distribution and quantification of PI incorporating cells (PI⁺) in all experimental conditions. g Retrogradely traced pMNs (green), do not incorporate PI (magenta) at 1 day (n = 5 zebrafish), 3 days (n = 3 zebrafish) and 7 days (n = 3 zebrafish) post-injury. h Reconstructed examples of neurobiotin-filled pMNs of segment 14 in different experimental conditions. i Sholl analysis of dendritic complexity with a decreasing radial distance from the pMN soma (epicenter). j Analysis of major dendritic tree properties of pMNs shows no significant morphological changes after injury. Asterisks indicate the pMN cell bodies. μ population mean, Σ summation (total), dpi days post-injury, HuC/D elav3 + 4, NeuN neuronal protein (Fox-3/Rbfox3), PI propidium iodide, pMN primary motoneuron, SCI spinal cord injury, sMN secondary motoneuron. Data are presented as mean ± s.e.m., and as violin plots. ***P < 0.001; ****P < 0.0001; ns, not significant. For detailed statistics, see Supplementary Table 1. Source data are provided as a Source Data file.