Histology
At the 16-week of age mice sacrificed with an overdose of pentobarbital and underwent transcardial perfusion with a 4% paraformaldehyde solution. Following perfusion, the lumbar segment of the spinal cord was carefully extracted, post-fixed for 2 hours, and subsequently cryopreserved in a solution of 30% sucrose in phosphate buffer (PB) at 4ºC. Then, transverse 20-µm thick sections of the spinal cord were serially cut with a cryostat. For assessing the number of MNs, slices of L4-L5 spinal cord segments separated 100 μm were stained with a solution of 3.1mM cresyl violet for 3 h. MNs were identified by their anatomical localization in the ventral horn of the spinal cord and counted following strict criteria of their size (diameters lager than 20µm) and morphological characteristics (polygonal shape and prominent nucleoli).