GDNF and amyotrophic lateral sclerosis: Previous studies have demonstrated that neural stem cells of mouse, rat or human origin are a rich source of extracellularly released trophic factors (such as NGF, BDNF, GDNF, EGF, IGF-1, and VEGF) in in vitro cultured cells and that these cell populations retain a high level of neurotrophin expression after in vivo grafting in naïve animals and in a variety of neurodegenerative models including spinal injury and transgenic ALS models [56-61].