NGF and Alzheimer disease: In comparison to conventional drug therapies, stem cell-based therapy may be more effective for AD because implanted stem cells can improve the brain microenvironment by supplying growth-promoting and growth-permissive factors for synaptogenesis and neurite repair, reduce oxidative stress by enhancing local antioxidant capacity, and trigger the sustained production of neurotrophic factors like brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) [33].