NGF and Alzheimer disease: Similar to MSCs, although the low differentiation rate of neurons also limits the neurogenic effect of NSC, its secretion of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and anti-inflammatory cytokines may have great therapeutic potential to increase the density of neurons, synapses, and nerve fibers and reduce AD pathology to improve cognitive impairment in AD (Duncan & Valenzuela, 2017; Han et al., 2020).