In the neurogenic niches, adult neural stem and/or progenitor cells (NSCs) generate functional neurons throughout life, which has been implicated in learning and memory and affective behaviors. During adult neurogenesis, newborn neurons release feedback signals into the niches to regulate NSC proliferation and differentiation. However, whether and how NSCs contribute to the niche governing newborn neuron development is still unknown. Using a combination of cell ablation, retrovirus-mediated single-cell labeling, and signaling pathway modulation, we show that adult hippocampal NSCs continuously supply pleiotrophin factor to the newborn neurons. Without this feedforward signal, the newborn neurons display defective dendritic development and arborization. Thus, our findings reveal that NSCs behave as a functional niche for newly generated newborn neurons to regulate their maturation.