Tagging proliferating cells with thymidine analogs is an indispensable research tool; however, the issue of the potential in vivo cytotoxicity of these compounds remains unresolved. Here, we address these concerns by examining the effects of BrdU and EdU on adult hippocampal neurogenesis and EdU on the perinatal somatic development of mice. We show that, in a wide range of doses, EdU and BrdU label similar numbers of cells in the dentate gyrus shortly after administration. Furthermore, whereas the administration of EdU does not affect the division and survival of neural progenitor within 48 h after injection, it does affect cell survival, as evaluated 6 weeks later. We also show that a single injection of various doses of EdU on the first postnatal day does not lead to noticeable changes in a panel of morphometric criteria within the first week; however, higher doses of EdU adversely affect the subsequent somatic maturation and brain growth of the mouse pups. Our results indicate the potential caveats in labeling the replicating DNA using thymidine analogs and suggest guidelines for applying this approach.

Tissue-specific somatic stem cells are characterized by their ability to reside in a state of prolonged reversible cell cycle arrest, referred to as quiescence. Maintenance of a balance between cell quiescence and division is critical for tissue homeostasis at the cellular level and is dynamically regulated by numerous extrinsic and intrinsic factors. Analysis of the activation of quiescent stem cells has been challenging because of a lack of methods for direct detection of de novo dividing cells. Here, we present and experimentally verify a novel method based on double labeling with thymidine analogues to detect de novo dividing stem cells in situ. In a proof of concept for the method, we show that memantine, a drug widely used for Alzheimer’s disease therapy and a known strong inducer of adult hippocampal neurogenesis, increases the recruitment into the division cycle of quiescent radial glia-like stem cells—primary precursors of the adult-born neurons in the hippocampus. Our method could be applied to assess the effects of aging, pathology, or drug treatments on the quiescent stem cells in stem cell compartments in developing and adult tissues.