Parallel impact of enriched housing conditions on hippocampal neurogenesis and Cajal–Retzius cell density. (A) Identification of proliferating cells in the subgranular zone (SGZ) of the dentate gyrus by Ki-67 immunoreactivity. Left panel: DAPI counterstaining, middle panel: Ki-67-labeled nuclei, and right panel: previous images superimposed. OML, outer molecular layer; GCL, granule cell layer. (B) Simultaneous identification of dentate gyrus structure (left: DAPI counterstaining), Cajal–Retzius cells (middle left: tdTomato fluorescence), dividing cells (middle right: Ki67), and superimposition of all the images (right: merge). SLM, stratum lacunosum-moleculare. (C) High resolution image of a Ki-67-labeled nucleus in the SGZ. Top, middle, and bottom panels show DAPI counterstaining, Ki-67 immunoreactivity, and the 2 images overlapped, respectively. (D1) Comparison of Cajal–Retzius cell densities in a control Wnt3a mouse (ctrl) versus an animal housed in environmental enriched conditions (EE). Notice the increased numbers of Cajal–Retzius cells in the image from the treated mouse (n = 32 vs. 41, respectively). (D2) Same as in (D1), but for PDE1c animals. (E1) Summary plots showing the densities of Ki-67-positive nuclei of neural progenitor cells (NPC) (left panel) and of Cajal–Retzius cells (in the hippocampus: Hip and in layer 1 of the adjacent cortex: L1, right panel) of Wnt3a mice housed in control (ctrl) and environmental enriched (EE) conditions (left panel). (E2) Same analysis as in (E1) carried on in sections from PDE1c mice.