Fig. 1. Developmental regulation of spontaneous [Ca²⁺]_i oscillations in the embryonic and postnatal hippocampus. (A) Horizontal hippocampal slice from an E18 embryo loaded with fura-2-AM. The principal cells (pyramidal and granular neurons) and some interneurons show intense fura-2 fluorescence. Scale bar: 400 µm. (B,C) Representative plots of F/F over time showing spontaneous [Ca²⁺]_i transients in CA1 hippocampal neurons at E17 (B) and P2 (C). The onset of each [Ca²⁺]_i oscillation is indicated by a bar on the x axis. Note the greater number of spontaneous Ca²⁺ events at postnatal ages than at embryonic ages. (D,E) Histograms illustrating the developmental regulation of the number of spontaneously active cells/2,000 µm² (D) and their activation rate ([Ca²⁺]_i oscillations/cell/10⁴ seconds) (E) from E16 to P5. Both parameters increase around birth and peak at P2-P5. Values are shown as mean ± s.e.m. CA1 and CA3, hippocampal regions; DG, dentate gyrus; h, hilus; gcl, granule cell layer; ml, molecular layer; slm, stratum lacunosum moleculare; so, stratum oriens; sp, stratum piramidale; sr, stratum radiatum.