Quantitative analyses on the neurogenesis and fate of pioneer neurons in the hippocampus. Data are expressed as the number of labeled cells per section. A,B, Number of calretinin-positive Cajal-Retzius cells (A) in the stratum lacunosum-moleculare and of GABA-positive neurons (B) in the stratum radiatum at different postnatal ages (P0–P21) and in the adult. Solid bars are crude cell counts, whereas hatched barsare values corrected for a factor of cortical growth (see Materials and Methods). C, Laminar distribution of BrdU-positive cells at P0 after a BrdU pulse injection at E10, E11, E12, or E16. Pioneer neurons in the stratum radiatum and stratum lacunosum-moleculare are generated at E10–E12, whereas the far more numerous pyramidal neurons are born later. D, E, Comparison of the numbers of BrdU-positive cells in the stratum lacunosum-moleculare and stratum radiatum in littermates aged P0 and P45 after BrdU injections administered at E10, E11, and E12. Values at P45 are corrected for a factor of cortical growth as in A and B(** indicates p ≤ 0.01; ANOVA, Scheffé’s test).AD, Adult; CALR, calretinin;SLM, stratum lacunosum-moleculare; SO, stratum oriens; SP, stratum pyramidale;SR, stratum radiatum.