Ontogeny of progesterone receptor (PR) expression in dentate gyrus. (a) Nuclear progesterone receptor immunoreactivity (PR-ir) was observed within the molecular layer (MOL) of both the infra- and supra-pyramidal blades of the dentate gyrus (DG) of the neonatal rat. Nuclear PR-ir was not detectable in the granule cell layer (gc), nor in other regions of the hippocampus (e.g., CA1-3). (b) The number of PR-ir cells in the supra-pyramidal MOL on postnatal days 1 (P1) through P28. PR-ir cells (c) were first visible on the day of birth (P1), (d) peaked on postnatal day 7, (e) were significantly diminished by P21 and were undetectable by P28. * p<0.001; (a) Scale bar: 300Om; (c,d,e,) Scale bar: 100Om.