Development of Parvalbumin-Expressing Basket Terminals in Layer II of the Rat Medial Entorhinal Cortex

Position of regions of interest used for immuno-electron microscopy. a, Caudal view of rat brain, right hemisphere, without cerebellum, made by using the Waxholm Space Atlas (Kjonigsen et al., 2015; Papp et al., 2014). Red line signifies cutting plane, showing region from which parasagittal sections used in the study were taken. a, Example of parasagittal section of the caudal part of rat brain. Left: Waxholm Space Atlas representation showing subdivisions. Middle: Waxholm Space Atlas representation superimposed onto PV⁺ immunolabeled section used in study. Right: A corresponding PV⁺ immunolabeled vibratome section. Black lines delineate MEC LII. Red boxes show the dorsal and ventral positions from which the tissue was processed for further analysis. Scale bars: 1 mm.

Typical neuronal somata within the dorsal and ventral portion of MEC LII at P10, P15, and P30. PV⁺ terminals (blue) forming synaptic contact onto somata (pink) are indicated by red arrows. The boxes indicate PV⁺ terminals shown in the enlarged figures just below the low-magnification images of the somata. Note that the ventral soma at P10 is apposed by only one PV⁺ terminal; thus, the first close-up image is of a PV⁺ labeled profile in the vicinity. so, soma; mit, mitochondria. Scale bars: images of somata, 5 µm; images of PV⁺ terminals, 500 nm.

Box-and-whisker plots showing cross-sectional area of somata and number and density of PV⁺ perisomatic terminals per cell per thin section at P10, P15, and P30 in dorsal and ventral MEC LII. a, c, e, Changes in average cross-sectional soma area and number and density of PV⁺ terminals, respectively, during ontogeny in dorsal and ventral LII. b, d, f, Dorsoventral differences in average cross-sectional soma area and number and density of PV⁺ terminals, respectively, per time point. The middle bar of each box plot represents the population median, and the 25th and 75th percentiles are represented by the lower and upper boundaries of the boxes, respectively. The minimum and maximum values are represented by the lower and upper ends of the whiskers. Dots represent outliers, i.e., values that exceeded the 75th percentile + 1.5 × (75th percentile – 25th percentile). *, p ≤ 0.05; **, p ≤ 0.01; ***, p ≤ 0.001.

Cross-sections of typical PV⁺ terminals in dorsal and ventral part of MEC LII at P10, P15, and P30. Arrows (red) indicate synaptic contacts between PV⁺ terminals (blue) and somata (pink). so, soma; mit, mitochondria. Scale bars: 500 nm.