Distribution and numbers of neurons in the hippocampus. Confocal laser scanning micrographs were taken from serial horizontal sections through the dorsal (A) and the ventral hippocampus (G) that were immunostained for NeuN to detect the nuclei of neuronal cells. To quantify cytoarchitectural variations in the structure of neuronal cell layers, the numbers of neighbours for any given cell were determined as a measure of density of cell packing. In the dorsal hippocampus of WT animals (n = 6), fewer cells were surrounded by a high number of neighbours, i.e. neurons showed a more sparse pattern (C-F). By contrast, in Ctsk-/- mice (n = 6) the ventral part of the hippocampus showed a significantly more sparse neuronal distribution (G-L). This patterning difference was most pronounced in the DG (C and I), CA2 (E and K) and CA1 (F and L), although all regions exhibited significant differences in the patterning of the neuronal layer between the two genotypes (p < 0.001, Kolmogorov-Smirnov test). (B) Numbers of neurons were increased in DG, CA3 and CA2 regions in the dorsal part of the hippocampus of Ctsk-/- compared to WT mice, whereas neuronal numbers were decreased in the CA1 region of Ctsk-/- mice. (H) Fewer neurons were observed in DG and CA1 region in the ventral area of the hippocampus of Ctsk-/- mice. Bars - 200 μm (A, G). Fluorescence micrographs are displayed in reverse contrast.