(A, B) Representative images of the brain section from the HC (A) or striatum (B) of young (2 mo) or old (18–24 mo) Cx3cr1+/+ or Cx3cr1eGFP/eGFP male or female mice immunoreacted with anti-Iba1 (purple) and anti-Pu.1 (brown). Scale bar: large image, 200 μm; inset, 50 μm. (C, D) Quantification of microglial cell numbers and morphology in the HC (C) and the striatum (D). Microglial cell numbers were quantified as the number of Pu.1-positive nuclei per unit area. Microglial morphology was assessed as the total Iba1 immunoreactive area per Pu.1+ nucleus, dendritic process area per Pu.1+ nucleus, and dendritic process perimeter per Pu.1+ nucleus. Only processes attached to nuclei were included in the dendritic process calculations. Statistics: two-way ANOVA (for genotype and age) and Tukey’s post hoc test. Only select significant intergroup comparisons are shown. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. HC cell number: genotype: F(1,32) = 2.159, P = 0.1515; age: F(1,32) = 8.952, P = 0.0053; interaction: F(1,32) = 0.1583, P = 0.6934. HC Iba1 area: genotype: F(1,32) = 11.1, P = 0.0022; age: F(1,32) = 5.603, P = 0.0241; interaction: F(1,32) = 1.685, P = 0.2036. HC process area: genotype: F(1,32) = 2.69, P = 0.1108 Age: F(1,32) = 20.21, P < 0.0001; interaction: F(1,32) = 1.566, P = 0.2199. HC process perimeter: genotype: F(1,32) = 3.972, P = 0.0548; age: F(1,32) = 16.07, P = 0.003; interaction: F(1,32) = 1.647, P = 0.2086. Str cell number: genotype: F(1,32) = 0.005704, P = 0.9403; age: F(1,32) = 14.24, P = 0.0007; interaction: F(1,32) = 0.9689, P = 0.3323. Str Iba1 area: genotype: F(1,32) = 5.419, P = 0.0264; age: F(1,32) = 1.819, P = 0.1870; interaction: F(1,32) = 8.379, P = 0.0068. Str process area: genotype: F(1,32) = 5.746, P = 0.0225; age: F(1,32) = 34.86, P < 0.0001; interaction: F(1,32) = 10.53, P = 0.0027. Str process perimeter: genotype: F(1,32) = 8.114, P = 0.0076; age: F(1,32) = 27.4, P < 0.0001; interaction: F(1,32) = 10.9, P = 0.0024.