Cerebral ischemia and CX3CR1 deficiency increased microglial activation in the hippocampus. (A) Significantly increased phagocytic microglia was detected in ischemic rats with CX3CR1 deficiency in comparison to the sham rats and the ischemic control animals. Expression of phagocytic microglia was also significantly increased in the ischemic control rats in relation to the sham group with the greatest increased seen in the CX3CR1 deficient rats. In contrast, the number of ED1-positive cells in the sham rats was negligible and no group difference was seen. Upper panel shows representative photomicrograph of ED1 staining showing larger cell bodies and very short processes. (B) Significantly increased OX-42 microglia phenotype was detected in ischemic control rats compared to the ischemic animals with CX3CR1 deficiency and sham groups. OX-42-positive cells were also significantly increased in ischemic animals with CX3CR1 deficiency in comparison to the sham groups. The number of OX-42-positive cells in the sham rats was negligible and no group difference was seen. Upper panel shows representative photomicrograph of OX-42 staining showing large cell bodies and short processes. (C) Representative photomicrographs of the different microglial phenotype. Ramified microglia or ‘resting microglia’ have smaller cell bodies and longer processes while hypertrophic microglia, also considered as moderately activated, shows bushy appearance, larger cell bodies, and shorter processes. Meanwhile, the ameboid phenotype, considered as highly activated, shows large cells bodies with almost no processes evident. Scale bar = 100 μm. (D) When microglial phenotype was quantified, the ‘resting’ form was significantly increased in the sham groups compared to the ischemic animals but microglial activation was seen in both ischemic groups. The hypertrophic phenotype (E) was significantly increased in the ischemia control rats while expression of the (F) ameboid phenotype was significantly higher in the ischemic animals with CX3CR1 deficiency. The number of hypertrophic and ameboid microglial phenotype in the sham groups were negligible. *P <0.05 and **P <0.01.