Border-associated macrophages (BAMs), including meningeal, perivascular and choroid plexus macrophages, play essential roles in neuroimmune surveillance and responses in homeostasis and neuropathology. However, compared with microglia, the innate parenchymal macrophages in the central nervous system (CNS), the ontogeny and functions of BAMs in health and disease remain relatively unknown. This project focuses on the turnover of the macrophages by circulating monocyte-derived immune cells at the CNS border. We adopted a transgenic model, SclCreERT2 x R26-ZsGreen mice, by which only bone marrow-derived BAMs rather than resident BAMs will be irreversibly marked when the mice are treated with tamoxifen. It prevents invasive surgeries like whole-body irradiation followed by Bone marrow transplantation inducing strong immune responses. Using this method, we quantitatively analysed the peripheral monocyte-based BAM turnover of adult mice under the homeostatic conditions.