Abstract: We discovered microglia in the adult brain are dependent on signaling through the colony-stimulating factor 1 receptor (CSF1R), and identified several CSF1R inhibitors that crossed into the brain, leading to the elimination of most of the microglia. This remarkable phenomenon has been widely replicated and is now a standard in the field to explore microglial function in health and disease, and clinical trials are being conducted/planned as a result. We also found that we could eliminate microglia for as long as we continued treatment, but upon drug withdrawal, repopulation of the microglial tissue occurred rapidly from proliferating cells throughout the brain that formed a new microglial tissue in ~14 days. We found we could use this to “reset” the inflamed microglial tissue after injury or in aging, and promote functional recovery/cognition. In this continuation, we seek to understand the source and properties of these repopulating cells that become microglia, and study how they modulate neuronal gene expression to rejuvenate the aged brain and fully restore long-term potentiation to that of a young animal. In addition, we describe a second slower source of microglial repopulation, that originates in specific brain niches – the rostral migratory stream (RMS) and associated projecting axonal tracts. This “alternative” repopulation is only unmasked by the complete elimination of microglia. These “alternative” cells arise from unknown cells within these brain niches, and eventually can break out from the white matter tracts and fill the cortex/brain. These cells never attain the numbers, morphologies, or gene expression of microglia, but resemble microglia found in the RMS, which have pro-neurogenesis and increased phagocytotic capabilities than other microglia. We will determine the source of these “alternative” cells, and the consequences of filling the brain with them, including if they have any therapeutic potential, in a mouse model of Alzheimer's disease.