Interestingly, the slow astrocytic colonization of hGPC-engrafted an chimeric brains provided a revelation of its own, in that it suggests that adult human astrocytes may differ from their murine counterparts in their origin, as well as in their functional repertoires. Human glial chimeras preterminally tagged with the mitotic marker BrdU revealed that while new human astrocytes are continuously recruited to these brains from engrafted human GPCs, astrocytic production from murine progenitors is essentially nil; Windrem et al identified no mouse GFAP+ cells that incorporated BrdU, at either 4 or 8 months of age. This observation suggests a fundamental distinction in the origin of new astroglia in the brains of adult rodents and humans; whereas in mice resident astrocytes have been reported as the principal source of new astrocytes in adulthood (Ge et al. 2012), with GPCs serving principally as oligodendrocyte progenitors (Bu et al. 2004; Kang et al. 2010; Nishiyama et al. 2009; Tripathi et al. 2010), human glial progenitors are notably bipotential for both oligodendrocytes and astrocytes in the adult brain (Nunes et al. 2003; Sim et al. 2011;
