Their cell-autonomous maturation and morphologies notwithstanding, the engrafted human cells rapidly integrated with murine host cells. The gap junction tracer Alexa 594 (MW 760), once injected into EGFP+ human cells, spread rapidly into multiple neighboring EGFP- host cells, suggesting the competence of interspecies gap junctions linking human and mouse astroglia, likely derived from the apposition of human and mouse connexin 43 hemi-channels (Fig. 2F). A large number of hNuclei+ cells failed to express GFAP, but did express a human specific isoform of the chondroitin sulfate proteoglycan, NG2 (Fig. 2G), a prototypic marker of parenchymal glial progenitor cells (Mangin and Gallo, 2011; Robel et al., 2011). Of note, hGFAP+ and hNG2+ human cells often co-existed in close proximity, although their relative ratios exhibited considerable variation across regions, as well as between individual mice (Fig. 2G). Transferrin immunostaining failed to detect any human oligodendroglia, consistent with our prior assessment of glial progenitor cell fate upon transplantation to normally-myelinated brain (Windrem et al., 2009) (Figs. S2A-B): Whereas a large proportion of engrafted human GPCs differentiate into oligodendrocytes in hypomyelinated shiverer mice, essentially no human oligodendrocytes were found in similarly engrafted wild-type mice (Windrem et al., 2009).
