It bears emphasizing that the competitive advantage of human over murine glial progenitors is evident in wild-type as well as in shiverer mice; in both recipient environments, the human glial progenitors typically out-competed their murine counterparts. The selective expansion of the human glial population in the mouse appears at least in part to be a product of the more sustained proliferation of the transplanted human OPCs, which appear to retain cell-autonomous regulation of expansion, and expand in numbers by over 40-fold in the corpus callosum alone over the first year of life. Yet the competitive advantage of human GPCs on the mouse environment is clearly based on more than preferential expansion, since the murine GPCs are completely replaced by their human counterparts over time. Rather, the process of hGPC colonization seems overtly competitive. Human GPCs typically expand outwards from their periventricular and callosal points of introduction, in advancing waves that seem to repulse resident murine progenitors, which then die, both in situ and upon retreat to the cortical surface (Figure 2). The human GPCs ultimately attain a relatively uniform distribution,
