Specification of cerebellar progenitors after heterotopic-heterochronic transplantation to the embryonic CNS in vivo and in vitro.

The different cerebellar phenotypes are generated according to a precise time schedule during embryonic and postnatal development. To assess whether the differentiative potential of cerebellar progenitors is progressively restricted in space and time we examined the fate of embryonic day 12 (E12) or postnatal day 4 (P4) cerebellar cells after heterotopic-heterochronic transplantation into the embryonic rat brain in utero or into organotypic CNS explants in vitro. Donor cells, isolated from transgenic mice overexpressing the enhanced-green fluorescent protein under the control of the beta-actin-promoter, engrafted throughout the host brainstem and diencephalon, whereas they rarely incorporated into specific telencephalic structures. In any recipient site, the vast majority of transplanted cells could be recognized as cerebellar phenotypes, and we did not obtain clear evidence that ectopically located cells adopted host-specific identities. Nevertheless, the two donor populations displayed different developmental potentialities. P4 progenitors exclusively generated granule cells and molecular layer interneurons, indicating that they are committed to late-generated cerebellar identities and not responsive to heterotopic-heterochronic environmental cues. In contrast, E12 precursors had the potential to produce all major cerebellar neurons, but the repertoire of adult phenotypes generated by these cells was different in distinct host regions, suggesting that they require instructive environmental information to acquire mature identities. Thus, cerebellar precursors are able to integrate into different foreign brain regions, where they develop mature phenotypes that survive long after transplantation, but they are committed to cerebellar fates at E12. Embryonic progenitors are initially capable, although likely not competent, to generate all cerebellar identities, but their potential is gradually restricted toward late-generated phenotypes.