Figure 6. Competence transitions during mammalian neurogenesis.

A. Cortical progenitors lose competence to specify early-born neuronal phenotypes over time. Heterochronic transplantation experiments, in which neural progenitors isolated from one developmental stage (donor) is placed in a similar environment of a different stage (host), show that early progenitors (blue) transplanted into an older host can give rise to later-born phenotypes (red); however older progenitors (red) transplanted into the young embryo do not give rise to early born (layer VI) phenotypes.

B. Changes in chromatin structure at neuronal and gliogenic genes as development progresses contribute to the neurogenic to gliogenic competence transition in the embryo. In early progenitors, regulatory DNA sequences of key gliogenic genes (such as GFAP) are hypermethylated and silenced. In older progenitors, these DNA regions become hypomethylated and are now competent for transcriptional activation. Neural progenitors cultured from older embryos (red and green embryos) are more competent to respond to gliogenic signals to give rise to glial cells (green stars).