The specification of sensory cortex: lessons from cortical transplantation.

The mammalian neocortex is functionally organized into numerous specialized "areas." The distinct functional properties characteristic of each area are in large part due to connectional and architectural differences among the areas. However, these "area-specific" features which distinguish mature areas are not apparent early in the development of neocortex. We have used heterotopic cortical transplantation to examine whether these area-specific features are prespecified or emerge as a result of epigenetic interactions. Here, we review our studies in which late fetal rat cortex was transplanted heterotopically into the cortex of newborn rats to test its capacity to differentiate features normally unique to other cortical areas. We find that regions of the developing neocortex have similar potentials to differentiate the connectivity and functional architecture that distinguish neocortical areas in the adult. We conclude that the neocortical neuroepithelium generates comparable populations of cells across its extent, and when exposed to the same extrinsic cues, these populations can differentiate in comparable ways. These studies support the concept that the neocortical neuroepithelium generates a "protocortex" (20), specified to have fundamental cortical features but lacking a rigid specification of "area-specific" features.