THE mammalian neocortex is subdivided into functionally distinct areas
differing in cytoarchitecture and connectivity. Areal specification i
s thought to occur late in development1-3 and to be controlled by extr
insic cues, particularly thalamic afferents4. We have produced a trans
genic mouse line in which beta-galactosidase expression in the neocort
ex is largely restricted to layer-IV neurons of the somatosensory area
. Transgene expression in these mice may be considered as an intrinsic
marker of a somatosensory cortex identity. We investigated whether th
e fate of pieces of embryonic cortex from transgenic embryos is modifi
ed after transplantation to ectopic locations. Parietal or occipital c
ortex obtained on embryonic days 14-16 maintained their characteristic
s with respect to transgene expression after heterotopic transplantati
on to the cerebellum or neocortex of newborn hosts. This shows that th
e specification of neocortical areas involves a previously unsuspected
early step of areal determination.