Early commitment of embryonic neocortical cells to develop area-specific thalamic connections.

In this study we examined the thalamic connectivity developed by grafts of embryonic (E16) parietal or occipital cortex placed homo- or heterotopically into the neocortex of newborn rats. We also examined the cytoarchitectonic organization developed by the grafts. Our findings indicate that E16 parietal cortex grafted into the parietal cortex of newborn recipients develops reciprocal connections with the host thalamic ventrobasal complex (VB) but does not establish connections with the host dorsal lateral geniculate nucleus (DLG). When implanted into the occipital cortex, these grafts are still able to establish connections with the VB. In contrast, E16 occipital cortex grafted into the parietal cortex establishes only a few connections with the VB. These grafts are, however, able to develop a substantial system of connections with the host DLG. At 16 days of embryonic age, graft cells are committed to establish thalamic connections appropriate to their tangential locus of origin. In addition, our results show that E16 parietal or occipital cortical cells do not possess the capacity to differentiate and maintain barrel organization even though they are grafted into the terminal field of developing VB axons.