Cooperation of intrinsic and extrinsic signals in the elaboration of regional identity in the posterior cerebral cortex.

Understanding the compartmentalization of the neocortex (isocortex) of the mammalian brain into functional areas is a challenging problem [1-3] . Unlike pattern formation in the spinal cord and hindbrain, it does not involve the specification of distinct cells types: distinct areas differ in their patterns of connectivity and cytoarchitecture. It has been suggested that signals intrinsic to the neocortical neuroepithelium specify regional fate [3]. Alternatively, spatial patterning might be imposed by extrinsic cues such as thalamocortical projections [4-6]. Recent results highlight the ability of early precursor cells of the telencephalic neuroepithelium to 'remember' their spatial position from times before thalamic innervation [7,8] [9-12]. An influence from the thalamus, however, cannot be ruled out as there is a precise invasion of the correct cortical areas by the corresponding projections [13,14]. Furthermore, cortical neuronal progenitors have been proposed to adopt new connection patterns after transplantation [6,7], as well as when the thalamic input is rerouted [15,16]. Here, we describe the transient expression of the homeobox gene Otx2 in the posterior, prospective visual, neocortex and use it to analyze the establishment of posterior cortical fate. The results suggest that whereas intrinsic cortical information is sufficient to specify regional fate, extrinsic signals from the thalamus are involved in the expansion or maintenance of the population of cells expressing Otx2 but not in regionalization.