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 doe s not involve the specification of distinct cells types: distinct area s differ in their patterns of connectivity and cytoarchitecture. It ha s been suggested that signals intrinsic to the neocortical neuroepithe lium specify regional fate [3]. Alternatively, spatial patterning migh t be imposed by extrinsic cues such as thalamocortical projections [4- 6]. Recent results highlight the ability of early precursor cells of t he telencephalic neuroepithelium to 'remember' their spatial position from times before thalamic innervation [7-12]. An influence from the t halamus, however, cannot be ruled out as there is a precise invasion o f the correct cortical areas by the corresponding projections [13,14]. Furthermore, cortical neuronal progenitors have been proposed to adop t new connection patterns after transplantation [6,7], as well as when the thalamic input is rerouted [15,16]. Here, we describe the transie nt 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 in formation is sufficient to specify regional fate, extrinsic signals fr om the thalamus are involved in the expansion or maintenance of the po pulation of cells expressing Otx2 but not in regionalization. (C) Curr ent Biology Ltd ISSN 0960-9822.