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What then are the cellular and molecular mechanisms involved in the establishment of the regional organization of the forebrain. Central to this problem is identifying when cells within forebrain are committed to a specific regional phenotype. To address this question, we previously initiated a series of in vivo grafting experiments using forebrain progenitors from a variety of developmental stages from early to late neurogenesis. (Fishell, 1995, Na et al., 1998). We observed that while individual progenitors maintain their anterior-posterior (A-P) identity after transplantation, their dorsal-ventral (D-V) identity remains plastic. This suggests that forebrain progenitors contain intrinsic A-P information that dictates their response to more ubiquitous D-V cues, such as Shh and BMPs.
Establishing D-V pattern within telencephalon involves two complementary processes: the expression of regional phenotypes and a concommitant change in cell proliferation. How these two processes are coordinately regulated is at present poorly understood. Nonetheless, candidate molecules for regulating each of these processes have been identified. We have begun to experimentally test the role of both genes with regional patterns of expression; as well as genes in the lateral signalling pathway , which have been shown to be involved in patterns of proliferation and differentiation. Genes involved in regional patterning which we are investigating include the transcription factors Dlx2 and Emx1 and the signaling molecules sonic hedgehog and FGF8. Members of the lateral signaling pathway that we are currently examining includes, Notch, Delta, JkRBP and Numb. We are currently using gene targeting methods as a means of examining both gain of function and loss of function phenotypes in the telencephalon.. In addition, we are presently testing the function of both of these classes of genes through ectopic expression studies, using both in vivo and in vitro approaches. To this end, we have devised methods to use viral vectors to etopically express genes in mouse embryos in utero, using a novel high resolution ultrasound-guided injection system (developed by Dr. Dan Turnbull). This has allowed us to gain access to the forebrain at stages before neurogenesis (embryonic day 9.5, E9.5) As a complement to this work we are also using in vitro forebrain explant cultures from stages prior to when in utero manipulations are possible.
It is clear that regional patterning in the CNS results from an active interplay of regional patterning, combined with control of local patterns of proliferation. Our present experiments are allowing us to take an experimental approach, aimed at dissecting the mechanisms by which these processes are regulated within the telencepahlon.