Regionalization within the mammalian telencephalon is mediated by changes in responsiveness to Sonic Hedgehog

The cortex and basal ganglia are the major structures of the adult brain de rived from the embryonic telencephalon. Two morphologically distinct region s of the basal ganglia are evident within the mature ventral telencephalon, the globus pallidus medially, and the striatum, which is positioned betwee n the globus pallidus and the cortex. Deletion of the Sonic Hedgehog gene i n mice indicates that this secreted signaling molecule is vital for the gen eration of both these ventral telencephalic regions. Previous experiments s howed that Sonic Hedgehog induces differentiation of ventral neurons charac teristic of the medial ganglionic eminence, the embryonic structure which g ives rise to the globus pallidus. In this paper, we show that later in deve lopment, Sonic Hedgehog induces ventral neurons with patterns of gene expre ssion characteristic of the lateral ganglionic eminence. This is the embryo nic structure from which the striatum is derived. These results suggest tha t temporally regulated changes in Sonic Hedgehog responsiveness are integra l in the sequential induction of basal telencephalic structures.