Sonic hedgehog signaling at gastrula stages specifies ventral telencephalic cells in the chick embryo

A secreted signaling factor, Sonic hedgehog (Shh), has a crucial role in th e generation of ventral cell types along the entire rostrocaudal axis of th e neural tube. At caudal levels of the neuraxis, Shh is secreted by the not ochord and floor plate during the period that ventral cell fates are specif ied. At anterior prosencephalic levels that give rise to the telencephalon, however, neither the prechordal mesoderm nor the ventral neural tube expre sses Shh at the time that the overt ventral character of the telencephalon becomes evident. Thus, the precise role and timing of Shh signaling relevan t to the specification of ventral telencephalic identity remains unclear. B y analysing neural cell differentiation in chick neural plate explants we p rovide evidence that neural cells acquire molecular properties characterist ic of the ventral telencephalon in response to Shh signals derived from the anterior primitive streak/Hensen's node region at gastrula stages. Exposur e of prospective anterior prosencephalic cells to Shh at this early stage i s sufficient to initiate a temporal program of differentiation that paralle ls that of neurons generated normally in the medial ganglionic eminence sub division of the ventral telencephalon.