Shh dependent and independent maintenance of basal midbrain.

Sonic hedgehog (Shh) is well known as the molecule responsible for the induction and maintenance of ventral neural tube structures. Recent data have shown that ventral neuronal populations react differentially to the amount of this morphogen not only in the spinal cord, but also in more rostral parts of the brain, like the midbrain. A dorsal expansion in the Shh expression domain modifies the differentiation program in this territory. The lack of Shh produces alterations in the development of this area as well. Here, for the first time, we analyze in detail the development of the different mesencephalic basal nuclei in the absence of Shh. We report that the oculomotor complex is lost, the dopaminergic populations are strongly affected but the red nucleus is maintained. These results point out that not all the midbrain neuronal populations are dependent on Shh for their maintenance, as previously thought. Based on our results and recently published data, we suggest the existence of a specific genetic pathway for the specification of the mesencephalic red nucleus. Foxa2 could be the candidate gene that might control this genetic pathway.