Regional specification during cerebellar development

At early phases of development (stage HH12 of chick and quail embryos) the rostral part of the cerebellar anlage extends into the caudal third of the mesencephalic vesicle. This region is characterized by its high level of expression of the Wnt-1 and engrailed genes, which encode, respectively, for a secreted molecule and for a homeodomain protein. In order to analyze the potentialities of various parts of the cerebellar anlage, pieces of quail or mouse embryonic neural tubes have been transplanted heterotopically in the prosencephalon of HH12 chick embryos. These experiments indicated that signals operating in the plane of the neural tube are still operating in the donor avian and murine embryos. The met- mes- and prosencephalic territories of HH12 chick embryos are still competent to change their fates under the influence of these signals. It was also found that the Wnt-1 expression domain, the caudal mesencephalic vesicle, has striking organizing properties until late stages of development in avian and rodent embryos. The exact role of Wnt-1 in these inductive interactions is still unknown. Wnt-1 influence on cell fate could be mediated by the engrailed genes. Observations suggesting a later role of en-2 in the regionalization of the cerebellar cortex are also reported.