AHRENS, K.*; SCHLOSSER, G.: Induction of Placodes at Experimentally Created Neural Plate Boundaries

It has long been controversial, whether different types of vertebrate ectodermal placodes (e.g., adenohypophyseal, olfactory, profundal/trigeminal, epibranchial, dorsolateral placodes) originate from a common primordium or are separately induced at various locations in the embryo. However, recent data on the expression of Six1 and Eya1 and several other genes support the existence of a common placodal primordium. At neural plate stages this horseshoe shaped primordium borders the neural plate rostrally and the cranial neural crest laterally. This suggests that early steps of placode induction are shared by all placodes, followed by more localized inductions specific for certain placode types. Moreover, the close lateral apposition of the early placodal primordium to the neural crest forming region suggests that a common dorsoventral patterning mechanism may be involved in the early induction of both neural crest and placodes. In order to test this hypothesis and to gain insights into how such a common patterning mechanism may operate, we performed iso- and heterochronic transplantations in Xenopus embryos: Pieces of the anterior neural plate from GFP-labeled host embryos were grafted to the belly of donor embryos at various stages of development. Neural crest markers were previously shown to be induced along the ectopic neural plate boundaries created in such experiments. Here, we show by in situ hybridization that general placodal markers such as Six1 are also expressed at ectopic neural plate boundaries. We currently study the exact spatiotemporal expression profile of crest and placodal markers along ectopic neural plate boundaries in order to further analyze commonalities or differences in crest vs. placode induction.