Untersuchungen zur Frühentwicklung und Induktion neurogener Plakoden beim Krallenfrosch Xenopus laevis

It has been suggested that different types of placodes arise from a common panplacodal primordium. Here, I describe the expression of a marker gene, Eya1, which demarcates this panplacodal primordium and is expressed in all neurogenic placodes during subsequent development, similar to Six1 and Six4. Furthermore, markers of subtypes of placodes are already expressed within subdomains of this panplacodal primordium. By means of transplantation, explantation, bead implantation, microinjections and in situ hybridisation, I investigated the time window of the induction of the panplacodal primordium (as revealed by Six1 expression) and the tissues and molecules that mediate this induction. The anterior neural plate is sufficient to induce expression of the panplacodal marker Six1 and necessary for its induction in vivo. In contrast, lateral endomesoderm is not sufficient to induce Six1 expression, but also necessary for its induction in vivo. Both FGF8 and BMP-inhibition are necessary for the induction of Six1 expression. Moreover, FGF8 is able to induce ectopic Six1 expression when BMP is inhibited. Because of its expression pattern, I propose that FGF8 mediates the inductive activity of the anterior neural plate, whereas BMP-inhibition may come from the underlying lateral endomesoderm. The results obtained in this study do not support the BMP gradient model for placode induction and a different model, which emphasizes the competence of ectodermal tissue is proposed here.