Meeting Abstract

S2-1.4  Thursday, Jan. 3  Segmentation of neurogenic placodes SCHLOSSER, Gerhard; Brain Research Institute, University of Bremen, 28334 Bremen, Germany gschloss@uni-bremen.de

One of the distinguishing features of the vertebrate head is its branchiomeric segmentation with matching segmental units in pharyngeal endoderm and ectoderm (e.g. neural crest streams, rhombomeres). Here I review briefly how segmentation is established in the placodal ectoderm. Cranial placodes give rise to a variety of cell types and contribute to many cranial sensory organs and ganglia. Recent evidence suggests that all cranial placodes originate from a common precursor region located around the anterior neural plate. The transcription factors Six1 and Eya1 are initially expressed in this panplacodal primordium and continue to be expressed in most placodes later on suggesting that they may regulate generic placodal developmental processes shared by different placodes. We have recently shown that placodal Six1 and Eya1 expression is induced during neural plate and fold stages in Xenopus by signals from anterior neural plate and dorsolateral endomesoderm. Moreover, we have demonstrated that Six1 and Eya1 synergistically promote neurogenesis in all neurogenic placodes. Already during neural plate stages several subregions of the panplacodal primordium can be distinguished based on their differential expression of various transcription factors including a posterior placodal region. The latter is then further subdivided into otic, epibranchial and lateral line placodes by unknown mechanisms. The segmental arrangement of this posterior group of neurogenic placodes and their derivative ganglia appears to be imposed by interactions with the underlying pharyngeal pouches and the neural crest. I propose that these tissue interactions do not induce neurogenic placodes de novo but rather serve to stabilize a neurogenic bias in the placodal ectoderm conferred by Six1/Eya1 expression in a segmentally repetitive pattern.