The ascidian embryo, with its relative morphological simplicity, is an excellent system in which to study cell fate specification. The Cionaneural plate, for example, contains six rows and eight columns of aligned cells, each with a unique molecular signature. Hudson and colleagues now report that sequential and combinatorial inputs from Nodal, Delta2/Notch and FGF/MEK/ERK signalling pathways establish this grid-like organization of distinct cell identities (see ). To study cell fate specification in the posterior-most two rows of the neural plate,the researchers used a combination of morpholino-based gene knockdown,dominant-negative genes and pharmacological inhibitors. They show that Nodal signalling first defines the medial and lateral neural plate domains. Delta2 signalling then subdivides each of these domains to generate four columns of cells. Finally, FGF/MEK/ERK signalling along the anteroposterior axis promotes row I fates and represses row II fates. Future studies, suggest the researchers, have the potential to uncover the gene regulatory networks that control the fate of each and every neural plate cell.
