Vertebrate anteroposterior patterning: the Xenopus neurectoderm as a paradigm

This review discusses formation of the vertebrate anteroposterior (AP) axis, focusing on the dorsal ectoderm, which gives rise to the nervous system, using the frog Xenopus as a model. After summarizing classical models of AP neural patterning, we describe recent molecular studies that are encouraging re-examination of these models. Such studies have shown that AP ectodermal patterning occurs by the onset of gastrulation, much earlier than previously thought. The identity of tissues that determine AP pattern is discussed, and the definition of the Organizer is reconsidered. The activity of factors secreted by inducing tissues in early patterning decisions is assessed and formulated into a revised model for Xenopus AP neural patterning. Finally, AP ectodermal patterning in Xenopus dorsal ectoderm is compared to that of other germ layers, and to other vertebrates.