Axial mesendoderm refines rostrocaudal pattern in the chick nervous system

There has long been controversy concerning the role of the axial mesoderm i n the induction and rostrocaudal patterning of the vertebrate nervous syste m. Here we investigate the neural inducing and regionalising properties of defined rostrocaudal regions of head process/prospective notochord in the c hick embryo by juxtaposing these tissues with extraembryonic epiblast or ne ural plate explants, We localise neural inducing signals to the emerging he ad process and using a large panel of region-specific neural markers, show that different rostrocaudal levels of the head process derived from headfol d stage embryos can induce discrete regions of the central nervous system. However, we also find that rostral and caudal head process do not induce ex pression of any of these molecular markers in explants of the neural plate. During normal development the head process emerges beneath previously indu ced neural plate, which we show has already acquired some rostrocaudal char acter. Our findings therefore indicate that discrete regions of axial mesen doderm at headfold stages are not normally responsible for the establishmen t of rostrocaudal pattern in the neural plate. Strikingly however, we do fi nd that caudal head process inhibits expression of rostral genes in neural plate explants. These findings indicate that despite the ability to induce specific rostrocaudal regions of the CNS de novo, signals provided by the d iscrete regions of axial mesendoderm do not appear to establish regional di fferences, but rather refine the rostrocaudal character of overlying neuroe pithelium.