INITIAL ANTEROPOSTERIOR PATTERN OF THE ZEBRAFISH CENTRAL-NERVOUS-SYSTEM IS DETERMINED BY DIFFERENTIAL COMPETENCE OF THE EPIBLAST

Analyses using amphibian embryos proposed that induction and anteropos terior patterning of the central nervous system is initiated by signal s that are produced by the organizer and organizer-derived axial mesod erm, However, we show here that the initial anteroposterior pattern of the zebrafish central nervous system depends on the differential comp etence of the epiblast and is not imposed by organizer-derived signals . This anteroposterior information is present throughout the epiblast in ectodermal cells that normally give rise both to neural and non-neu ral derivatives, Because of this information, organizer tissues transp lanted to the ventral side of the embryo induce neural tissue but the anteroposterior identity of the induced neural tissue is dependent upo n the position of the induced tissue within the epiblast, Thus, otx2, an anterior neural marker, was only ever induced in anterior regions o f the embryo, irrespective of the position of the grafts, Similarly, h oxa-1, a posterior neural marker was induced only in the posterior reg ions. Furthermore, the boundary of each ectopic expression domain on t he ventral side was always at an equivalent latitude to that of the en dogenous expression of the dorsal side of the embryo. The anteroposter ior specification of the epiblast is independent of the dorsoventral s pecification of the embryo because neural tissues induced in the ventr alized embryos also showed anteroposterior polarity. Cell transplantat ion and RNA injection experiments showed that non-axial marginal mesod erm and FGF signalling is required for anteroposterior specification o f the epiblast, However, the requirement for FGF signalling is indirec t in that cells with compromised ability to respond to FGF can still r espond to anteroposterior positional information.