Establishment and maintenance of the border of the neural plate in the chick: involvement of FGF and BMP activity

We have investigated the cell interactions and signalling molecules involve d in setting up and maintaining the border between the neural plate and the adjacent non-neural ectoderm in the chick embryo at primitive streak stage s. msx-1, a target of BMP signalling, is expressed in this border at a very early stage. It is induced by FGF and by signals from the organizer, Hense n's node. The node also induces a ring of BMP-4, some distance away. By the early neurula stage, the edge of the neural plate is the only major site o f BMP-4 and msx-1 expression, and is also the only site that responds to BM P inhibition or overexpression. At this time, the neural plate appears to h ave a low level of BMP antagonist activity. Using in vivo grafts and in vit ro assays, we, show that the position of the border is further maintained b y interactions between non-neural and neural ectoderm. We conclude that the border develops by integration of signals from the organizer, the developi ng neural plate, the paraxial mesoderm and the non-neural epiblast, involvi ng FGFs, BMPs and their inhibitors. We suggest that BMPs act in an autocrin e way to maintain the border state. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.