ON THE FUNCTION OF BMP-4 IN PATTERNING THE MARGINAL ZONE OF THE XENOPUS EMBRYO

Bone morphogenetic protein 4 (BMP-4) is expressed in the ventral margi nal zone of the gastrulating embryo. At late gastrula stage this gene is expressed in the ventral-most part of the slit blastopore and in ti ssues that derive from it. At tailbud stages BMP-4 is expressed in the spinal cord roof plate, neural crest, eye and auditory vesicle. The i nteractions of BMP-4 with dorsal genes such as goosecoid (gsc) and Xno t-2 were studied in vivo. In embryos ventralized by UV irradiation and suramin treatment, BMP-4 zygotic transcripts accumulate prematurely a nd the entire marginal zone expresses this gene. The patterning effect of BMP-4 on ventro-posterior development can be revealed by a sensiti ve assay involving the injection of BMP-4 mRNA in the ventral marginal zone of embryos partially dorsalized with LiCl, which leads to the co mplete rescue of trunk and tail structures, The experiments presented here argue that BMP-4 may act in vivo as a ventral signal for the prop er patterning of the marginal zone, actively interacting with dorsal g enes such as gsc and Xnot-2. A model is proposed in which the timing o f expression of various marginal zone-specific genes plays a central r ole in patterning the mesoderm.