BONE MORPHOGENETIC PROTEIN-4 EXPRESSION CHARACTERIZES INDUCTIVE BOUNDARIES IN ORGANS OF DEVELOPING ZEBRAFISH

We have cloned and examined the expression pattern of zebrafish bone m orphogenetic protein-4 (BMP4) as a start to evaluating signals which m ight participate in the fashioning of organ systems in this geneticall y tractable species. The predicted sequence of the mature zebrafish pr otein is more than 75% identical to that of other vertebrates and 66% identical to Drosophila decapentaplegic (Dpp). As in other species, BM P4 is ex pressed ventrally during gastrulation, but the zebrafish is u nusual in having an additional dorsal domain of expression. Subsequent BMP4 expression is especially prominent in sensory organs, fm buds, a nd in the gut, kidney, and heart. In al these sites, it becomes partic ularly enriched in regions of inductive demarcations. For example, exp ression initially extends through the entire heart tube but then becom es limited to the boundaries between cardiac chambers (sinus venosus-a trial junction, atrioventricular junction, and aortic root) prior to c ushion formation. In early pectoral fin development, BMP4 is at first expressed uniformly but then becomes restricted to the mesenchyme subj acent to the apical ectodermal ridge. This suggests that among its rol es in development, BMP4 serves as a signal in primordial outgrowth and also as a signal demarcating the borders within organs or structures where subspecializations occur.