Expression of amphioxus neural plate border specifier genes is effected by exogenous zebrafish Bmp4 (zBmp4) protein. Anterior is perpendicular to the plane of the page in the blastopore view and at the top in the dorsal view. The medial boundaries of gene expression in dorsal ectoderm are indicated by arrows (A) or arrowheads (B). Scale bar, 50 μm. (A) In normal late gastrula stage embryos, AP2, Dlx, and Msx are expressed throughout the epidermal ectoderm. Msx is also strongly expressed at the neural plate border. Zic and Pax3/7 expression in the dorsal ectoderm marks the neural plate border, and SoxB1-a expression is in the entire neural plate. During the early neurula stage, the edges of the epidermal ectoderm dissociate from the neuroectoderm and migrate toward the midline. This process is shown in the blastopore view of early neurula embryos. (B) In zBmp4 treated embryos, expression of epidermal markers AP2 and Dlx is greatly expanded, suggesting that the entire dorsal ectoderm is transformed to epidermal fate. The expression of neural plate border markers Msx and Zic is converged to the dorsal midline; expression of another neural plate border marker Pax3/7 and neural plate marker SoxB1-a is lost. In addition, no neurulation process can be observed in zBmp4 treated embryos.
