Neural pattern in vertebrates has been thought to be induced in dorsal ectoderm by ‘vertical’ signals from underlying, patterned dorsal mesoderm. In the frog Xenopus laevis, it has recently been found that general neural differentiation and some pattern can be induced by ‘planar’ signals, i.e. those passing through the single plane formed by dorsal mesoderm and ectoderm, without the need for vertical interactions. Results in this paper, using the frog Xenopus laevis, indicate that four position-specific neural markers (the homeobox genes engrailed-2(en-2), XlHbox1 and XlHbox6 and the zincfinger gene Krox-20) are expressed in planar explants of orsal mesoderm and ectoderm (‘Keller explants’), in the same anteroposterior order as that in intact embryos. These genes are expressed regardless of convergent extension of the neurectoderm, and in the absence of head mesoderm. In addition, en-2 and XlHbox1 are not expressed in ectoderm when mesoderm is absent, but they and XlHbox6 are expressed in naive, ventral ectoderm which has had only planar contact with dorsal mesoderm. en-2 expression can be induced ectopically, in ectoderm far anterior to the region normally fated to express it, suggesting that a prepattem is not required to determine where it is expressed. Finally, the mesoderm in planar explants expresses en-2 and XlHbox1 in an appropriate regional manner, indicating that A-P pattern in the mesoderm does not require vertical contact with ectoderm. Overall, these results indicate that anteroposterior neural pattern can be induced in ectoderm soley by planar signals from the mesoderm. Models for the induction of anteroposterior neural pattern by planar and vertical signals are discussed.

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