Signals from the dorsal blastopore lip region during gastrulation bias the ectoderm toward a nonepidermal pathway of differentiation in Xenopus laevis.

Epi 1, a monoclonal antibody, was generated against an epidermal specific epithelial antigen; it does not stain neural epithelium. We have used Epi 1 as a marker to determine when the spatial patterns delineating neural from nonneural epithelium become established. We used ventral ectoderm in a sandwich assay to show that signals from the central blastopore lip region, passing through the plane of the ectoderm sheet, define the pattern and boundary characteristics of Epi 1 expression. The dorsal blastopore lip at stages 10 and 12 are the strongest in inhibiting Epi 1 expression. The involuted chordamesoderm has only a limited inhibitory effect on Epi 1 expression in ventral ectoderm recombinates and does not appear to establish pattern boundaries. We suggest that the blastopore lip region establishes a preneural bias in the adjacent ectoderm prior to the interaction of the latter with chordamesoderm.