Planar Polarity in the Ciliated Epidermis of Xenopus Embryos

The coordinated orientation of ciliary beat in the larval epidermis of amphibians, evident in an organized streamline pattern, suggests a planar polarity of the epithelium, i.e., a polarity within the plane of the cell sheet. It has been proposed that the direction of ciliary beat is determined at mid gastrula by a gradient of a diffusible factor produced by the mesoderm. To analyze whether ectoderm in isolation can establish a uniform direction of ciliary beat, and at what stage its polarity is specified in the embryo, ectoderm of Xenopus laevis embryos of different stages was cultured in vitro on substrates. On concanavalin A, ectoderm isolated at early gastrula stages, i.e., prior to any contact with mesoderm, can autonomously coordinate the direction of ciliary beat, at least in small regions. A uniform planar polarity is expressed by ectoderm explanted from the early mid gastrula onward. On fibronectin, which promotes migration, the direction of movement correlates well with the direction of ciliary beat, and directional migration can even override the inherent polarity specified prior to explantation. Embryos which lack dorsal mesoderm nevertheless develop a highly organized streamline pattern, excluding a strict requirement for dorsal mesoderm for the determination of planar polarity. However, in spite of the early specification of planar polarity found for isolated tissue, rotated ectodermal transplants in situ can readjust their polarity in accordance with that of the host.