Halfway through Drosophila development, at the end of germband retraction, a large elliptical hole remains on the dorsal epidermis of the embryo. Closure of this hole (dorsal closure) requires complex cell-shape changes and cell movements within the epidermis and amnioserosa, and on p. 3273, Morel and Martinez Arias provide some surprising insights into this process. During dorsal closure, the dorsal-most epidermal (DME) cells are polarized and acquire an asymmetric distribution of cell membrane-associated proteins and cytoskeletal components. The researchers show that DME cell polarisation involves `canonical' Wingless (Wg) signalling, and results in the activation of β-catenin (Armadillo in Drosophila). This is unexpected because DME cell polarization is reminiscent of planar cell polarity (PCP), a pathway associated instead with Armadillo-independent non-canonical Wg signalling. Indeed, Morel and Martinez Arias find that PCP has no major role during dorsal closure, leading them to re-evaluate the notion of the PCP pathway.

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