Breaking cellular symmetry along planar axes in Drosophila and vertebrates.

In many organs, epithelial cells are polarized not only along the apicobasal axis, but also along a second axis within a plane. Acquisition of the latter polarity, known as planar cell polarity (PCP) or tissue polarity, is crucial for specialized cellular functions. Genetic programming of PCP has been most thoroughly studied in Drosophila, which has allowed identification of a number of regulatory molecules that are evolutionally conserved. One group of the regulators is responsible for interpreting a hypothetical polarity cue and directing local cytoskeletal reorganization. This group includes a seven-pass transmembrane cadherin known as Flamingo (also known as Starry night), other receptors, and downstream components; and many of those molecules are redistributed to restricted subcellular compartments. Recent studies on a trio of cell-surface molecules challenge a previous hypothesis about the identity of the polarity cue and prompt a novel hypothesis about a global input. Studies on vertebrate systems support the notion that the molecular mechanisms demonstrated in Drosophila are applicable to at least two classes of polarized behaviors of vertebrate cells: sensory hair morphogenesis in the inner ear epithelium, and convergent extension movements during gastrulation.