Gradients and the Specification of Planar Polarity in the Insect Cuticle

In addition to specifying cell fate, there is a wealth of evidence that molecular gradients are also primarily responsible for specifying cell polarity, particularly in the plane of epithelial sheets (“planar polarity”). The first compelling evidence of a role for gradients in specifying planar polarity came from transplantation experiments in the insect cuticle. More recent molecular genetic analyses in the fruit fly Drosophila have begun to give insights into the molecular nature of the gradients involved, and how they are interpreted at the cellular level.Development requires the coordinated specification of at least three attributes: cell fate, tissue size, and cell polarity. In both theory and practice, all three can be specified by the action of gradients. This article examines the experimental evidence for gradients acting to specify cell polarity in developing tissues, considers the mechanisms by which they are thought to act, and discusses what remains unknown. The problem of how cell polarity is specified in the plane of a tissue (“planar polarity”) is addressed. The tissues discussed are all formed from epithelial sheets that also show apicobasal cell polarity.For more than half a century, the preeminent system for studying the regulation of planar polarity in epithelia has been the insect cuticle. This lends itself to the study of the problem by virtue of often being adorned by structures such as hairs, scales, ridges, or other protrusions that reveal the polarity of the underlying cells. However, the lack of polarized structures on the surface of other epithelial-derived tissues should not be taken as evidence that the cells are not planar polarized, because often such polarity is cryptically expressed and only becomes apparent when the cells participate in a polarized process, such as cell division or cell intercalation.