PCP and the refinement of tissue patterns. (A and B) Examples in development in which PCP spatially regulates actomyosin to refine the positioning of an epithelial structure. (A) The orientation and number of Drosophila wing hairs are controlled by the PCP pathway. Loss of core PCP proteins leads to the misorientation of wing hairs, whereas ROCK activity downstream of Dsh regulates the number of hairs by restricting actin bundling activity to a single site. PCP similarly controls the global AP angling of mammalian hair follicles, which may involve apical constriction of cells along one side of a follicle. (B) In the Drosophila eye, the PCP pathways controls the rotation of ommatidia, which establishes an axis of mirror symmetry along the dorsal–ventral axis in the eye imaginal disc. Here, PCP likely regulates the remodeling of specific cell–cell junctions in ommatidial precursors to control the degree of cluster rotation. (C) The core PCP pathway. After their apical localization, PCP components distribute into mutually antagonistic complexes, consisting of Frizzled, Dishevelled, and Diego at the distal end, and Strabismus (Vangl2) and Prickle at the proximal end in Drosophila wing cells. PCP proteins are recruited into the region of the adherens junction by the atypical cadherin Flamingo (Celsr1). (D) PCP spatially controls myosin II activity, local actin remodeling, and adhesion through tissue-specific effectors. In vertebrate systems such as the chick neural tube and Xenopus mesoderm, this involves activation of ROCK downstream of Dsh and DAAM1. Other systems use different effectors that may either be binding partners of Dsh or may function independently. The PCP pathway is also known to control differentiation in the Drosophila eye and leg through Notch signaling, both by transcriptional expression of Delta and regulation of Notch receptor endocytosis, which has the potential to transcriptionally regulate the cytoskeleton. Illustration in C based on Seifert and Mlodzik (2007).