Screens for mutants with misoriented wing hairs in Drosophila have identified a set of genes that is critical for planar cell polarity. Some of these genes code for
Figure 21–50 Convergent extension and its cellular basis. (a) Schematic diagram of cell behaviors that underlie convergent extension. the cells form lamellipodia, with which they attempt to crawl over one another. alignment of the lamellipodial movements along a common axis leads to convergent extension. the process depends on the Wnt–Frizzled/ planar-cell-polarity signaling pathway and is cooperative, presumably because cells that are already aligned exert forces that tend to align their neighbors in the same way.
(B–G) the pattern of convergent extension of dorsal mesoderm during zebrafish gastrulation at 8.8 (B, e), 9.3 (c, F), and 11.3 (D, G) hours after fertilization. cells that will give rise to the notochord are labeled in green, and cells that will give rise to somites and muscle are labeled in blue. the notochord and somite domains are spatially separate from the start of the recording (B, e), but their boundaries are at first barely visible and only a little later become obvious. convergence narrows the notochord domain to a width of about  two cells at the last time point (D, G). (a, after J. Shih and r. Keller, Development 116:901–914, 1992; B–G, after
N.S. Glickman et al., Development 130:873–887, 2003. With permission from the company of Biologists.)
Figure 21–51 Planar cell polarity. (a) Wing hairs on the wing of a fly. each cell in the wing epithelium forms a small, spiky protrusion or “hair” at its apex, and all the hairs point the same way, toward the tip of the wing. this reflects a planar polarity in the structure of each cell. (B) Sensory hair cells in the inner ear of a mouse similarly have a well-defined planar polarity, manifest in the oriented pattern of stereocilia (actin-filled protrusions) on their surface. the detection of sound depends on the correct, coordinated orientation of the hair cells. (c) a mutation in the gene Flamingo in the fly, coding for a nonclassical cadherin, disrupts the pattern of planar cell polarity in the wing. (D) a mutation in a homologous Flamingo gene in the mouse randomizes the orientation of the planar cell polarity vector of the hair cells in the ear. the mutant mice are deaf. (a and c, from J. chae et al., Development 126:5421–5429, 1999. With permission from the company of Biologists; B and D, from J.a. curtin et al., Curr. Biol. 13:1129–1133, 2003. With permission from elsevier.) components of the Wnt signaling pathway, others code for specialized members of the cadherin superfamily, while the functions of others are uncertain. These components of planar-cell-polarity signaling are assembled at cell–cell junctions in the epithelium in such a way as to exert a polarizing influence that can propagate from cell to cell. Essentially the same system of proteins controls planar cell polarity in vertebrates; mice deficient in homologs of the Drosophila planar polarity genes have a variety of defects, including incorrectly oriented hair cells in the inner ear, making them deaf (Figure 21–51).