The distinct and coordinated cellular polarity in the inner ear sensory organs () offers an excellent paradigm for cellular and molecular mechanisms in vertebrate PCP signaling. Together with findings from other model systems, the study of inner ear PCP revealed a potentially conserved mechanism operating in the vertebrates to regulate the formation of reiterative pattern of polarized structures in cells across a tissue field. In particular, several genes conserved from Drosophila, including homologs of core PCP genes Vang, Fz, Fmi, Dvl, Diego, Pk, are also required for planar polarization in the vertebrates (). In addition, the vertebrate PCP genes may have also retained their function in regulating cell adhesion. Such a conserved function for PCP genes could underlie their evolved essential role in the CE process in vertebrates.
The vertebrate PCP pathway, predictably, has also evolved novel mechanisms to comply with special requirements under different cellular and tissue contexts. Scrb1 and PTK7 are novel core PCP genes in vertebrates, vertebrate Wnts may indeed be bona fide PCP genes, and the primary cilia are identified as a unique component of the vertebrate PCP signaling pathway (). These new members of the vertebrate PCP pathway provide insights into the mechanisms operating in the vertebrates. For instance, the primary cilia are shown to be essential for intrinsic cellular polarity in the inner ear sensory cells during PCP signaling, and the associated structure of the primary cilia, the basal body, is likely a key component of the vertebrate PCP signaling pathway that collaborates with core PCP complexes to regulate morphological polarization (). The requirement for Wnts in PCP-regulated cellular processes in vertebrates is met with mechanisms, including coreceptors of Wnts, dual functions of PCP genes, and the primary cilia and basal body, to suppress the β-catenin-mediated canonical Wnt signaling for specification of PCP signaling ().
The emerging model of vertebrate PCP signaling () has evoked several key questions. The identity and action of directional cues are currently unknown; the interactions among core PCP proteins are yet to be delineated; the link between core PCP proteins and the primary cilia or the basal body is missing; the activation of downstream pathways to impact the cytoskeletal machinery for changes in cell shape and formation of polarized structures is not understood; and the mechanism underlying the role of PCP signaling in CE is not clear. The combination of advancement in research tools and further understanding of the morphogenesis of various inner ear sensory organs will undoubtedly start to address some of these issues.