Abstract
Planar polarity describes the organization and orientation of polarized cells or cellular structures within the plane of an epithelium. The sensory receptor hair cells of the vertebrate inner ear have been recognized as a preeminent vertebrate model system for studying planar polarity and its development. This is principally because planar polarity in the inner ear is structurally and molecularly apparent and therefore easy to visualize. Inner ear planar polarity is also functionally significant because hair cells are mechanosensors stimulated by sound or motion and planar polarity underlies the mechanosensory mechanism, thereby facilitating the auditory and vestibular functions of the ear. Structurally, hair cell planar polarity is evident in the organization of a polarized bundle of actin-based protrusions from the apical surface called stereocilia that is necessary for mechanosensation and when stereociliary bundle is disrupted auditory and vestibular behavioral deficits emerge. Hair cells are distributed between six sensory epithelia within the inner ear that have evolved unique patterns of planar polarity that facilitate auditory or vestibular function. Thus, specialized adaptations of planar polarity have occurred that distinguish auditory and vestibular hair cells and will be described throughout this review. There are also three levels of planar polarity organization that can be visualized within the vertebrate inner ear. These are the intrinsic polarity of individual hair cells, the planar cell polarity or coordinated orientation of cells within the epithelia, and planar bipolarity; an organization unique to a subset of vestibular hair cells in which the stereociliary bundles are oriented in opposite directions but remain aligned along a common polarity axis. The inner ear with its complement of auditory and vestibular sensory epithelia allows these levels, and the inter-relationships between them, to be studied using a single model organism. The purpose of this review is to introduce the functional significance of planar polarity in the auditory and vestibular systems and our contemporary understanding of the developmental mechanisms associated with organizing planar polarity at these three cellular levels.
Highlights
Planar polarity is the coordinated organization of cellular polarities within the plane of an epithelial layer
The hair cells are distributed between two groups that are aligned along a common polarity axis but differ in that their stereociliary bundles are oriented in opposite directions
As I will describe, the mechanisms generating planar bipolarity would be better viewed as tissuespecific modulators of intrinsic cell polarity. These three aspects of inner ear planar polarity are mediated by separate signaling pathways encoded by different groups of polarity genes, but their coordinated activities are necessary for building functional auditory and vestibular sensory organs
Summary
Planar polarity is the coordinated organization of cellular polarities within the plane of an epithelial layer. The third level of planar polarity is only found in the otolithic organs (utricle and saccule) of the vestibule (Deans, 2013) and the lateral line neuromasts of fish, where it was recently described as planar bipolarity (Kozak et al, 2020) In these systems, the hair cells are distributed between two groups that are aligned along a common polarity axis but differ in that their stereociliary bundles are oriented in opposite directions. As I will describe, the mechanisms generating planar bipolarity would be better viewed as tissuespecific modulators of intrinsic cell polarity These three aspects of inner ear planar polarity are mediated by separate signaling pathways encoded by different groups of polarity genes, but their coordinated activities are necessary for building functional auditory and vestibular sensory organs. This review will contain a combination of anatomical and physiological observations relating to the functional significance of planar polarity as well as molecular mechanisms that have been identified that build these systems, with differences and similarities between auditory and vestibular highlighted throughout
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