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Key Words
deafness, development, hair cells, inner ear, lateral line organ, mechanotransduction
Abstract
The zebrafish is an excellent model system for studying the molecular basis of inner ear development and function. The eggs develop ex utero and the ear is transparent for the first few weeks of life. Forward genetic screens and antisense technology have helped to elucidate the signaling pathways and molecules required for inner ear development and function. This review addresses the most recent advances in our understanding of how the ear forms and discusses the molecules in hair cells that are essential for sensing sound and movement in the zebrafish.
THE INNER EAR RECEPTOR: THE SENSORY HAIR CELL
The inner ear detects motion and sound. The sensory patches dedicated to this task are termed maculae, cristae, or the organ of Corti. Maculae contain receptors coupled to dense crystal structures known as otoconia or otoliths. Cristae are situated in the semicircular canals. The organ of Corti is a specialization for hearing found only in higher vertebrates. The sensory receptors of the inner ear, the hair cells, are exquisitely sensitive to the mechanical stimuli of movement and sound (20). Their highly specialized apical surfaces consist of unusual processes, termed stereocilia. Each stereocilium is densely packed with actin filaments in a paracrystalline array. The hair-like appearance of the bundle of apical stereocilia gave rise to the name "hair cells." Within each hair bundle, stereocilia are arranged with remarkable precision in rows, with increasing height toward one end of the bundle. Next to the tallest stereocilia in vestibular or lower vertebrate hair cells is a true cilium known as the kinocilium. Its role appears to couple the bundle of stereocilia to the overlying extracellular matrix or the otolithic membrane. In mammalian auditory hair cells, kinocilia appear during development but are reabsorbed in mature hair cells. In the cochlea, the tallest stereocilia of the outer hair cells are instead attached to the tectorial membrane, a gelatinous membrane very similar to the otolithic or cupula membranes present in maculae or cristae. All hair cells are bathed in a special fluid, the endolymph, a high potassium fluid unique to the ear. When hair cells are mechanically stimulated, stereocilia pivot about their bases and move together as a unit...