Sculpting the labyrinth: Morphogenesis of the developing inner ear

Abstract The vertebrate inner ear is a precision sensory organ, acting as both a microphone to receive sound and an accelerometer to detect gravity and motion. It consists of a series of interlinked, fluid-filled chambers containing patches of sensory epithelia, each with a specialised function. The ear contains many different differentiated cell types with distinct morphologies, from the flask-shaped hair cells found in thickened sensory epithelium, to the thin squamous cells that contribute to non-sensory structures, such as the semicircular canal ducts. Nearly all cell types of the inner ear, including the afferent neurons that innervate it, are derived from the otic placode, a region of cranial ectoderm that develops adjacent to the embryonic hindbrain. As the ear develops, the otic epithelia grow, fold, fuse and rearrange to form the complex three-dimensional shape of the membranous labyrinth. Much of our current understanding of the processes of inner ear morphogenesis comes from genetic and pharmacological manipulations of the developing ear in mouse, chicken and zebrafish embryos. These traditional approaches are now being supplemented with exciting new techniques—including force measurements and light-sheet microscopy—that are helping to elucidate the mechanisms that generate this intricate organ system.