Morphogenetic mechanisms of inner ear development

Abstract

The vertebrate inner ear is one of the most complex three-dimensional sense organs of our head. This anatomical complexity reflects its different functions as the organ responsible for the senses of hearing and balance: it detects the direction and speed of head rotation and the wide range of sound wave frequencies. During embryonic development, specialized cells (hair cells) originate in distinct domains of the inner ear, the sensory patches, whose topological organisation and orientation is fundamental for proper sensory function. Hair cells have the ability to convert mechanical stimuli into electrical activity that is then transmitted to the brain by sensory neurons. The major sensory patches comprise the three cristae (for angular movement detection), the saccule and utricule (for gravity detection) and the auditory sensory patch, the organ of Corti in mammals or basilar papilla in birds (for auditory detection). For sensory cells to be born in appropriate locations, inner ear patterning and cell fate specification must be coupled with morphogenesis of the entire organ. While excellent reviews have summarized the pathways involved in inner ear patterning (Fekete in Curr Opin Neurobiol 6(4):533-541, 1996; Whitfield et al. in Off Publ Am Assoc Anat 223(4):427-458, 2002; Torres and Giráldez in Mech Dev 71(1-2):5-21, 1998; Fekete and Wu in Curr Opin Neurobiol 12(1):35-42, 2002; Barald and Kelley in Development (Cambridge, England), 131(17):4119-4130, 2004; Alsina et al. in Int J Dev Biol 53(8-10):1503-1513, 2009) morphogenetic events have received little attention and in particular the cross-talk between patterning and morphogenetic cues is poorly understood. In this chapter we will review the morphogenetic mechanisms regulating inner ear shape, size and sensory organization. A wide array of cell behaviours contributes to the final size and shape of all organs. These include cell migration, modulation of cell division or cell death, oriented cell division, epithelial to mesenchymal transition, cell intercalation and remodelling and convergent extension movements. Many of these operate in the inner ear and we will review how each contributes to sculpting the inner ear into its final form.