4 - Morphogenesis and Genetics of Inner Ear Development and Malformation

Abstract

This chapter reviews the morphogenesis of the membranous and bony labyrinths of the inner ear and the genetics of inner ear development. The inner ear develops from cephalic surface ectoderm that forms the otic placode. Each otic placode thickens, and then invaginates to form the otic vesicle, the epithelial primordia from which the vestibular and cochlear regions of the membranous labyrinth develop. The otic capsule appears initially at 6 weeks of human embryonic development as a condensation of mesodermal mesenchyme around the developing otocyst. By 8 weeks of gestation, the condensed mesenchyme has formed a fully chondrified otic capsule that serves as a template for the subsequent formation of the endochondral bony labyrinth. Patterning of the inner ear into prospective vestibular and auditory sensory areas is associated with restriction of gene expression domains during the early stages of otic development. A final consideration in the genetic patterning of the inner ear is the role of the sequential expression of growth factor genes and in specifying the development of both the otic capsule and the perilymphatic spaces, which act to protect and nourish the membranous labyrinth, ultimately forming a fully chondrified otic capsule, with its perilymphatic spaces, which serves as the template for subsequent formation of the bony labyrinth, scala vestibuli, and scala tympani of the adult inner ear, and temporal bone.