Forming and Patterning the Rudiments of the Inner Ear

Abstract

The inner ear begins its development as a plate of cells (the otic placode) formed from the surface ectoderm of the embryo, which quickly internalizes to form a vesicle called the otocyst. The otocyst in turn forms the developing membranous labyrinth, which subdivides into dorsal vestibular and ventral auditory components. Two aspects of inner ear development will be discussed: induction of the otic placode; and initial dorsal patterning of the otocyst. Induction of the otic placode involves a cascade of growth factors secreted during gastrulation/early neurulation by all three germ layers: the neuroectoderm (secretes WNT8a and FGF3), the mesoderm (secretes FGF3, 4, and 19), and endoderm (secretes FGF8). Gain‐of‐function and loss‐of‐function experiments demonstrate the importance of these factors in establishing a hierarchy of interactions that lead to otic induction. Initial dorsal patterning of the otocyst‐‐manifested by rapid thinning and expansion of the otocyst wall‐‐is controlled by BMP signaling that results in a dramatic change in cell shape. Loss of adherens junctions and redistribution of E‐Cadherin play important roles in this process. As a result of these changes, dorsoventral regionally specific morphogenesis of the otocyst occurs, leading to structural and functional differences that define the vestibular and auditory components of the developing inner ear.