Development of the Mammalian 'Vestibular' System: Evolution of Form to Detect Angular and Gravity Acceleration

The mammalian ear consists of a hearing organ (the cochlea) and a gravity- and angular acceleration-perceiving vestibular ear. The vestibular ear consists of a labyrinth of ducts and recesses with associated sensory epithelia that provide sensory input to neurons that project such information to the hindbrain (vestibular nuclei and cerebellum). The hindbrain incorporates these sensory impulses with other sensory inputs from the eyes and limbs to maintain balance. In this chapter, we provide evolutionary insights into the origins of the vestibular ear and discuss the genes that are needed for its formation and function. We will provide first an overview of evolutionary changes in the vestibular ear, such as the addition of an extra lateral (horizontal) canal and segregation of a single otoconia-bearing macula into two or three distinct endorgans, each located in their distinct recess. Subsequently we discuss atavistic phenotypes identified in loss of function mouse mutants, which we interpret as evidence of how the addition of those genes could have driven the evolution of morphological changes. In addition, we discuss how neurons form, how they migrate, and lastly, what keeps them alive. Our discussion will incorporate the newest theories of inner ear development based on comparative phylogenetics and genetically manipulated mice that allow us the unique opportunity to look back in time while keeping our vision on the future. As our knowledge of these complicated processes is incomplete, it is imperative that additional research builds on the past and enlightens our way to understanding the molecular definition of inner ear evolution and its development.