Gestational Development of the Human Auditory System Including the Cochlea and the Central Auditory Pathways

Abstract

Embryologically, in the human, the cochlea and its auditory sensory cells (inner and outer hair cells) begin to develop just after the fourth week of pregnancy and both are completely developed at the end of gestation whereas the vestibular organs and their sensory cells begin development much earlier. The neuronal myelination of both systems begins around the fourth month of pregnancy. Phylogenetically, the auditory system is a more recent development than the older vestibular system. Compared across species, the overall neuronal development and myelination of these systems begin later in human embryos than in other species. In human, complete development of both systems continues after birth and progresses through infancy and into puberty. In the central auditory pathways, the developmental rate of axonal myelination does not proceed uniformly across the different auditory relay nuclei. The neuronal axonal tracts only become fully functional after they are completely myelinated. Myelination and function are closely related. Because the gestational onset of the vestibular neurons (axons) precedes that of the auditory organs, complete myelination of the vestibular neurons and relay nuclei tends to occur sooner in the vestibular system versus the auditory system. This supports the concept that the auditory system and its’ central neuronal pathways is, phylogenetically, a more recent development compared to the older vestibular system.