Early innervation and differentiation of hair cells in the vestibular epithelia of mouse embryos: SEM and TEM study.

Early afferent innervation and differentiation of sensory vestibular cells were studied in mouse embryos from gestation day (GD) 13 to 16. Afferent neurites were found as early as GD 13 in the epithelium when there were no clearly differentiated sensory cells. By GD 14 the earliest sensory cells which exhibited short hair bundles at their luminal pole were then contacted by afferent endings at their basal part. On GD 15 nerve endings establishing specialized synaptic contacts, characterized by asymmetrical membrane densities and synaptic bodies, were observed. At this stage, microtubules contacting the presynaptic membranes, as well as coated vesicles were found. On GD 16 the hair cells were multi-afferented and numerous synaptic bodies were found. These results showing a concomitance between the hair cell differentiation and the establishment of nerve contacts are discussed with particular respect to nerve-hair cell interactions during sensory differentiation. This study does not point to a primary induction of vestibular hair cell differentiation by nerve endings, but it is consistent with the possibility that the ingrowth of nerve fibers is one of many factors that influence the differentiation of receptor cells. With respect to synapse formation, it is assumed that the location of synaptic bodies at presynaptic densities is determined by the arrival of afferent nerve endings.