Principles in embryonic development and differentiation of vestibular hair cells.

The early development and maturation of vestibular hair cells in the CBA/CBA mouse were analyzed at the ultrastructural level with conventional transmission electron microscopy and freeze fracturing. Cells differentiating into future hair cells pass their terminal mitosis close to the otocyst lumen. The earliest morphologic sign of differentiation into future hair cells was the arrangement of microvilli in a regular fashion. Hair cell cytodifferentiation occurred with a gradient from the hair cell surface to the base. In parallel with the maturation of sensory hairs a structural intracellular rebuilding occurred: the number of polyribosomes and amount of rough endoplasmic reticulum decreased, the nucleus moved basally, and mitochondria accumulated in the supranuclear region. Both tight junctions and gap junctions occurred initially on developing hair cells. From the sixteenth gestational day on, gap junctions disappeared, indicating an uncoupling phenomenon. Afferent nerve terminals developed before efferent nerve endings. Maturation of innervation occurred, for the most part, postnatally, and after that hair cells reached mature morphology.