Cytodifferentiation of cochlear hair cells.

Cytodifferentiation of a limited number of cochlear hair cells in the mouse starts on the 14th gestational day. One day later, the number of identified hair cells increases considerably. Cytodifferentiation apparently occurs in a gradient from the hair cell surface to the base. First, the irregular microvilli covering the future hair cell surfaces begin to show a regular pattern but are of the same thickness and length as microvilli on supporting cells. Second, a polarization of sensory hairs occurs with a stepwise increase in stereocilia length in the different rows toward the kinocilium. Finally, the cuticle is formed, giving an anchorage for sensory hair rootlets in the hair cell. At birth some hair cells can be found with immature surface morphologic features, e.g., stereocilia of the same length on the entire hair cell surface and lack of a cuticular plate. The onset of hair cell differentiation takes place without morphologic contact with ingrowing nerve fibers.