The development of stereociliary bundles in the cochlear duct of chick embryos.

The differentiation of hair cell stereociliary bundles was investigated during early stages of embryonic development in the chick cochlear duct. The ultrastructural characteristics of the differentiating stereocilia and the position of the hair cells at the time of their differentiation were determined with scanning (SEM) and transmission (TEM) electron microscopy. Stereocilia were first identifiable with SEM as early as embryonic day 6 (stage 29) in only the distal region of the basilar papilla. By embryonic day 7.5 (stage 32) stereocilia were detected with both SEM and TEM on hair cells located in the distal two-thirds of the basilar papilla. Stereociliary bundles were recognizable throughout the entire basilar papilla by embryonic day 9 (stage 35). At this stage the hair cells exhibited a distal-to-proximal gradient in the cell surface area and the number of stereocilia on each hair cell. These results suggest that there is a distal-to-proximal wave of hair cell differentiation which occurs at a very early time period in the development of the chick cochlea. Both the timing and the direction of the stereociliary differentiation contrast with previous ultrastructural reports of avian hair cell development, yet compare favorably with the patterns of functional development in the auditory system.