Ultrastructural observations on regenerating hair cells in the chick basilar papilla.

This experiment was designed to investigate cellular and subcellular maturational changes in regenerated immature sensory cells and support cells of the chick basilar papilla following gentamycin treatment. Scanning and transmission electron microscopy were used. The experimental animals received one subcutaneous injection of gentamycin sulfate daily (50 mg/kg) for five or 10 days. The animals receiving five days of injection were sacrificed the following day. The remaining animals were allowed to survive either seven or 28 days before sacrifice and preparation for electron microscopy. The initial lesion consisted of total degeneration of hair cells within 500 microns of the proximal tip providing the opportunity to study a 'pure' population of regenerating sensory cells. Sensory cell regeneration could be identified by one day after terminating gentamycin treatment. Early in development sensory cell precursors were morphologically very similar to supporting cells. A density gradient, based on cytoplasmic staining characteristics, was established which increased from cells displaying low density at the base of the supporting cell layer to high density cells at the luminal surface. These changes in density were equated to increase in number of and types of cytoplasmic organelles. In contrast to the empty appearing cytoplasm of the support cell, the cytoplasm of the hair cell precursor contained numerous mitochondria, clusters of ribosomes, and vesicles. As the cell approached the surface, mitochondria became more numerous as did smooth and coarse endoplasmic reticulum and Golgi apparatus. This gradient suggested that determination of the cellular phenotype occurred at the level of the basal membrane followed by migration to the surface, during which time differentiation was characterized by an increase in number and complexity of cellular organelles. Luminal surface modifications occurred as soon as the cell erupted. The development of stereocilia, rootlet, cuticular plate and cellular polarization followed the normal embryogenetic pattern. At 28 days, stereocilia organization was still incomplete as was the orientation of the bundle. To the extent that proper orientation of hair cells or bundles is necessary for normal transduction, mature function at 28 days would not be anticipated. Innervation of the presumptive hair cell precursors could be observed one day after treatment, early in the course of hair cell differentiation. Synaptogenesis followed the normal embryogenetic sequence; however, afferent and efferent nerve terminals remained immature appearing at 28 days. This observation may have physiological implications manifested by delay of hearing