Maturation of kinocilia in amphibian hair cells: growth and shortening related to kinociliary bulb formation.

New hair cells are added to the amphibian sacculus throughout life, primarily at its outer edge. The stereociliary bundles near that edge are heterogeneous, but eventually develop the more homogeneous morphology of the overwhelming majority of mature cells near the center of the epithelium. During their development the kinocilium grows and a kinociliary bulb forms. It has been proposed that initial elongation of the kinocilium is followed by shortening, and that the bulb may form during shortening. To test those hypotheses, amphibian sacculi were examined by scanning electron microscopy and the length of the kinocilium, the width of the kinociliary bulb, and the length of the tallest stereocilia were measured for 159 hair cells. The length of the tallest stereocilia on each hair cell was used as an indicator of the relative maturity of that cell, so that changes in the structure of each cell's kinocilium could be related to that cell's stage of development. Results indicate that initial elongation of the kinocilium is followed by shortening. Kinociliary bulbs first appear and increase in volume as shortening proceeds. These findings support the hypotheses. Models are proposed to explain how the formation of the bulb could result from depolymerization of apical cytoskeletal elements, and how kinocilium growth and shortening may contribute to hair bundle reorientation in the developing ear.