The development of links between stereocilia in hair cells of the chick basilar papilla.

Auditory papillae of chicks (embryonic age 6-21 days) were examined by scanning and transmission electron microscopy, in order to trace the development of the tip links between the stereocilia, and in order to trace the development of the spatial organisation of the tip links. In the most immature bundles, stereocilia were not graded in height, while strands of tenuous material interconnected adjacent stereocilia, this material being concentrated in a band near the tips of the stereocilia. The material joined the stereocilia in all directions, with no preferential direction for the interconnecting material being visible. Similarly, no columnar organisation of the stereocilia was visible. As soon as a gradation in height of the stereocilia began to appear, material could be seen running upwards from the shorter stereocilia to the adjacent lengthening stereocilia. There was a continuum in appearance between (i) the material running laterally between short immature stereocilia, (ii) the material running upwards between stereocilia which were developing a gradation in height, and (iii) the tip links seen in more mature bundles. It is suggested that tip links are a specialisation of the links which join immature stereocilia laterally near their tips. It is also suggested that the orientation of tip links, parallel to the hair cell axis of bilateral symmetry, is produced by the gradient in growth of the stereocilia.