Changes in the structure and the composition of microtubules within supporting cells of the organ of hearing during development.

The auditory organ is a highly specialized structure composed by specific cellular types. The sensory cells are characterized by stereocilia at their apex and are necessary for the sound perception. These cells are supported by supporting cells which contain up to 3000 microtubules and are formed by 15protofilaments (15pf) instead of the canonical 13, a unique fact among vertebrates. It was shown that a β-tubulin isoform can specify the microtubule architecture, such as the expression of the Moth β2 in the Drosophila testes imposed the 16pf structure on the corresponding subset of Drosophila microtubules, which normally contain 13pf.
To determine the role of these tubulins in the auditory organ and their possible involvement in the formation of the unusual 15pf microtubules of supporting cells, we studied the spatiotemporal localization of five -tubulin isotypes (β1 to 5) as well as acetyl-α-tubulin within the hearing organ during development in rodents. By using confocal microscopy, we showed that with the exception of the β3-tubulin isoform that was specific to nerve fibres, all the different β-tubulin isoforms and acetyl-α-tubulin were mainly present in the supporting cells. Contrary to β1-4-tubulins, we also found that the β5-tubulin isoform appeared only at a key stage of the postnatal development.
By using transmission electron microscopy, we examined the fine structure of microtubules at an early and a late postnatal stage. Our TEM study indicated that these cells are composed by 13pf microtubules at P2, but by 15pf microtubules at P25. We revealed further that this developmental stage coincide with the formation of two separate bundles of microtubules from a unique one in these supporting cells.
In conclusion, the architecture and composition of microtubules present in the supporting cells change during development of the organ of Corti. Further experiments are now required to determine if these changes are related to the appearance of β5-tubulin.