Nicolas Johnen

Evidence for a partial epithelial–mesenchymal transition in postnatal stages of rat auditory organ morphogenesis

The epithelial–mesenchymal transition (EMT) plays a crucial role in the differentiation of many tissues and organs. So far, an EMT was not detected in the development of the auditory organ. To determine whether an EMT may play a role in the morphogenesis of the auditory organ, we studied the spatial localization of several EMT markers, the cell–cell adhesion molecules and intermediate filament cytoskeletal proteins, in epithelium of the dorsal cochlea during development of the rat Corti organ from E18 (18th embryonic day) until P25 (25th postnatal day). We examined by confocal microscopy immunolabelings on cryosections of whole cochleae with antibodies anti-cytokeratins as well as with antibodies anti-vimentin, anti-E-cadherin and anti-β-catenin. Our results showed a partial loss of E-cadherin and β-catenin and a temporary appearance of vimentin in pillar cells and Deiters between P8 and P10. These observations suggest that a partial EMT might be involved in the remodelling of the Corti organ during the postnatal stages of development in rat.

Spatio-temporal dynamics of β-tubulin isotypes during the development of the sensory auditory organ in rat.

Abstract There are different β-tubulin isoforms in microtubules of vertebrate tissues. However, their functional significance is still largely unknown. In the present study, we investigated the localization of five β-tubulin isotypes (β1–5) within the hearing organ during development in rat. 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 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 post-natal development in specific cell types (pillar cells and Deiters’ cells). By using transmission electron microscopy, we revealed further that this developmental stage coincided with the formation of two separate bundles of microtubules from a unique one in these supporting cells. Together, these results suggest that the β5-tubulin isoform might be involved in the generation of new microtubule bundles from a pre-existing one.

Differentiation of Boettcher’s cells during postnatal development of rat cochlea

Contrary to the highly specialized epithelial cells of the mammalian auditory organ, little is known about the surrounding cells and, in particular, Boettcher’s cells (BC). Our morphological studies show that, in rats, these cells began their differentiation around postnatal day 8 (P8) reaching maturity around P20, when they are completely covered by Hensen’s and Claudius’ cells. Tight junctions were noted near the apex of BC, providing that they were in direct contact with the endolymphatic space, between approximately P8 and P16. We observed gap junctions between BC and adjacent cells before the end of the covering process suggesting the additional involvement of BC in potassium recycling into the endolymph. Adherens junctions were also seen between BC throughout their maturation. Importantly, we noticed cytoplasmic secretory granules and an accumulated material, probably a secretion, in the intercellular space, between P8 and P25. These results indicate that BC could basally take part in the secretion of the extracellular matrix of the basilar membrane. Finally, we show that the basolateral interdigitations of BC are longer and more tighlty grouped at maturity and harbour urea transporters as early as P18. Our observations thus support the view that BC perform several functions.