Morell, M., Lenoir, M., Jauniaux, T., Dabin, W., Ferreira, M., Maestre, I., Degollada, E., Cazevieille, C., Fortuño, J., Puel, J., André, M. Imaging of odontocete cochlea through scanning and transmission electron microscopy Proceedings of the 25th European Cetacean Conference, Cadiz, Spain, p.94, Mar 2011 Résumé: The morphological study of the Odontocete organ of Corti as well as possible alterations associated to sound exposure represent a key conservation issue to assess the effects of acoustic pollution on marine ecosystems. In addition, since odontocetes produce species-specific acoustic signals, morphological differences in the cochlea are expected to be detected among species. Through the collaboration with stranding networks and rehabilitation centres from several European countries that followed an ear extraction and fixation protocol (defined at the Necropsy Workshop 2009 in Liège, Belgium), 117 ears from 13 species of Odontocetes that stranded in the Mediterranean Sea, North Atlantic and North Sea were processed. Due to technical and experimental constraints, all the cochlea were chemically fixed post-mortem, at least 6 hours after death. Here, we present scanning (SEM) and transmission (TEM) electron microscopy images of several cochlea structures of Delphinus delphis, Phocoena phocoena, Stenella coeruleoalba and Hyperoodon ampullatus. Using SEM, these structures include hair cell stereociliary bundles and outer hair cell (OHC) stereocilia imprints in the tectorial membrane. We focused on hair cells, supporting cells and spiral ganglion neurons through TEM. Typical characteristics of high-frequency hearing species like Horseshoe Bat’s were observed: small size of OHCs, high development of OHC cuticular plate and supporting cell cytoskeleton, robust cup formation of the Deiters cell body which houses the bottom of the OHC and short and thick basilar membrane. By contrast with the rapid decomposition process of the sensory epithelium after death, spiral ganglion neurons and tectorial membrane appeared to be more resistant to post-mortem autolysis. Interestingly, the tectorial membrane structure still remains in acceptable condition for analysis when the cochlea was fixed more than 20h post-mortem. The analysis of the stereocilia imprints on the tectorial membrane is allowing gaining insights in odontocete hair cell stereocilia organization and detecting possible ultrastructural alterations. Projet: eCREM, Effects and Control of Anthropogenic Noise in Marine Ecosystems