Formation of junctional complexes in otocysts developed in vitro. A freeze-fracture study

Ornithine decarboxylase activity during development of the mouse inner ear in vivo and in vitro

Ornithine decarboxylase activity was determined during the development of the peripheral auditory system in the murine otocyst with the goal of understanding the role of this enzyme in the morphological and functional maturation of the inner ear. At gestational days 11 and 12 enzyme activity was more than 10-fold higher than adult levels. A sharp decline occurred between day 12 and 13 after which activity rose to a peak around day 15. Activity then dropped continuously until near-adult levels were reached at birth. A lower specific activity of ODC but a similar time-course was seen in otocysts explanted at gestational day 13 and subsequently cultured for 6 days. For two stages of development, enzyme activity and binding of 3H-alpha-difluoromethylornithine were compared. The four-fold difference in enzymatic activity on gestational days 15 and 17 was paralleled by a similar difference in binding. Ornithine decarboxylase activity during inner ear development therefore seems primarily regulated at the level of protein synthesis. Ornithine decarboxylase activity correlates with major inductive events in the morphogenesis of the cartilagenous otic capsule that serves as a template for the formation of the bony labyrinth. The pattern of activity may reflect the changes in the head mesenchyme that is recruited by the otocyst to aggregate and form its protective otic capsule.

Intermediate filament proteins in the embryonic inner ear of mice under normal conditions and after exposure to ototoxic drugs

The expression of the intermediate filament subclasses cytokeratins (CKs), vimentin, and neurofilament triplet proteins (NFs) was analysed in the embryonic inner ear of the CBA/CBA mouse, in vivo and in vitro, using well-defined monoclonal antibodies. Some of the cultured inner ears were exposed to 1-10 micrograms/ml of the ototoxic drugs gentamicin, ethacrynic acid or cisplatin. There was no difference in the expression of IF pattern in the cultured inner ears as compared with the in vivo developed labyrinths. Exposure to the ototoxic drugs did not affect the expression of CKs, vimentin, or NFs. CKs 8 and 18 were identified in all inner ear epithelia. In the mature hair cells, only the apical surfaces showed immunoreactivity to CKs. In addition, CKs 7 and 19 were visualized in the epithelia involved in maintaining endolymph homeostasis. The ganglion cells showed co-expression of CKs, vimentin and NFs, each having a characteristic localization in the cells.

The effect of gentamicin-induced hair cell loss on the tight junctions of the reticular lamina

Freeze fracture has been used to examine junctional complexes within the gentamicin damaged guinea pig organ of Corti during the period of structural reorganisation associated with hair cell loss. When examined up to 28 days after a regime of chronic gentamicin administration, no significant alterations to the pattern or nature of the complex tight junctions associated with the hair cell apex could be detected. In contrast, marked abnormalities of the junctions between adjacent supporting cells, and evidence of formation of new gap junctions on the lateral membranes of Deiter's cell was seen. The use of lanthanum as an electron dense tracer in thin sections also showed that no breakdown of permeability barriers occurred in the reticular lamina. These results support the concept of a controlled response by the organ of Corti, with conservation of the tight junction of the hair cell and maintenance of permeability barriers. Early changes to the membranes of the stereocilia and lateral cisternae of the outer hair cells were also identified. These may indicate direct effects of gentamicin upon membrane structure.