A Comparative Study of Tenascin Expression in the Inner Ear

Sensorineural hearing loss affects millions of people worldwide, yet we do not know enough about hearing development to remedy this loss. By studying the interactions within the inner ear, specifically the cochlea, we hope to gain a better understanding of how we develop the ability to hear.

Extracellular matrix molecules can have either attractive or repulsive interactions on growing axons. Tenascin glycoproteins are of particular interest in developing neural tissues because they can exert both inhibitory and stimulatory effects on growing axons. Tenascin is present around cochlear hair cells during development. However, the role that tenascin plays during development of neural connections in the inner ear remains a mystery. Tenascin could play a role in establishing either efferent or afferent neural connections to the hair cells. Using a comparative approach, we decided to explore the similarities and differences of tenascin expression in the inner ear of the chick, mouse, and rat. Mammals only grow one set of hair cells in their lifetime; therefore, they only need to make neural connections to the hair cells once. Yet nonmammalian vertebrates, such as birds, will grow new hair cells after trauma (e.g., noise, ototoxicity, etc.) to the inner ear. This regenerative ability in birds means that they also need to be able to establish new neural connections. If tenascin is involved in making new neural connections, then one might expect tenascin expression in the chick cochlea to be on-going whereas it would be transient in the mammalian cochlea.

Previous studies in the lab have shown that tenascin expression appears in the rat cochlea at embryonic age 18 (E18) and disappears by postnatal age 10 (P10). This transient expression corresponds to the period when efferent neural connections are being established in the cochlea. To confirm these results as well as to investigate expression across species, we studied tenascin immunoreactivity in the inner ears of chick and mouse. Immunostaining was used to show the relationship between tenascin, efferent fibers and terminals, and the hair cells by using specific antibody markers. Labeled tissues were visualized with confocal microscopy and digital images were collected, rendered, and analyzed.

In the chick inner ear, we found tenascin immunostaining in both P14 and P21. Tenascin cups the bottoms of the hair cells, forming a basket around them, and the efferents are present on the same side of the hair cells as tenascin. There was no apparent difference between tenascin expression in P14 and P21 chick cochleas. In the mouse cochlea, tenascin immunostaining was present at E18 but was not present at P10. These findings in the mouse are similar to the expression of tenascin in the rat cochlea, suggesting that tenascin expression in the inner ear is the same across mammalian species.

Our results are consistent with our hypothesis that the chick inner ear would demonstrate a different pattern of tenascin expression than found in the mammalian inner ear. This means that tenascin may play a role in establishing efferent or afferent neuron connections in the inner ear. From here we will go in a couple of different directions. We need to look at the chick cochlea at different ages, from E18 to P10, to see how tenascin expression develops in the chick. We also need to see the interaction between tenascin and regrown hair cells within the chick cochlea, and compare it with tenascin’s interaction with the original hair cells. Hopefully this research will give us a better idea of tenascin’s role in the development of neural connections in the inner ear as well as elsewhere in the developing or regenerating nervous system.