Proliferation and Regeneration of Sensory Hair Cells

Figure 1

Figure 2

Several years ago we demonstrated

that fishes, like elasmobranchs, continue to add sensory hair cells to the ear for at least several years after birth (Lombarte and Popper, 1994). In one study of the hake, Merluccius merluccius (a relative of the cod), we demonstrated that proliferation continues in all three otolithic end organs until fish are at least nine years old. As shown figure 1, the increase in number of hair cells is far greater in the saccule than in the utricle and lagena. Figure 1 shows that the total number of sensory hair cells in all three end organs of a single ear is over 2 million by the time the fish is 70 cm long (or about nine years of age). A newly proliferating sensory hair cell is shown in the SEM (figure 2). Note the size of this bundle as compared to the more mature surrounding bundles.

In a more recent study, Higgs and Popper (in preparation) have shown that proliferation may stop once a fish reaches its full size, in those species where growth does not continue for the life of the fish.

In addition to demonstrating cell proliferation, we have also found that hair cells are regenerated after damage with the ototoxic drug gentamicin sulphate (Lombarte et al, 1993). Gentamicin is known to damage sensory hair cells of the ears of mammals, and it also damages hair cells in fishes. In figure 3, the top SEM shows the utricle from a normal oscar. The SEM at the bottom shows a utricle from a fish after several days of treatment with gentamicin. A band can be seen (the striola region) that is now devoid of sensory hair cells. Figure 4 shows tissue ten days after treatment with gentamicin. The upper SEM shows a low magnification and the area in the square is enlarged in the image below it.

Figure 3	Figure 4

After 15 days, however, regeneration has occurred and the epithelium appears to have a complete set of hair cells. The top SEM in figure 5 shows a low power view of the striola region of the utricle, and the low figure shows the area in the square at higher magnification. There is complete regeneration. The time course for regeneration in experimental and control animals is shown in figure 6.

	Figure 6
Figure 5