Proliferation and Regeneration of Sensory Hair Cells
Several years ago we
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.
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.