Biology Reference
In-Depth Information
FPO
FIGURE 8.3. Scanning electron micrographs showing two types of hair cell abnor-
mality. (B) Outer hair cells of Snell's waltzer (
Myo6
sv
/Myo6
sv
) mutants at 7 days
after birth, in which the stereocilia are disorganized and fused together. (D) Outer
hair cells of the shaker1 (
Myo7a
816SB
/ Myo7a
816SB
) mutant at 3 days after birth. The
stereocilia are not arranged in the characteristic W-shape seen in controls, but
instead clump together in clusters on the hair cell surface. (A) and (C) show litter-
mate controls.
Several of the mutants show abnormal differentiation of sensory hair
cells. The shaker1, tailchaser, and waltzer mutants all have disorganized hair
bundles around the time of birth (Fig. 8.3) (Self et al. 1998; Kiernan et al.
1999; Yonezawa et al. 1996; R. Holme, personal communication 2000). In
these mutants, hair cells are present in their normal arrangement in the
organ of Corti, and they extend many microvilli from their upper surface,
which develop into stereocilia. However, the normal V-shaped arrangement
of stereocilia within each bundle either does not form correctly, or becomes
progressively disorganized. Hair cells eventually degenerate, as is generally
the case in hearing-impaired mutants, but it is clear from the timing that
this must be secondary to the early abnormalities in hair cell development.
This emphasizes the importance of studying development in a model
species to determine the primary defect.
A different hair bundle defect is observed in Snell's waltzer mutants. In
these hair cells, adjacent stereocilia fuse together, starting from the base
near the insertion into the cuticular plate of the hair cell at around the time
of birth, and forming giant protrusions from the top of each cell within a
few days (Self et al. 1999). A third type of stereocilia defect is seen in the
