Figure 1.
Diagrammatic cross-section of the organ of Corti to illustrate main cytoskeletal components.
Structures containing actin filaments and microtubules are lightly and darkly shaded, respectively. Numbers of microtubules were counted from transverse sections at 4 different levels: a) the axial region of inner pillar cells (IPC) b) the axial region of outer pillar cells (OPC) c) the basal part of each row of Deiters' cells (DC1–3) d) the phalangeal processes of each row of Deiters' cells. The bases and apices of both pillar cells and Deiters' cells included dense actin meshes into which the microtubules were embedded. The axial regions of all cells were composed of parallel arrays of both microtubules and actin filaments. IHC – inner hair cell, OHC - outer hair cells, RL – reticular lamina, BM – basilar membrane, TM – tectorial membrane, a-d – levels of section illustrated in figure 2.
Figure 2.
Electron micrograph of a section cut in the same plane as the basilar membrane.
The section passes at a slightly oblique angle to cut supporting cells transversely across their axes at different levels along the organ of Corti between the basilar membrane and the reticular lamina. The insets are representative of the regions to which they refer.IPC(a) – inner pillar cell at level equivalent to that labeled a in figure 1 OPC(b) – outer pillar cell at level equivalent to that labeled b in figure 1 DC(c) – Deiters' cell at level equivalent to that labeled c in figure 1 DC(d) – Deiters' cell phalangeal process at level equivalent to that labeled d in figure 1 Note that inner pillar cell shafts are oval in cross-section and lie adjacent to the inner hair cells, that outer pillar cell shafts are rounded in cross-section with no mechanical support from adjacent cells, that the basal portion of Deiters' cells contain relatively small bundles of microtubules with a much higher proportion of cytoplasm than that seen in PCs, and that the phalangeal processes of Deiters' cells are thin and rounded in cross-section with no mechanical support from adjacent cells. A - Dense actin networks at the bases and apices of the pillar cells IHC – inner hair cells OHC – outer hair cells (distorted during preparation) Scale bar = 10 µ (insets = 500 nm).
Figure 3.
Diagram of the key measurements made from the organ of Corti to illustrate the differences between dimensions at the base and apex of the cochlea.
The profiles illustrated in this figure were taken from histological sections cut near to the apex and close to the base of a guinea pig cochlea. The profile from the apex is labeled to indicate the basilar membrane (bm), reticular lamina (rl), inner and outer pillar cells (ipc, opc), a single Deiters' cell (Dc) and a single outer hair cell (ohc). The lower profile from the base illustrates the angle between the upper ends of ipc and opc (α), and inside the arch of Corti between the ipc and bm (β) and between the opc and the bm (χ). The distance w defines the separation between the bottom ends of the ipc and opc. Note that the greatest measured changes in cell length were in the ohc and opc with the length of the Dc remaining almost constant. Whilst w decreased towards the base the associated changes in cell length meant that α remained almost constant.
Figure 4.
Cell length measurements plotted against Greenwood frequency along the cochlear duct.
The mean length of inner pillar cells (a) decreased towards the basal high frequency end of the cochlea and a similar but more pronounced decrease was measured for outer pillar cells (b). The arch of Corti was consequently smaller at the high frequency end of the cochlea. The mean length of the outer hair cells decreased more dramatically (c) whilst there was only a relatively small decrease in the mean length of the Deiters' cell bodies (d). Dashed boxes indicate verification of measurements obtained from dissociated cells from individual cochlear turns. Lines are linear regression fits to raw data. Error bars are two standard deviations about the mean. All correlation coefficients are highly significant. Mean values were derived from three animals.
Figure 5.
Number of microtubules plotted against Greenwood frequency along the cochlear duct.
Inner (a) and outer (b) pillar cells contained more microtubules in the basal, higher frequency regions. There was no obvious relationship with frequency for numbers of microtubules in the Deiters' cell bodies (c) or phalangeal processes (d). Vertical and horizontal lines indicate measurement uncertainty as determined by two standard deviations about the mean from repeated measurement.
Table 1.
Axial, bending, and buckling stiffness of supporting cells at selected frequency regions of the cochlea.
Figure 6.
The data from Table 1 plotted to show relative stiffness values between cells.
Figure 7.
Structure and buckling of outer pillar cells.
Outer pillar cells are normally straight (a) and span the space between the reticular lamina (RL) and the basilar membrane (BM). In many histological sections of the organ of Corti the central shaft is deformed (b) as if the structure has been subjected to a compressional force (arrow). Under these conditions the shapes of the actin-rich apical and basal cones remain unchanged (b). This implies that the cones are relatively stiff compared with the shaft and that the effective length of the cell is considerably shorter than the full length in terms of our microtubule model.