Fine structural features of mammalian otoconia shown by different EM techniques. (a) SEM image of adult mouse otoconia showing typical barrel-shaped morphology and faceted ends; (b) freeze-etching of guinea pig otoconium showing pointed tip and rounded bodies with abundant filaments covering the surface (arrowhead); (c) TEM of a mouse otoconium showing both outer cortex (OC) and inner core (IC); (d) corresponding high magnification image showing fibrillar meshwork of inner core (IC) with several mineralized plaques (arrow), and outer cortex (OC) with parallel aligned mineralized fiber bundles (arrowhead); (e) freeze-etch image of cross-fractured guinea pig otoconium exposing parallel-aligned lamellae consisting of fused crystallites. On the right-hand side of the image, the rims of the cleavage planes exhibit sharp zigzagging patterns, representing the pointed tips of crystallites (arrowheads); (f) ultra-thin section of the densely mineralized cortex with parallel-arranged rows of iso-oriented columns of crystallites (arrowheads). The spaces between crystallites are widened due to sectioning procedure. Bars = 2 μm (a); 1 μm (b); 0.2 μm (c), 1 μm (d), 0.1 μm (e), 0.2 μm (f).