Characterization of inner ear sensory hair cells after rapid-freezing and freeze-substitution.

In order to determine the ultrastructural organization of normal cells and to understand better the anatomical substrates for outer hair cell motility, cryofixation was performed on the sensory epithelium of the inner ear of guinea pigs prior to substitution of frozen water with organic solvents containing chemical fixatives. In this way cells would not be altered by the direct application of the chemicals commonly used for preservation, which are also known to cause fixation-induced shape changes in outer hair cells. Following rapid freezing and freeze-substitution, preservation of cells within the isolated sensory epithelium containing the organ of Corti was similar to that seen in conventionally fixed cells. However, in rapidly frozen and freeze-substituted outer hair cells the cytoplasm and the cellular membranes differed from those seen in conventionally fixed preparations. The cytoplasmic matrix was densely packed with filaments and stained homogeneously, suggesting better preservation of the cytoskeleton and less extraction of the soluble ground substance. Cell membranes were smooth, indicating that fixation-induced shape changes and shrinkage had been avoided. The subsurface cisternal system of intracellular membranes lining the lateral wall of the outer hair cells was composed of continuous, tightly packed, parallel rows of membranous lamellae. Thus rapid-freezing and freeze-substitution are important techniques by which structural alterations correlated with outer hair cell motility can be separated from fixation-induced cell shape changes.