Scanning electron microscopic observations on the distended Reissner's and saccular membranes in the guinea pig.

The surface ultrastructures of membranous labyrinths in normal and hydropic guinea pig inner ears were studied. In normal specimens differences between Reissner's and saccular membranes were observed. The endolymphatic epithelial cells of the saccule were smaller in size and their microvilli varied more in size and distribution. Mesothelial cells of the saccule were often spherical in shape and bulged toward the vestibule. In hydropic guinea pig inner ears, distension of Reissner's and saccular membranes occurred by irregular enlargement of epithelial cells. Microvilli were reduced in size and number. On the severely distended Reissner's membrane, outpouchings, infoldings and holes were observed. Transmission electron microscopic examination revealed no significant difference between the tight junctions of epithelial cells in normal or hydropic Reissner's membranes. Mesothelial cells of Reissner's membrane were more severely degenerated than those of the saccule. Cochlear sensory cells were often degenerated in the apical turn where mesothelial cells were lacking, but saccular sensory cells showed very little change in their surface structures.