The human round window membrane. An electron microscopic study.

The normal adult human round window membrane was examined by transmission electron microscopy. The membrane consists of the following three layers: (1) an outer squamous epithelial layer with an underlying basement membrane; (2) a middle fibrous layer containing collagen, elastin, fibrocytes, vessels, and nerves; and (3) an inner layer of mesothelial cells. Mucosal membrane veils that cover the round window membrane, forming "false" round window membranes, are also described. These membranes are also three layered, including (1) an outer epithelial layer with an underlying basement membrane, (2) a middle fibrous layer containing collagen, elastin, fibrocytes, vessels, and nerves, and (3) an inner epithelial layer with an underlying basement membrane. Ultrastructural differences between these two structures are discussed.