Ultrastructural studies of the round-window membrane of the cat.

An ultrastructural study of eight round-window membranes of four normal cats disclosed three basic layers: an outer epithelium (middle ear), a middle core of connective tissue, and an inner epithelium (inner ear). Morphologic evidence suggests that these layers participate in absorption and secretion of substances to and from the inner ear, such that the entire membrane could play a role in an inner ear "defense system." A comparison of morphologic features of round-window membranes from cats, rodents, and human beings suggests that the cat is a superior animal model for experimental studies. Cationic ferritin, placed for two hours in the round-window niche of four normal cats, was observed to traverse the round-window membrane through pinocytotic vesicles into the connective tissue layer. Evidence of exocytosis of tracer by the inner epithelial layer into the scala tympani is presented. When placed in perilymph, this same tracer was incorporated by inner epithelial cells, suggesting absorptive capabilities of the round-window membrane.