ROUND WINDOW MEMBRANE - STRUCTURE-FUNCTION AND PERMEABILITY - A REVIEW

The ultrastructure of the round window membrane of humans, monkeys, fe lines, and rodents discloses three basic layers: an outer epithelium, a middle core of connective tissue, and an inner epithelium. Interspec ies variations are mainly in terms of thickness, being thinnest in rod ents and thicker in humans. Morphologic evidence suggests that the lay ers of the round window participate in absorption and secretion of sub stances to and from the inner ear, and that the entire membrane could play a role in the defense system of the ear. Different substances, in cluding antibiotics, local anesthetics, and tracers such as cationic f erritin, horseradish peroxidase, and 1 mu latex microspheres, are plac ed in the middle ear side traverse the membrane. Cationic ferritin and 1 micron microspheres placed in perilymph become incorporated by the inner epithelial cells of the membrane. Permeability is selective; fac tors include size, concentration, liposolubility, electrical charge, a nd thickness of the membrane. Passage of substances through the round window membrane is by different pathways, the nature of which is seemi ngly decided at the outer epithelium of the round window membrane. (C) 1997 Wiley-Liss, Inc.