ABSTRACT
The tectorial membrane, the otoconial membranes, and the cupulae are specialized extracellular matrices of the inner ear that are associated with the apical surfaces of the sensory epithelia. The tectorial membrane lies over the organ of Corti in the cochlea, and the otoconial membranes cover the maculae of the vestibule. These membranes provide a structure against which the stereocilia bundles of the hair cells can react when the epithelia are displaced in response to sound waves, as in the cochlea, or head motion, as in the vestibule. A cupula sits on top of the crista in each of the ampullary organs of the semicircular canals and acts like a sail, transmitting the fluid motion caused by head movements to the sensory hair bundles. These three types of extracellular membrane differ considerably in their structure and molecular composition (1). The mammalian tectorial membrane contains collagenase sensitive polypeptides that react with antibodies to type II, type V, and type IX collagen (2, 3), and three noncollagenous glycoproteins, αtectorin, β-tectorin, and otogelin (4, 5). The otoconial membrane contains α-tectorin, βtectorin and otogelin (1, 4). The cupula contains otogelin (1, 4). Other, as yet unidentified components, either collagens or noncollagenous glycoproteins/ proteoglycans, may also be present in these structures.
