DEVELOPMENT OF THE MAMMALIAN EAR - COORDINATE REGULATION OF FORMATIONOF THE TYMPANIC RING AND THE EXTERNAL ACOUSTIC MEATUS

The tympanic membrane in mammals is a trilaminar structure formed by t he apposition of two epithelial cell layers, along with an intervening layer of cells derived from pharyngeal arch mesenchyme. One epithelia l layer is contributed by the external acoustic meatus, a derivative o f the first pharyngeal cleft. The other epithelial layer is contribute d by the tubotympanic recess, a derivative of the first pharyngeal pou ch. We demonstrate here an absolute correlation between formation of t he external acoustic meatus and formation of the tympanic ring, a firs t arch-derived membrane bone that anchors the tympanic membrane. Exper imental loss of the tympanic ring by retinoic acid treatment, or dupli cation of the ring in Hoxa-2 null mutant embryos, resulted in correspo nding alterations in formation of the external acoustic meatus. We sug gest that the tympanic ring primordium induces formation and morphogen esis of the external acoustic meatus, and that expression of the Hoxa- 2 and goosecoid genes may be involved in regulating the formation and morphogenesis of these structures.