1209 MESODERMAL INDUCTION DEFECT AS A POSSIBLE CAUSE OF EAR MALFORMATIONS

Abstract

Deafness due to inner ear anomalies is rarely associated with malformation of the auricles. Two brothers born of consanguineous parents, with profound congenital sensorineural deafness, malformed auricles, abnormal vestibular function and delayed motor development, had low-set, cupped, pointed auricles with folded helices, narrow external auditory canals and normal tympanic membranes. Caloric stimulation failed to elicit nystagmus. Temporal bone laminography revealed bilateral absence of the cochlea and normal ossicles. Since the external and inner ear originate from distinctly separate structures, the embryogenesis of this malformation association is less clear than the more common association of external and middle ear anomalies. In the latter, the first and second branchial arches are the common derivative of both structures. The association of auricular and inner ear anomalies, with sparing of the middle ear, can be explained on the assumption that mesodermal induction is responsible for differentiation of both otocyst and branchial arch ectoderm. A recessive mutant gene may be responsible for the production of a deficient mesodermal inductor substance or a deficient cell surface receptor of the target tissue, rendering it unresponsive. A similar mechanism may be involved in other multiple malformation syndromes, whereby a mutant gene acting during a specific period of organogenesis causes disruption of the normal induction competence relationship.