Surface aspects of the developing human organ of Corti.

Thirteen cochleas from human fetuses ranging in age from week 9 to week 22 were observed by scanning electron microscopy. The classical 'base-to-apex' and 'internal-to-external' gradients of maturation were confirmed by surface observations of the developing organ of Corti. The tectorial membrane begins to be secreted around week 9, i.e. 2 weeks before the onset of the ciliogenesis. Its structure appears to be first amorphous and then fibrillar. The surface of the organ of Corti looks mature around week 22, but, in fact, its complete maturation, especially at outer hair cell level, probably occurs during the last trimester of pregnancy.