Scanning Electron Microscopy of the Echidna: Morphology of a Primitive Mammalian Cochlea

Abstract

Cochleae of three echidnae (Tachyglossus) were examined by scanning electron microscopy. There were 4 – 6 rows of outer hair cells, and 3 – 5 rows of inner hair cells. In spite of this ‘reptile-like’ number of hair cells across the cochlear width, the distinction between inner and outer hair cells was clear. Many inner hair cells had only two rows of stereocilia, sometimes with a third row of very small stereocilia which did not make contact with the others. As many as four pillar cells were seen in a cross-section of the organ of Corti. However, there was some variation from specimen to specimen in the numbers of inner hair cells, rows of stereocilia, and pillar cells. It is suggested that the feature that distinguishes the mammalian cochlea is the conformation of the outer hair cells, possibly associated with their particular form of motility.