Extra internal hair cells. A scanning electron microscopic study.

The extra internal hair cell (EIHC) of the human cochlea was observed by means of a scanning electron microscope. The EIHC was found not infrequently in all turns of the human cochlea. It was located medial to the IHC row. The inner pillar cells showed an abnormal structure. The anatomical relationships between the displaced IHC and EIHC, and the inner pillar cell were classified into five types. The origin of these anomalies is discussed form an embryological viewpoint.