The sensory epithelia of the human labyrinth. A freeze-fracturing and transmission electron microscopic study.

The end-organs of the human vestibular system were studied with transmission electron microscopic and freeze-fracturing techniques. The general morphology of the sensory epithelia was comparable to that of animals. The hair cells consisted of flask-shaped type I hair cells with the surrounding nerve calyx and rod-shaped type II hair cells contacted by several small bouton-shaped nerve endings. The hair cells were coupled to neighboring supporting cells by tight junctions at the lateroapical aspect. These were regular in appearance and of the moderately tight type. This type of tight junction has been found elsewhere in the inner ear and is believed to be competent to maintain its unique ion composition. The lateral surfaces of the supporting cells were coupled by gap junctions. The afferent nerve terminal of the type I hair cell, the so-called calyx, totally encompassed the hair cell. It could not be concluded on the basis of the present material whether the type I hair cell-calyx complex mediates electrical or chemical synaptic contact. The synapses of afferent nerves on type II hair cells, however, displayed typical signs of being chemically mediated. Synaptic bodies, similar to those seen in animals, were associated with the presynaptic membrane inside the hair cell. Membrane specializations comparable to those of animals were also seen in these synapses. Efferent nerve endings and synapses were not readily identified and were thus not discussed in this study.