The fine structure of the inner ear in caiman crocodilus.

Abstract

The auditory organ in young Caimans (50 to 80 cms length) was studied with SEM and TEM. The auditory organ contains the basilar membrane with the papilla basilaris and the 3 internal ear cavities. A “fourth cavity” bordering on the scala tympani extends into the medial fibrocartilaginous plate. It contains the first nodal region of the myelinated afferent nerve fibres. The efferent nerve fibres occurring in this portion are not myelinated. The sensory cells may be divided into the inner and outer hair cells. The inner hair cells are medially positioned on the basilar membrane above the “fourth cavity”. They are of a high prismatic shape and have relatively long stereocilia. The quantity of the inner hair cells increases from the region near the window towards the lagena. The outer hair cells are calotte shaped and, for this reason, have a relatively large free surface. For the most part, they lie on the tympanic portion of the basilar membrane. The stereocilia of the outer hair cells are remarkably short in contrast to those of the inner hair cells. Seen as a whole, the length of the stereocilia of both types increases from the region near the window towards the lagena. The afferent and efferent innervation of the inner and outer hair cells is on principle the same. In contrast to the outer hair cells, the inner hair cells have more synapses with afferent axons. The efferent axons form axosomatic and axodendritic synapses. Within the region of the tympanic portion of the basilar membrane, the efferent axons also form synaptic contacts with the supporting cells and hyaline cells.