The Tympanic Covering Layer. An Electron Microscopic Study in Guinea Pig

The tympanic covering layer in guinea pig was studied with transmission and scanning electron microscopy. The tympanic border cells looked like bipolar ganglion cells with long processes. There was often a kinocilium on the cell body. The cells were rather similar structurally but their arrangement varied in the different coils. In the basal coil the cells were lying close together and oriented strictly longitudinally. In the upper coils both longitudinally and radially oriented cells were seen. There were fewer cells in the apical coils and the perilymphatic space between the cells, as well as the area of contact between the perilymph and the basilar membrane, became larger. On every level in the cochlea the tympanic covering layer was thinnest under the tunnel of Corti. The significance of the appearance of the cells and their arrangement are discussed. The tissue on the lower side of the basilar membrane is called the tympanic lamella or the tympanic covering layer and its cells have been called tympanic border cells, basilar membrane cells or cells of the tympanic covering layer. They are of mesothelial origin and were already noticed by Claudius (1856), Hensen (1863), Boettcher (1869) and Retzius (1884). Little interest was paid to them until Iurato (1962) described their submicroscopic anatomy in detail in his investigations of the rat's inner ear. In the studies of the cochlea using tracer substance it has been observed that these cells in the guinea pig have a high phagocytic ability (Angelborg & Engström, 1973; Angelborg, in press). Further studies of the tympanic border cells showed some new features in their anatomy which had not been described earlier. In this investigation their ultrastructure in guinea pig will be presented.