The ultrastructure of spiral ganglion cells in the mouse.

In the adult mouse, three types of spiral ganglion cells can be distinguished on an ultrastructural basis. The first type, TI cells, corresponds to the most commonly encountered cells in the ganglion, which represents 92 to 94% of the whole population. This type of cell is characterized by numerous cytoplasmic organelles that give a dark and a granular appearance, while the nucleus is more lightly coloured. Moreover, the perikaryon is surrounded by a myelin sheath composed of several myelin lamellae that can be either loose or compact. The periodicity of compact myelin around the perikaryon is larger than that found in nerve fibres. The T II ganglion cells, corresponding to the second type, are less numerous and represent only 6 to 8% of the cell population. They present a clear perikaryon with few cytoplasmic organelles and large areas composed of filamentous structures. The perikaryon is not surrounded by a myelin sheath, although several Schwann cell processes can be observed. The third type is composed of few ganglion cells that are ultrastructurally very close to T I cells, but without a myelin sheath. Desmosome-like junctions have been observed between myelin lamellae and between myelin lamellae and the plasmalemma, but no synaptic contacts have been observed between auditory nerve fibres and ganglion cells.