[Medullated ganglion cells in the guinea pig labyrinth]

The pericellular structures of the three ganglia situated in the petrous bone were investigated by light and electron microscopy in the guinea pig. All perikarya of the ganglion spirale cochleae and the ganglion vestibulare are surrounded by a myelin sheath which consists of 10-15 layers. Relative to their size, the cells of the ganglion spiral are characterized by a thicker myelin sheath than those of the ganglion vestibulare. The peripheral and central processes of the ganglion cells show a stronger myelination than the corresponding perikarya. The decrease in myelin from the nerve fibre to the perikarya does not proceed continuously but occurs only after the last interanular segment. At the peripheral processes, the myelin sheath ends a few micrometres before reaching the sensory epithelial layer. The perikarya of the ganglion geniculi nervi facialis are unmyelinated.