Qualitative and quantitative observations of spiral ganglion development in the rat.

The postnatal development of the spiral ganglion cells in the rat was studied from birth until the adult stage. At birth, a single population of ganglion cells is present. Some of them are surrounded by one or two layers of satellite cell processes. With maturation, the satellite cell processes increase in number around the cell body and its processes. At the end of the first postnatal week, two important events occur. The first is the appearance of myelin lamellae between the 4th and the 6th postnatal day in both ganglion cell processes, and between the 6th and the 8th day in the cell body. The second event is the appearance of a new type of cell (the Type II spiral ganglion cell) on the 6th to the 8th day postpartum. At this stage, the Type II cell is mainly characterized by densely packed neurofilamentous structures in the cytoplasm. Comparison between the myelination of the cell body and its processes reveals three main differences! There is a time lag of approximately 2 days between the onset of myelination in the cell body and in its processes. The kinetics of myelination are different in the cell processes and in the cell body. The myelination of the cell body starts slowly, whereas it is very fast in the processes. Later, the kinetics of myelination decrease in the processes, and increase in the cell body. At all stages including the adult, the fibers have a myelin sheath composed of more lamellae than the cell body. These observations are discussed with respect to development in other species.