Early stages of myelination in the spiral ganglion cells of the kitten during development.

Myelinated cell bodies of spiral ganglion in kittens during development can first be distinguished by light microscopic observation at the end of the first postnatal week. By mean of electron microscopic observation, the first signs of myelinated perikarya can be observed around the time of birth. Myelination is preceded by the ensheathment of the ganglion cell body by two or three layers of Schwann cell cytoplasmic processes. At birth, the first sign of myelination is visible at the basal part of the cochlea with thin thickenings which contain two or three major dense lines. At this stage, these thickenings cover only a small perikaryal area. Later on, the thickness and the number of thickenings increase and cover the major part of the cell body. The onset of myelination of the processes seems to precede the perikaryal myelination.