Postnatal increase of unmyelinated axon profiles in the feline ventral root L7

The proportion of unmyelinated axon profiles and possible age-related, degenerative and regenerative alterations were studied ultrastructurally in the L7 ventral root of 25 cats ranging form 3 weeks to 11 years of age. For comparison, the ventral root C6 was examined in 5 of these animals. The sections were taken from a level midway between the proximal and distal ends of the roots. The proportion of unmyelinated axon profiles in the L7 ventral root increased from about 14% to around 31% between 4 and 7 months after birth. Simultaneously, the average number of unmyelinated axon profiles per Schwann cell was doubled from 2-3 to 4-5. Thereafter, these figures remained largely unchanged for at least a decade. The total number of myelinated axons was similar in kittens and in cats. In the C6 root the proportion of unmyelinated axon profiles was about 20%, both in kittens and in young or old adult cats. At both segmental levels in the oldest cats, some unmyelinated axons showed degenerative changes and medium-sized and large axons had features characteristic of demyelination and remyelination.