Observations on the morphology at the transition between the peripheral and the central nervous system in the cat. IV. Unmyelinated fibres in S1 dorsal rootlets.

Unmyelinated (d.r.C) fibres have been investigated electronmicroscopically at their passage from the peripheral to the central nervous system in feline S1 dorsal rootlets. The earlier demonstrated rearrangement of C-fibres during their passage along the transitional region (TR) was confirmed by the observation of extensive fibre divergence. The transitional region Schwann cells was calculated to be 10-20 times as high in the TR as in the more distal parts of the rootlets. Analysis of consecutive ultra-thin sections showed that PNS unmyelinated axons 0.8 micrometer or more acquired a myelin sheath as they entered the CNS. Fibres 0.6-0.8 micrometer in diameter appeared to be segmentally myelinated in the CNS. Fibres less than 0.6 micrometer remained unmyelinated at their CNS entrance. A few fibres showed degenerative signs and terminated in the transitional region.