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

Myelinated nerve fibres were analysed at their passage of the PNS-CNS borderline in the transitional region (TR) of feline S1 dorsal rootlets by means of light and electron microscopy. At the borderline, PNS internodal length was 30% shorter than noted further distally in the root. Analysis of variance based on the numerical estimation of various fibre parameters did not support the current idea that a myelinated nerve fibre decreases in diameter as it passes from the PNS into the CNS. There was a rough rectilinear correlation between axon size and number of myelin lamellae in small and medium sized fibres. In large fibres the number of myelin lamellae was highly variable and could not be predicated from axon size. The nodes of Ranvier found at the borderline showed an organization that could be described as a compound node where the PNS-CNS demarcation line runs across the nodal axon segment in the midst of the node gap. Branching at the borderline nodes was a very rare phenomenon. The length of the nodal axon segment, the nodal axon membrane area and the amount of paranodal mitochondria were similar to earlier presented data of feline ventral root fibres. The hypothesis that the elements of the paranodal apparatus of PNS fibres compensates for the lack in the PNS of nodally connected astrocytes is presented and discussed. A small number of the fibres lacked a borderline node. In these cases the CNS myelin sheath projected uninterrupted for hundreds of micrometer into the PNS compartment where it was enclosed by a Schwann cell and PNS myelin. This rare arrangement was referred to as a transitional internode.