Observations on the morphology at the transition between the peripheral and the central nervous system in the cat. II. General organization of the transitional region in S1 dorsal rootlets.

The aproximal segment of feline S1 dorsal rootlets were investigated light and electronmicroscopically. A one shaped protrusion of CNS tissue was found to extend distally into each rootlet for a distance of 100-1000 micrometer. That free segment of a rootlet which when cross sectioned revealed both CNS and PNS constituents was denoted the transitional region (TR). The TR was subdivided into an axial CNS compartment and a surrounding PNS compartment. The CNS compartment consisted of a core zone that showed the organisation of a CNS fiber tract, and of an outer astrocytic mantle zone which corresponded to the external glial limiting membrane of the brain and the spinal cord. The outer aspect of the mantle zone constituted the PNS-CNS borderline. The mantle zone was rich in astrocytes and projected a large number of thin processes--the glial fringe--distally into the endoneural space of the PNS compartment. There were large fenestration defects in the basement membrane that demarcated the endoneural space from the glial space. The mantle zone contained many large myelinoid bodies and some encapsulated small dark unclassified cells. Similar cells, some covered by a basement membrane, were at hand in the adjacent endoneural space. The myelinated nerve fibres were equipped with a node of Ranvier at their borderline passage.