An electron-microscopical study of the developing transitional region in feline S1 dorsal rootlets.

The organization of the PNS-CNS transitional region in S1 dorsal roots was studied electron-microscopically in cat foetuses and in kittens. The adult organization pattern was recognized first during the 5th-6th postnatal week. Before this date the transitional region underwent a period of conspicuous remodelling. In 25- to 47-days-old foetuses the transitional region was characterized by large clusters of Schwann cells clinging to bundles of unmyelinated axons. This part of the root then remained unmyelinated after the more distal PNS part and the more proximal CNS part had acquired myelin. Axons of the transitional region started to myelinate first around the 60th day after conception. At this stage the transitional region was characterized by its cellularity: Schwann cells, glioblasts and fibroblasts were abundant. The CNS compartment started to grow out into the root during the 1st postnatal week. Concomitant with the distal expansion of the CNS compartment - calculated to be about 5 micrometer/day during the 1st month - there appeared in the adjacent PNS compartment numerous extraordinarily short internodes carrying myelin sheaths. A glial fringe began to develop and encapsulate PNS-borderline paranodes. The observations are discussed with respect to the adult ultrastructure. It is suggested that there is a conspicuous reorganization of the proximal part of the root. The need for supplementary quantitative data is emphasized.