The dorsal root transitional zone model of CNS axon regeneration: morphological findings

The Summer Meeting of the Anatomical Society of Great Britain and Ireland was held at University College Dublin, from 10 to 12 July 2001. It included a symposium on ‘The respiratory system’. The following are abstracts of communications and posters presented at the meeting. The dorsal root transitional zone (DRTZ) model was used to investigate axon degeneration and regeneration along the DR and into the CNS. Three protocols were used regeneration in the presence of neurotrophin-3 (NT3) (protocol A); regeneration in the absence of neurotrophin (protocol B). Protocol C provided background data on tissue changes in experimental controls in which axons were absent following DR ganglionectomy. In all protocols glial fringe complexes occurred at PNS levels. These showed extensive apposition of Schwann cells and astrocytic processes in common sleeves of basal lamina. In the normal TZ such apposition is very limited, being confined to the transitional node (Fraher and Kaar, J. Anat. 139, 1984). Complex formation involves astrocyte process outgrowth under the basal lamina at the erstwhile node gap. It indicates lessening of any repulsion between the cell types. A fine endoneurial cytoplasmic reticulum resembling that in cranial nerve rootlets (Fraher and O’Leary, J. Anat. 184, 1994) became increasingly prominent with time. In most axons at PNS levels of protocols A and B, myelination was absent. In a small number it was at an early stage. In protocol C, axons were absent, apart from a few small and occasional large myelinated axons in the PNS compartment. The latter were also seen in protocols A and B. The small axons may be autonomic, derived from perivascular plexuses. Some could represent distal outgrowths from the CNS (Carlstedt J. Anat. 190, 1997). The large axons were probably derived from vagrant plexiform connections with undamaged roots. That they were not regenerating is clear from known growth rates (Fraher, Brain Res. 105, 1976). In protocol A, axons traversed the CNS–PNS interface in a distinctive manner: they were commonly enfolded by a Schwann cell perikaryon immediately distal to it. This and its ensheathing processes were closely apposed to TZ astrocyte processes without intervening basal lamina. Within the CNS, axons frequently ran in groups separated by astrocyte processes from the persistent myelin debris.