Freeze-fracture study of the perineurium around frog dorsal root ganglia.

The perineurium around frog dorsal root ganglia consists of layers of flattened cells separated by extracellular connective tissue elements. The number of layers is smaller than that in the perineurium around adjacent peripheral nerves, and some of the layers are discontinuous, but in both cases, cells in the same layer overlap and form tight junctions with each other, sometimes accompanied by desmosomes or gap junctions. In freeze-fracture replicas the tight junctions between perineurial cells around peripheral nerves consist of 13-91 strands (mean: approximately 38). Some of these are parallel to the cell borders and some are oblique, forming elaborate meshworks. The overall width of each junction averages approximately 12 microns. In contrast, the tight junctions between perineurial cells around ganglia are much narrower, averaging approximately 2 microns in width, and they consist of only 1-14 strands (mean: approximately 6) with few anastomoses and many free ends. These structural differences provide a morphological basis for a less complete diffusion barrier around dorsal root ganglia.