Junctional complexes, perineurial and glia-axonal relationships and the ensheathing structures of the insect nervous system; A comparative study using conventional and freeze-cleaving techniques

When the ventral nerve cord of the locust or cockroach is examined after freeze-cleaving, the fine structure of the various tissues comprising the ganglia and connectives do not differ radically from conventional glutaraldehyde-fixed, sectioned material. The extracellular spaces, the extent of which is of considerable importance in calculating cation reservoirs, appear very similar in dimension after freezing unfixed tissue and after thin sectioning fixed material. Further comparisons between the structures revealed by freeze-cleaving briefly-fixed and unfixed nerve cords are described. The plane of freeze-cleavage along membrane faces yields new information about the extent of intercellular junctions and both the hemi-desmosomes and gap junctions in these tissues are found to be macular in outline. Evidence from the penetration of extracellular tracers as well as freeze-cleaving shows that the gap junctions are composed of subunits of the larger of the two types known so far to exist. Septate desmosomes are shown by both methods to be relatively disorganized in nature. ‘Tight’ junctions (zonulae or fasciae occludentes), found between perineurial cells and considered to be the morphological basis of the blood-brain barrier in insects, differ in freeze-cleave preparations from their vertebrate equivalent, being rather simpler in structure. Although basically very alike, a few relatively subtle differences are apparent in comparisons of insect tissues prepared by the two methods; these include variations in the cross-sectional appearance of the collagen-like fibrils in the neural lamella and differences in the diameter of microtubules.