Conventional chemical fixations induce artifactual swelling of dolipore septa

Toroidal swellings of dolipore septa of Laetisaria arvalis were typical of Homobasidiomycetidae septa when preserved by conventional chemical fixation procedures using glutaraldehyde followed by osmium tetroxide. The toroidal swellings were electron translucent and were delimited by a plasmalemma, both of which were very irregular in outline. Toroidal swellings of similar septa preserved by freeze-substitution were approximately one-third the size of conventional chemically fixed specimens. Furthermore, freezesubstituted-preserved toroidal swellings were moderately electron dense with a finely granular matrix delimited by a plasmalemma and were smooth in outline. Based on the results of the two preservation procedures as well as certain modifications of them, and on the results of light microscopic examinations of septal complexes before and after chemical fixation, it was concluded that conventional chemical fixation procedures induce artifactual enlargement of toroidal swelling.