The filamentous fungus Aspergillus nidulans partitions its cells by laying down septa at regularly spaced intervals in response to nuclear division. Physiological and genetic studies of the temperature-sensitive sep mutants have been especially useful in dissecting the regulation of septation. Electron microscopic studies of the sep mutants should be equally useful in dissecting the structural intermediates of septation. In preparation for ultrastructural studies of the sep mutants, we have examined septa in wild-type A. nidulans fixed by freeze substitution.

Dialysis membranes were placed on rich medium plates and inoculated with A. nidulans spore suspensions. After 12 hours at 30°C, the dialysis membranes with adhering fungal hyphae were cut into square pieces measuring approximately 5mm on each side. The pieces were plunged into liquid propane and processed according to the procedures of Hoch. Serial sections were cut using a diamond knife and post stained with uranyl acetate and lead citrate.