For TEM analysis, toluidine blue-stained semithin sections were preliminarily observed for general morphological examination and as survey sections to localize the areas of the samples for ultrathin sectioning. We focused in particular on the region at the interface between sterile and fertile veins, where paraphyses are organized in a palisade delimiting the sterile vein and where significant TUNEL positivity had been detected (Fig. 4a); in this zone we could observe the presence of altered nuclei (Fig. 4d–f), with irregular condensed chromatin masses reminding those found sometimes in (animal) apoptotic cells, within sterile hyphae showing cytoplasmic emptying and vacuolization (Fig. 4d). Another remarkable feature observed in these altered nuclei was the recurrent presence of a (generally) single, fairly large dilation of the nuclear envelope lumen (Fig. 4e, f).