Electron microscopy of PHF-tau in Tg30 and Tg30xtau−/− mice. A–D: Immunolabeling in electron microscopy of abnormal filaments present in the sarkosyl-insoluble fraction of Tg30 (A and B) and Tg30xtau−/− (C and D) mice. A, C: Anti-human tau antibody (BR21). A strong labeling was observed both in Tg30 and in Tg30xtau−/− mice. B, D: Anti-mouse tau antibody (mTau5). A weak labeling was observed in filaments from Tg30 mice but was absent in Tg30xtau−/− mice. Scale bar = 50 nm (A–D). E, F: Ultrastructural aspects of abnormal filaments in Tg30xtau−/− mice (subiculum). E: A dilated neurite contains bundles of 19-nm straight filaments, admixed with degraded organelles and lysosomal vacuoles. Scale bar = 0.5 μm. F: Under higher magnification, straight filaments and a PHF-like filament (arrow) can be identified. Scale bar = 0.25 μm.