2P-modified and P301S tau increase microtubule number and density in axonal profiles in the mouse cortex. Representative electron micrographs from layer II/III of mouse neocortex neuropil, 7 months post-vector injection. Images are taken close to the site of injection. (a,b) In the mouse injected with AAV-WT, the neuropil shows normal morphology with microtubules scattered in axon terminals and dendrites, similar to the ultrastructure observed in a non-injected mouse (normal cortex). (c,d) In mice injected with either AAV-P301S or AAV-2P, there is an abundance of cytoskeletal elements, with high densities of aligned microtubules (indicated by white arrowheads) located in axons. These are also seen in the axonal boutons (indicated by *). Scale bars: 500 nm. (e) Quantification of the number of microtubules per axon in each condition. (f,g) Quantification of microtubule density in individual axons. Note the significant increase in microtubule number and density in the AAV-P301S and AAV-2P conditions. Statistical analysis: one-way ANOVA followed by Tukey’s post-hoc test, n = 29–30 axons per condition, *p < 0.05; **p < 0.01; ***p < 0.001. (h) Representative electron micrographs from the neocortex, 7 months post-injection of the AAV-P301S tau vector. Enlarged axons containing high densities of aligned microtubules (indicated by #) are present, as well as axonal swellings containing large accumulations of degenerated organelles including whorls, membrane inclusions and degenerating mitochondrial profiles (white arrows). Scale bars: 1 µm (left panels) and 500 nm (right panel).