In vivo neurotoxicity of tau oligomers. A-D) Nuclear staining (Hematoxylin) of the hippocampal region CA1 from the hemispheres of mice injected with tau preparations or PBS (Scale bar = 20 μm). E-H) Neuronal staining using the antibody NeuN show neuronal damage in the area injected with tau oligomers (E) in comparison with the other hemispheres injected with fibrils (F) monomers (G) and PBS (H) (scale bar = 10 μm). (I-L) Higher toxicity of tau oligomers versus fibrils, monomers and PBS. Neurodegenerative pyramidal cells in the CA1 hippocampal region were stained with Fluoro-Jade B, emitting green fluorescence (scale bar = 10 μm). M) Number of Fluoro-Jade B positive cells/Total number of cells in the CA1 region, 63% of cells showed signs of damage in the group injected with tau oligomers, compared to less than 6.5% in the group injected with tau fibrils. Data are represented as the mean ± SE. *p < 0.01.