Ultrastructural analysis of the mouse cortex by TEM also revealed typical features of degenerating neurons. In particular, in the AAV-P301S mouse, we noticed axonal structures containing large abnormal accumulation of defective organelles or autophagic material at 7 months post-injection (Fig. 6h). For histological examination of neurodegeneration in the mouse forebrain, cresyl violet staining was used to reveal neuronal cell bodies in sagittal brain sections. In the animals injected with AAV vectors encoding either WT tau (4 out of 6 animals) or P301S tau (all the animals), there was an evident thinning of the cortical regions near the site of vector injection (Supplementary Fig. S2a). The hippocampus remained mostly intact. Degeneration of the mouse cortex was already observed at 1.5 and 3 months after vector injection. In contrast, there were no apparent morphological changes in the mice injected with the control AAV-maxFP vector at month 3, indicating that this effect was mainly caused by the human tau protein and not by mechanical injury (Supplementary Fig. S2a). In the AAV-2P injected group, only one out five mice showed a noticeable degeneration of the cortex. In the mice from the AAV-WT and AAV-P301S groups which displayed cortical neurodegeneration, it appeared that the most superficial layers of the cortex (layer I-V) were the most affected, as shown with a NeuN immunostaining to reveal cortical layers (Supplementary Fig. S2b).