Clustering of filaments polymerized from phosphorylated tau. Samples from the 25 μM ARA-induced phosphorylated tau reaction (20 h) shown in Figure 3, were diluted five-fold and prepared for transmission electron microscopy (TEM). Panels A, C and E show representative clusters of filaments at a magnification of 50,000 × (scale bar represents 500 nm). Panels B, D and F show the same clusters at a magnification of 100,000 × (scale bar represents 100 nm). In the higher magnification micrographs, white arrowheads denote non-touching filaments; white asterisks, branching filaments; and black arrowheads, tendril-like fibrils that appear to laterally connect larger adjacent filaments.