MAPT and tauopathy: 6 The precise mechanisms of tau spread are not yet fully understood, however, the species of tau involved appears to be critical,2 as are the extracellular vesicles such as exosomes in which tau can be compartmentalised in different phosphorylation states.7, 8, 9 Synaptic connectivity, rather than the proximity of neurons, appears to be critical for tau propagation,10 and therefore understanding the routes by which tau is released from neurons is important for elucidating how pathological tau spreads through the brain in AD and other tauopathies.