Tau lesions (pretangles, neuropil threads, neurofibrillary tangles) that develop in a few types of nerve cells in the brain are essential to the pathogenesis of Alzheimer's disease (AD). The formation of non-argyrophilic pretangles marks the beginning of the pathological process and is of increasing interest because it is temporally closer to the prevailing conditions that induce the pathological process underlying AD in contrast to late-stage disease. Not all of the pretangle material, however, converts into argyrophilic neurofibrillary lesions. The propensity to develop tau lesions may be related to the exceptionally protracted myelination of late developing portions in the human brain.