The dendritic spines on pyramidal cells represent the main postsynaptic elements of cortical excitatory synapses and they are fundamental structures in memory, learning and cognition. In the present study, we used intracellular injections of Lucifer yellow in fixed tissue to analyse over 19 500 dendritic spines that were completely reconstructed in three dimensions along the length of the basal dendrites of pyramidal neurons in the parahippocampal cortex and CA1 of patients with Alzheimer's disease. Following intracellular injection, sections were immunostained for anti-Lucifer yellow and with tau monoclonal antibodies AT8 and PHF-1, which recognize tau phosphorylated at Ser202/Thr205 and at Ser396/404, respectively. We observed that the diffuse accumulation of phospho-tau in a putative pre-tangle state did not induce changes in the dendrites of pyramidal neurons, whereas the presence of tau aggregates forming intraneuronal neurofibrillary tangles was associated with progressive alteration of dendritic spines (loss of dendritic spines and changes in their morphology) and dendrite atrophy, depending on the degree of tangle development. Thus, the presence of phospho-tau in neurons does not necessarily mean that they suffer severe and irreversible effects as thought previously but rather, the characteristic cognitive impairment in Alzheimer's disease is likely to depend on the relative number of neurons that have well developed tangles.

Neurons showing each of these phospho-tau patterns were analysed for possible alterations to their dendritic spines. As summarized in Fig. 9, no changes were found in the number, length or volume of the dendritic spines in neurons with Pattern I, nor in dendrite diameter, whereas several significant alterations were found in neurons showing Pattern II. At the intermediate level of a neurofibrillar pathology (Pattern IIa), we found different morphological alterations depending on the patients, mostly a reduction in dendritic spine volume and length (Patient P9) and a reduction in dendritic spine density (Patients P13 and P14). At late stages (Pattern IIb), there was a dramatic loss of spines, even in the proximal portion of the dendrites. This sequential loss of dendritic spines appears to be distinct to that found in the presence of amyloid-β plaques, where the dendritic regions close to the extracellular plaques are depleted of dendritic spines irrespective of whether they are proximal or distal to the soma (Knafo et al., 2009a). However, there seems to be a sequence related to the loss of dendritic spines in association with the intracellular tau pathology, occurring first in the distal and then in the more proximal regions (Fig. 9).Figure 9