Final answer:
The collapse of the microtubule assembly leads to the formation of neurofibrillary tangles, which are a result of abnormal tau protein accumulation in neurons, commonly seen in Alzheimer's disease.
Step-by-step explanation:
The formation of neurofibrillary tangles involves the aberrant modification and accumulation of a protein known as tau. Ordinarily, tau protein helps stabilize microtubules within the neuron's cytoskeleton. However, in Alzheimer's disease, tau protein becomes hyper-phosphorylated, changing its structure and leading to a collapse of the microtubule assembly. As a result, the destabilized tau proteins aggregate into tangles, which stain deep purple when immunostained. These neurofibrillary tangles contribute to neuronal dysfunction and are hallmark features of Alzheimer's pathology.