The total number of myelinated nerve fibers is reduced in corpus callosum in brains from patients with Alzheimer's disease

The total number of myelinated fibers in corpus callosum (CC) in Alzheimer's disease is poorly described. Using a two-dimensional fractionator principle we estimated the total number of fibers in ten females patients with Alzheimer's disease (AD) (mean age 82.8 years) and 12 female control subjects (mean age 80.6 years) and found the mean number of myelinated fibers to be significantly 30% higher in the control group (90.0 × 106; coefficient of variation = SD/mean = 27%) than in the AD group (69.3 × 106; coefficient of variation = 20%) (p = 0.028). A significant loss of fibers (≥2.2 μm in diameter) was found in the anterior part of the CC in AD patients compared to control subjects (p = 0.045). For both groups, we found a positive correlation between the total number of myelinated fibers and CC cross-sectional area (AD, r = 0.81; control subjects, r = 0.73). There was a negative correlation in both groups between age and total number of myelinated fibers (AD, r = −0.81; controls subjects, r = −0.59). These results implicate a loss of intrahemispheric connections in the pathophysiology of AD.