Aging and the human neocortex.
Level 4 - case-series / case-control
Cross-sectional post-mortem stereological study
PubMed 12543266 · doi:10.1016/s0531-5565(02)00151-1
What was done
Neurostereological methods were applied to post-mortem human brain tissue to quantify total neocortical neuron and glial cell counts, neuronal size distribution, total myelinated fiber length in cerebral white matter, and total neocortical synapse numbers across adult aging (ages 20 to 90 years).
What was found
Total neocortical neuron loss was less than 10% across the 20 to 90 year age range. Total glial cell counts did not differ significantly between six young individuals (mean age 26.2 years; mean 39 billion glia) and six elderly individuals (mean age 89.2 years; mean 36 billion glia). Total myelinated fiber length ranged from 150,000 to 180,000 km in young individuals and showed a large reduction with age. Total neocortical synapses were estimated at approximately 0.15 × 10^15. Sex differences across structural elements were larger than the effects of aging.
Why it matters
This quantitative stereological work refutes the historical assumption that healthy aging causes massive cortical neuron death, indicating instead that structural brain aging is driven largely by loss of white matter myelinated connectivity.
Limits
The glial comparison was limited to a very small cohort (n = 6 per group), and total sample sizes for other stereological endpoints were not stated in the abstract. As a cross-sectional post-mortem study, it cannot demonstrate individual longitudinal trajectories or correlate structural findings with clinical cognitive function.