Synaptic density in human frontal cortex - developmental changes and effects of aging.
Level 4 - case-series / case-control
Postmortem cross-sectional histological study of human brain tissue
PubMed 427544 · doi:10.1016/0006-8993(79)90349-4
What was done
Synaptic profile density in layer 3 of the middle frontal gyrus was quantified in 21 normal human brains from individuals aged newborn to 90 years. Profiles were visualized using the phosphotungstic acid method in tissue fixed up to 36 hours postmortem.
What was found
Synaptic density was constant across adult life (ages 16–72 years) with a mean of 11.05 x 10^8 synapses/cu.mm (SEM 0.41). In the aged group (ages 74–90 years, n = 4), density declined slightly to a mean of 9.56 x 10^8 synapses/cu.mm (SEM 0.28, P < 0.05). Neonatal brains had synaptic densities in the adult range but showed immature morphology (irregular presynaptic dense bands rather than separate presynaptic projections). Density peaked at ages 1–2 years at approximately 50% above the adult mean, followed by a decline between ages 2 and 16 years that was accompanied by a slight decrease in neuronal density.
Why it matters
This study provided foundational evidence that the human frontal cortex undergoes massive postnatal synaptogenesis peaking in early infancy followed by extensive developmental synaptic elimination during childhood and adolescence.
Limits
The sample size is very small (21 total brains, including only 4 in the 74–90 age group), limiting statistical power and subgroup comparisons. The cross-sectional postmortem design cannot track individual developmental trajectories and is vulnerable to tissue artifacts, variable postmortem fixation delays (up to 36 hours), and unmeasured cause-of-death confounders. Analysis was restricted to layer 3 of a single cortical area.