Regional differences of neuron loss of rat brain in old age.

DNA content was measured in four areas of the brain (cerebral hemisphere, cerebellum, diencephalon, and lower brain stem) of young adult (5-month-old) and aged 24- to 30-month-old Long-Evans rats. There was no change in DNA content in all four areas in old age. The number of brain cells was counted in three areas of the brain (cerebral hemisphere, cerebellum and brain stem) with an ultrasonic separation method. In old age, there was significant loss in the total number of brain cells in the cerebellum but no cell loss in the cerebral hemisphere and the brain stem. The neuron number was significantly decreased and the glia/neuron ratio was increased in the cerebellum and cerebral hemisphere, but neither of them changed in the brain stem in old age. The cerebellum showed significantly more neuron loss than the cerebral hemisphere in old age. DNA content in individual brain cells in the cerebral hemisphere, cerebellum and brain stem does not change in old age; thus, calculation of brain cell number from DNA content in the brain of old rats is justified.