Effects of aging on formalin-fixed brain weight in mice and rats

In three mouse lineages it was found that during the 18- to 60-day interval after formalin fixation, brain weight was inversely proportional to the age at sacrifice, as was the decline in the weight of the brain during this period. In a second study using rat subjects it was confirmed that the aging difference is due to a reduction, in old age, of the swelling reaction of the brain when first placed in a formalin solution. It was suggested that this easily measured phenomenon might be employed as an operational index of brain aging.