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Anterior commissure and corpus callosum size
increase disproportionately with advancing age in rats.
Michael Noonan, Katherine Sion, Michael Viksjo & Katherine
Wescott
Presented at the Society for Neuroscience, Los Angeles,
November, 1998.
Introduction
The Corpus Callosum and Anterior Commissure are large bundles of fibers which
interconnect the two cerebral hemispheres. Considerable interest has been shown in recent
years in the question of whether these two commissures vary in size between the sexes.
Last year, we published a paper which reported a male-greater-than-female size difference
for the AC, even when the sexes were equated for overall brain size. At that time, we
hypothesized that the sex difference in the AC would be greater for its anterior limb than
for its posterior limb, and that this difference would vary with age. In the present study
we investigated these questions in a large sample of rats.
Procedures
Our subjects were 214 long-evans rats divided among 5 age groups. All subjects had been
previously utilized in other, unrelated behavioral investigations, but were
physiologically untreated. For this study, following intracardial perfusion, frozen
sagittal sections of their brains were taken beginning at the midline and stained for
myelin. The total areas encompassed by the CC, ACa, and ACp were assessed separately in
the first intact section. Representative sections are presented in Figs 1 and 2.
Results
Sex. Male and females differed significantly in overall brain weight, and this
male-greater-than-female sex difference was true at each age when analyzed separately
(Fig. 3). There was also a significant sex by age interaction reflecting the fact that
while the males continued to show an increase in overall brain size with age, females
showed a slight decrease in size beyond one year of age. Concomitant with the overall size
difference, the commissures were likewise larger in male than in female brains when these
areas were analyzed without adjustment.
However when the commissure sizes were adjusted for variations in overall brain size
(whether using ratio, regression, or log-transform methods), there were no significant sex
differences for either commissure taken as a whole (CC or AC), or for either subdivision
(ACa, ACp). This also held true when each age was analyzed separately. Similarly, there
were no sex by age interactions for any adjusted measure of commissure size, nor were any
significant sex differences found for any ratio relating CC, ACa, and ACp.
Age. The effect of age proved to be significant in all analyses of adjusted
commissure size (regardless of the method used to adjust for overall brain size). This
reflected the fact that for all three commissural divisions (CC, ACa, ACp) the areas
increased in size with age beyond that attributable to increases in overall brain size.
This effect was primarily due to increases in adjusted commissure size in our oldest age
group. This was true for both sexes.
Our last figure depicts three regression lines which best fit the slope of our
commissure area measures with age (centered around zero). The rate of increase with age is
greatest for the corpus callosum, next greatest for ACa, and least for ACp.
Discussion
We feel that the very large sample of rats contained in the present study should put to
rest the question of whether sex differences in commissure size exist in this strain of
rat. The commissures do not vary in size between the sexes when adjusted for
difference in overall brain size.
For both sexes, the commissures increase in size with advancing age at a rate beyond
that shown for the brain overall. We are presently endeavoring to determine whether these
increases in size are accompanied by a concomitant increase in the number of fibers.
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