Stereological and allometric studies on mammalian cerebral cortex with implications for medical brain imaging.

Design-based stereological methods (the Cavalieri principle and vertical sectioning) have been used to estimate the volumes, surface areas and thicknesses of the cerebral cortex. Cortices of individual hemispheres were analysed in a selection of 31 adult domestic mammals (horses, oxen, pigs, goats, dogs, cats and rabbits). There were 13 females and 18 males. After correcting for fixation shrinkage effects, results were tested for species, laterality and sex differences using linear regression and analysis of variance. Mean body weights of domestic mammals varied from 4 kg to 460 kg and brain volumes from 11 cm3 to 603 cm3. Hemisphere dimensions varied between species but, except for volume (which exhibited a species x sex interaction effect), no other differences were detected. It is concluded that these mammalian brains are, in terms of their gross anatomy, symmetric and not sexually dimorphic. Apparent cortical thickness (measured directly on slices) proved to be a satisfactory estimate of true thickness (estimated by dividing cortical volume by the mean of outer and inner cortical surfaces). This has implications for medical slice images on which mean cortical thickness can be estimated only from apparent local thicknesses.