Species Differences and Similarities in the Fine Structure of the Mammalian Corpus callosum

Abstract

A cross-species ultrastructural study of the corpus callosum was performed in six domestic species: the rat, the rabbit, the cat, the dog, the horse and the cow. The results indicate cross-species conservatism in callosal fiber composition with a good interspecies relation between fiber number and brain size. Across species, increases in both brain size and callosal area indicate more callosal fibers, although less than expected from the estimated increase in cortical cell number. Within each species, the correlation between fiber number and brain weight tends to disappear, although in most cases a larger callosum implies a larger number of callosal fibers. The median fiber diameter was conservative across species (0.11–0.2 µm), indicating the maintenance of conduction velocity of most callosal fibers regardless of interhemispheric distance. Nevertheless, the maximal fiber diameters tended to be higher in species with larger brains. Therefore, there is a population of coarse-diameter fibers that tend to increase their diameter and conduction velocity with increasing brain size. However, allometric calculations suggest that the associated increase in velocity in these large fibers may not be sufficient to maintain a constant interhemispheric transmission time in different species.

One hundred million years of interhemispheric communication: the history of the corpus callosum

“…Callosal regions connecting prefrontal and temporoparietal association areas are characterized by large proportions of poorly myelinated, small caliber, slow-conducting fibers, while in regions connecting primary and secondary sensorimotor areas there is a concentration of fast-conducting, highly myelinated fibers of more than 3 µm in diameter. However, in other mammals such as carnivores, rodents and ungulates, we did not observe regional differences in fiber size, at least in the posterior corpus callosum (10). This may partly reflect the more diffuse topographic arrangement of different cortical areas in the corpus callosum of these species (see Ref.…”

“…We and others have determined that callosal fiber composition tends to be conservative across species (10,49), indicating that interhemispheric transmission time can be significantly impaired in large-brained species (50). There is only a small contingent of fibers of very wide diameter whose caliber increases with increasing brain weight, but their increase in maximal diameter may not be sufficient to fully compensate for the increasingly long interhemispheric distances (10).…”

“…This may partly reflect the more diffuse topographic arrangement of different cortical areas in the corpus callosum of these species (see Ref. 10,11).…”

“…We and others have determined that callosal fiber composition tends to be conservative across species (10,49), indicating that interhemispheric transmission time can be significantly impaired in large-brained species (50). There is only a small contingent of fibers of very wide diameter whose caliber increases with increasing brain weight, but their increase in maximal diameter may not be sufficient to fully compensate for the increasingly long interhemispheric distances (10). Furthermore, across species the proportion of callosal fibers in relation to brain size or to the estimated numbers of cortical cells tends to decrease with increasing brain weight (10,51,52), thereby reducing the degree of interhemispheric connectivity.…”

“…There is only a small contingent of fibers of very wide diameter whose caliber increases with increasing brain weight, but their increase in maximal diameter may not be sufficient to fully compensate for the increasingly long interhemispheric distances (10). Furthermore, across species the proportion of callosal fibers in relation to brain size or to the estimated numbers of cortical cells tends to decrease with increasing brain weight (10,51,52), thereby reducing the degree of interhemispheric connectivity. Overall, this evidence suggests that reduced callosal transmission is related to increased brain size, which is consistent with the hypothesis of Ringo et al (48).…”