Cellular, morphometric, ontogenetic and connectional substrates of anatomical asymmetry.

Although anatomical cerebral asymmetry appears in all animals that have been examined, its link to functional lateralization is not clear. In an attempt to further elucidate this relationship between structure and function, we have compared, in rats and humans, brains that have asymmetric architectonic areas to those that are symmetric. We have found that (1) asymmetry is the result of the production of a small side rather than the production of a large side; (2) architectonic asymmetry is the result of changes in the total numbers of neurons rather than cell-packing density; (3) events occurring early in corticogenesis--specifically during the period of progenitor cell proliferation and/or death--are important for the formation of asymmetric cortical areas; and (4) symmetric brains have relatively greater numbers of callosal fibers and more patches of termination than their asymmetric counterparts. These results, taken together, suggest that if anatomic asymmetry underlies functional lateralization, it may have more to do with the different organization of symmetric and asymmetric brains, rather than simply which hemisphere (or brain region) is larger.