Development of the Corpus Callosum

Abstract

During the course of development, the areal distribution of neurons which project across the corpus callosum undergoes significant changes. In the mature animal, the neurons which contribute axons to the corpus callosum are not uniformly distributed throughout the neocortex. Rather, within any given cortical area they are distributed in a differential fashion which varies from cortical area to cortical area. Generally speaking, the distribution of callosal projection neurons is more restricted within primary sensory areas than it is in other cortical areas. This discrete adult distribution of callosal projection neurons emerges out of a more widespread distribution in the developing animal This phenomenon was initially reported by G. Innocenti and co-workers who described the initial widespread distribution of callosal projection neurons in the visual cortex of the neonatal cat. The term they used to refer to this initial widespread distribution was “exuberance.” This phenomenon has since been reported to occur in the neocortex of a wide variety of mammalian species and in a second major class of neocortical projection neurons (cortico-subcortical) suggesting that it is a general feature of neocortical ontogeny. Two major questions concerning this phenomenon are: 1) what is the mechanism by which it occurs? and 2) what is its adaptive significance? Before turning to these questions, however, it is necessary to describe the phenomenon.