Formation of cortical fields on a reduced cortical sheet

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiatio n of cortical fields (O'Leary, 1989; Krubitzer, 1995). In the present study , we examined how changing the size of the immature neocortex before the es tablishment of thalamocortical connections affects the subsequent developme nt and organization of the adult neocortex. This alteration in cortex is co nsistent with one of the most profound changes made to the mammalian neocor tex throughout evolution: cortical size. Removing the caudal one-third to t hree-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationship s between visual, somatosensory, and auditory cortical fields on the remain ing cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this a lteration allowed the maintenance of their original anteroposterior distrib ution. These results demonstrate the capacity of the cortical neuroepitheli um to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortic al fields appear to be invariant. The shifting of afferents and efferents w ith cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be suffici ent to explain much of the phenotypic variation in cortical field number an d organization in different mammals.