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The Journal of Neuroscience, November 15, 1999, 19(22):9939-9952

Formation of Cortical Fields on a Reduced Cortical Sheet

Kelly J. Huffman¹, Zoltán Molnár^{2, 3}, Anton Van Dellen², Dianna M. Kahn¹, Colin Blakemore², and Leah Krubitzer¹

¹ Center for Neuroscience and Department of Psychology, University of California, Davis, Davis, California 95616, ² University Laboratory of Physiology, University of Oxford, England OX1 3PT, and ³ Institut de Biologie Cellulaire et de Morphologié Faculté de Medecine, 1005 Lausanne, Switzerland

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiation of cortical fields (O'Leary, 1989; Krubitzer, 1995). In the present study, we examined how changing the size of the immature neocortex before the establishment of thalamocortical connections affects the subsequent development and organization of the adult neocortex. This alteration in cortex is consistent with one of the most profound changes made to the mammalian neocortex throughout evolution: cortical size. Removing the caudal one-third to three-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationships between visual, somatosensory, and auditory cortical fields on the remaining cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this alteration allowed the maintenance of their original anteroposterior distribution. These results demonstrate the capacity of the cortical neuroepithelium to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortical fields appear to be invariant. The shifting of afferents and efferents with cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be sufficient to explain much of the phenotypic variation in cortical field number and organization in different mammals.

Key words: cerebral cortex; visual cortex; cortical organization; development; evolution; electrophysiology; Monodelphis domestica

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Copyright © 1999 Society for Neuroscience  0270-6474/99/19229939-14$05.00/0

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