CONTRIBUTION OF THALAMIC INPUT TO THE SPECIFICATION OF CYTOARCHITECTONIC CORTICAL FIELDS IN THE PRIMATE - EFFECTS OF BILATERAL ENUCLEATION IN THE FETAL MONKEY ON THE BOUNDARIES, DIMENSIONS, AND GYRIFICATION OFSTRIATE AND EXTRASTRIATE CORTEX

Bilateral enucleation was performed at different fetal ages during cor ticogenesis, and the brains were prepared for histological examination . Early-enucleated fetuses (operated prior to embryonic day 77) showed morphological changes at the level of the thalamus and the cortex. In the thalamus, there was a loss of lamination and a decrease in size o f the lateral geniculate nucleus. There was a decrease in the size of the inferior pulvinar, but there was no change in the lateral pulvinar . The border of striate cortex was as sharp in the enucleates as it wa s in the normal monkeys. In three of the four early enucleates, we obs erved an interdigitation of striate and extrastriate cortex. In three of the early enucleates, we observed a small island of nonstriate cort ex near the striate border that was surrounded entirely by striate cor tex. Enucleation led to an age-related reduction of striate cortex. Th is reduction was greater in the operculum than in the calcarine fissur e. The reduction of striate cortex was accompanied by an increase in t he dimensions of extrastriate visual cortex, so that the overall dimen sions of the neocortex remained invariant. The extrastriate cortex in the enucleated animals presented a uniform cytoarchitecture and was in distinguishable from area 18 in the normal animal. There were changes in the gyral pattern that were restricted mainly to the cortex on the operculum. A deepening of minor dimples as well as the induction of a variable number of supplementary sulci led to an increase in the convo lution of the occipital lobe. These results are discussed with respect to the specification of cortical areas. They demonstrate that the red uction in striate cortex was not accompanied by an equivalent reductio n in the neocortex; rather, there was a border shift, and a large volu me of cor tex that was destined to become striate cortex appears to be cytoarchitectonically normal extrastriate cortex. (C) 1996 Wiley-Liss , Inc.