Experiments involving the heterotopic transplantation of neocortex during development have indicated that the efferent connections maintained by the transplant as well as other organizational features are appropriate to the transplant's regional locale within the host cortex, rather than to its site of origin within the donor cortex. These findings would seem to be consistent with the idea that developing cortical neurons lack any rigid regional specification. To examine this further, we made lesions in the rostral cortex of newborn rats and placed pieces of either rostral or occipital fetal cortex into the lesion site. Additional cases with similar lesions, but with no transplants, wore also prepared. When the animals matured, behavioral testing was done to identify any residual deficits. Compared with lesioned animals that had received no transplants, animals with homotopic transplants show a substantial sparing in certain forelimb placing and somatosensory tasks. In marked contrast, animals with heterotopic transplants did not show any sparing. Indeed, when placing reactions were elicited by vibrissal stimulation (vibrissae→ forelimb placing; extinction-placing) the animals with heterotopic transplants showed a greater impairment than the lesioned animals that received no transplants. These results indicate that while homotopic fetal cortical transplants may help ameliorate behavioral deficits that normally follow neonatal lesions of the rostral cortex, heterotopic transplants do not, and may in fact exacerbate the deficits, despite the fact that such heterotopic transplants have been shown to maintain projections that seemingly are appropriate to their locale. These findings indicate that at a stage when fetal cortical neurons retain the potential to extend axons to targets appropriate for a variety of cortical regions, their capacity for complete functional integration into heterotopic cortical regions may already be restricted. This may imply that a progressive determination of regional cortical attributes occurs during normal fetal development.

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