Transplants of Limbic and Neocortex Reveal Mechanisms of Generating Phenotypic Diversity in the Cerebral Cortex

Genetic and environmental factors influence the choices that cells make in expressing specific phenotypic features. We have examined the relative impact of these factors in controlling the fate of neurons comprising specific cerebral cortical areas using fetal tissue transplants/1,2/ and cell cultures of cortical stem cells. The limbic cortical phenotype is assayed in heterotopic and homotopic transplants by two methods: 1) evaluation of the expression of the limbic system-associated membrane protein (LAMP) and 2) mapping of limbic and nonlimbic afferents originating from the host thalamus using DiI labeling of the transplants. Grafts of presumptive limbic or sensorimotor cortical tissue, harvested after neurogenesis begins [embryonic day (E) 14 and 17], retain their appropriate molecular and connectivity features, even when placed in an inappropriate milieu. In contrast, cortical progenitor cells, isolated at E12, exhibit phenotypes that reflect the position of the transplant in the host. For example, stem cells destined to give rise to nonlimbic neocortex were induced to express LAMP and develop limbic thalamo-cortical connections when placed into the limbic cortical region of host brains. Stem cells destined to give rise to limbic cortex generated non-limbic neurons with appropriate thalamic connections when placed in the sensorimotor cortex of the host. In an effort to identify potential signals that could regulate the behavior of cortical progenitor cells, regions of the cerebral wall giving rise to limbic and non-limbic areas were isolated at E12 and placed in monolayer cell culture. Neuronal differentiation, monitored by expression of microtubule associated protein 2 (MAP2), was identical in both populations over a 4 day period. The expression of LAMP by these neurons, however, differed dramatically. Stem cells originating from regions destined to give rise to limbic cortex differentiated into LAMP+ neurons (>80%), while most progenitors originating from sensorimotor regions of the cerebral wall were LAMP (>75%). The transplant results suggest that there is an early critical period for the specification of the cells that give rise to certain cerebral cortical areas. While the results with the grafted stem cells indicate that they are uncommitted to a specific cortical phenotype, their behavior in vitro reveals that the progenitors are fated to produce neurons of a particular cortical area. The eventual commitment of a cell to a particular cortical fate is likely to be dependent upon the interaction of environmental signals with intrinsic genetic information of the progenitor.