Efferent connections of homotopic and heterotopic transplants of embryonic neocortical tissue placed in the occipital neocortex of newborn rats.

We examined (i) the capacity of transplants of embryonic neocortex to restore corticofugal systems disrupted following neonatal damage to the occipital cortex and (ii) the influence of the embryonic origin of the transplanted neurons on the reconstruction of the corticofugal circuitry. Transplants of embryonic occipital or frontal cortex were grafted homo- or heterotopically into the damaged occipital cortex of newborn rats. Several months after grafting, an anterograde tracer was injected into each category of transplants. Homotopic transplants developed a set of projections directed exclusively towards most of the cortical and subcortical visual targets normally contacted by occipital cortical neurons. Heterotopic transplants formed a hybrid system of efferent projections that reflected both their embryonic origin and their new location within the host cortex. These findings are consistent with previous results indicating that fetal frontal and occipital neurons are not interchangeable. Consequently, transplantations aiming at the reconstruction of neural circuits disrupted following neonatal damage affecting a given cortical area should only use fetal cortical cells taken from the same cortical locale.