ABNORMALITIES IN THE DEVELOPMENT OF THE TECTAL PROJECTION FROM TRANSPLANTS OF EMBRYONIC OCCIPITAL CORTEX PLACED IN THE DAMAGED OCCIPITAL CORTEX OF NEWBORN RATS

We have examined the degree of precision in the topographic arrangemen t of the the tectal projection developed by homotopic transplants of e mbryonic occipital cortex and tried to determine whether the developme nt of the corticotectal, projection is exclusively dependent on enviro nmental cues or is also controlled by intrinsic factors. Transplants o f embryonic (E16) occipital cortex were grafted into various areas of the occipital cortex (Oc1 or Oc2) of newborn rats and the organization of the tectal projection arising from the transplants was subsequentl y examined by injecting different neurotracers into the transplants. O ur results indicate that in most cases the laminar and tangential dist ributions of the tectal projections from the transplants were abnormal , Indeed, whatever the location of the transplant in the host occipita l cortex and whatever the placement of the injection into the transpla nt, a hybrid distribution of the tectal labeling was found, reminiscen t of the pattern observed following tracer deposits in both Oc1 and Oc 2 in intact animals. Since the grafts were composed of cells of both O c1 and Oc2 embryonic origin, it is likely that the hybrid pattern of e fferents reflects the heterogeneity of the embryonic origin of the cel ls composing the graft. These findings provide evidence that the devel opment of the topographic distribution of neocortical efferents is not only dependent on factors extrinsic to the cortex and further indicat e that even within one single cortical region, the occipital cortex, d ifferent areas (Oc1 vs Oc2) are not totally interchangeable. These fin dings might have important implications in transplantation experiments aiming; at the reconstruction of damaged neocortical circuitry where a precise ''point-to-point'' reconstruction of the circuitry is expect ed. (C) 1997 Academic Press.