DEVELOPMENT OF SPINAL-CORD PROJECTIONS FROM NEOCORTICAL TRANSPLANTS HETEROTOPICALLY PLACED IN THE NEOCORTEX OF NEWBORN HOSTS IS HIGHLY DEPENDENT ON THE EMBRYONIC LOCUS OF ORIGIN OF THE GRAFT

Previous experiments based on heterotopic transplantation paradigms ha ve indicated that the distribution of efferents developed by layer V p yramidal cells seems to be related to where in the neocortex the cells develop and not to where they were generated. The present study was u ndertaken in an attempt to obtain a quantitative estimation of the wei ght of extrinsic factors in the development of neocortical efferents. Fragments of embryonic (E15-E19) frontal or occipital cortex were graf ted homotopically or heterotopically into the frontal or occipital cor tex of newborn rats. As adults, the hosts received an injection of a r etrograde tracer into the pyramidal tract decussation, and the distrib ution of the subsequent cell labeling was examined in each category of transplant. The mean numbers of labeled cells were 725 in frontal-to- frontal transplants and 250 in frontal-to-occipital transplants. In oc cipital-to-frontal transplants, the numbers of labeled cells were extr emely low, ranging from 0 to 14. Finally, as expected, practically no cell labeling was found in occipital-to-occipital transplants. Thus, t ransplants of presumptive frontal origin systematically develop and ma intain in adulthood a spinal cord projection even though they are plac ed in the host occipital cortex. Conversely, transplants of presumptiv e occipital origin are practically incapable of maintaining a spinal c ord projection in adulthood even though they are placed in the host fr ontal cortex. It seems, therefore, that the generation of regional dif ferences in efferent connectivity found in the mature cortex depends o n early regional specification within the neocortical neuroepithelium. (C) 1996 Wiley-Liss, Inc.