Development of the striatal projection from embryonic neurons from the lateral or medial frontal cortex grafted homo- or heterotopically into the medial frontal cortex of newborn rats.

The present study was designed to further investigate the effects of intrinsic or extrinsic influences on the development of the efferent connectivity of frontal neocortical neurons. The lateral or medial parts of the frontal neocortex of embryonic (E) day 16 fetuses were grafted into homo- (medial-to-medial) or heterotopic (lateral-to-medial) position in the medial part of the left frontal cortex of newborn hosts. Three to four months after grafting, a retrograde neurotracer was injected into the dorsomedial or ventrolateral quadrant of the left caudate-putamen (CPU). The ensuing retrograde labeling in the transplants was then compared to that found in an equivalent cortical area in control animals. Medial-to-medial transplants developed a striatal projection whose mediolateral organization conforms to that of the projection arising from the medial part of the intact frontal cortex. The mediolateral distribution of the projection arising from lateral-to-medial transplants was not fundamentally different from that originating from medial-to-medial transplants, a finding which stands in marked contrast with what was found recently [7] with medial-to-lateral transplants. These results indicate that inside the frontal cortex, different subregions are not totally interchangeable, at least in terms of development of efferent connectivity.