DEVELOPMENT OF PROJECTIONS FROM TRANSPLANTS OF EMBRYONIC MEDIAL OR LATERAL FRONTAL-CORTEX PLACED IN THE LATERAL FRONTAL-CORTEX OF NEWBORN HOSTS

Several recent experiments using neocortical transplantation paradigms indicated that embryonic neurons grafted in a heterotopic locus retai n developmental characteristics corresponding to their site of origin. In the present study, limited portions of lateral (lateral-to-lateral ) or medial (medial-to-lateral) sectors of embryonic (E16) frontal cor tex were grafted into the lateral frontal cortex of newborn rats. A re trograde tracer was injected 3-4 months later into the dorsomedial or ventrolateral sectors of the host caudate-putamen (CPU). The results i ndicate that the mediolateral arrangement of the striatal projection d eveloped by lateral-to-lateral transplants is virtually identical to t hat found in intact rats. A very weak proportion of the transplanted c ells distribute fibers to the dorsomedial sector of the CPU. In marked contrast, the proportion of efferents from medial-to-lateral transpla nts projecting to the dorsomedial CPU is by far larger than the one di rected to the ventrolateral CPU. Our findings provide evidence that ev en within one single neocortical area (the frontal neocortex) some deg ree of prespecification (medial versus lateral patterns of efferent pr ojections) is already present at E16.