THE TOPOGRAPHIC DISTRIBUTION OF THE EFFERENTS FROM NEOCORTICAL NEURONS IS NOT ONLY DEPENDENT UPON WHERE IN THE NEOCORTEX THE CELLS DEVELOP - A TRANSPLANTATION STUDY WITHIN ONE SINGLE NEOCORTICAL REGION

It has been proposed that the distribution of efferents developed by n eocortical neurons depends upon where in the neocortex the cells devel op, not where they were generated. However, the capacity of diverse is ocortical areas to differentiate connectional characteristics belongin g to other isocortical areas has recently been questioned in several e xperiments using heterotopic transplantation paradigms. The present st udy was designed to determine whether the principle of multipotentiali ty is still valid within one single isocortical region. Mediolateral b ands of embryonic (E16) frontal neocortex were dissected out and graft ed into the left frontal cortex of neonate hosts according to either c orrect or inverted mediolateral orientation. Five to six months after grafting, a retrograde tracer was injected into the dorsomedial or ven trolateral left neostriatum of the host. The mediolateral distribution of the cell labeling within the transplant was then compared to that of an equivalent frontal cortical area (EGA) in control cases. The res ults indicate that strips of embryonic frontal neocortex transplanted according to a correct mediolateral orientation are able to develop a projection towards the host striatum whose mediolateral topographical distribution is not significantly different from that arising from the frontal neocortex of control animals. The percentages of transplant c ells labeled in the medial or lateral division of the grafts were not significantly different from those found medially or laterally in the ECA in control cases. Following inversion of the mediolateral orientat ion of the grafts at the time of transplantation, the percentages of c ells labeled in the medial or lateral division of the grafts were near ly equal whatever the site of tracer deposit within the host neostriat um. These results indicate that even within one single neocortical reg ion the principle of areal interchangeability is not entirely validate d and that the development of neocortical efferents is not only guided by extrinsic factors.