Striatal neurons in striatal grafts are derived from both post-mitotic cells and dividing progenitors

Transplants of embryonic striatal tissue are characteristically heterogeneous, containing patches (P-zones) of striatal medium spiny projection neurons. It is not yet known how this morphology develops, and whether the striatal neurons in the grafts are derived from post-mitotic neuroblasts in the embryonic brain or from striatal progenitors that continue to divide after transplantation. To address this question we labelled dividing cells in the transplants with bromodeoxyuridine (BrdU), either prior to or after transplantation into the adult lesioned rat striatum. Cells for transplantation were either pre-labelled in utero by intraperitoneal (i.p.) injections of BrdU, or post-labelled after transplantation by i.p. injections to the hosts. Either two or six months after transplantation the brains were processed using double immunohistochemical techniques to detect BrdU and calbindin-positive neurons in the transplants. In the transplants pre-labelled with BrdU, approximately 30% of calbindin-positive cells were heavily labelled with BrdU, suggesting these had undergone a final division prior to transplantation. In transplants where cells had been labelled post-transplantation, approximately 17% of calbindin cells were heavily BrdU labelled. These results suggest that whereas a proportion of striatal medium spiny neurons in the striatal grafts were post-mitotic at the time of transplantation, other striatal progenitor cells can continue to divide after transplantation, and then complete an appropriate neuronal maturation programme in the adult host brain environment.