HE primary goal of therapeutic intervention after TBI has been directed at ameliorating the destructive pro- cesses that begin with the injury. Although promising results have been demonstrated in animal models, the phar- macological agents that have been used have not displayed the same efficacy in clinical trials. Regardless of the select- ed therapeutic intervention, the initial trauma will always cause loss of neurons. Hence, successful restoration of the brain needs to involve the regeneration or transplantation of cells capable of differentiation and integration to restore functional connectivity. With the relatively recent discovery that neural stem and progenitor cells can be expanded in culture while retaining their capability to differentiate into neurons and glia, the possibility of therapeutic cell trans- plantation for a multitude of neurological disorders has be- come tangible. 7,37,47,49 Animal studies have demonstrated that xenografted hu- man neural stem/progenitor cells can migrate, differentiate, and respond to local cues when transplanted into uninjured adult and neonatal rodent brains. 10,13 In addition, the ability of neural progenitor cell lines to improve functional out- come after ischemic and excitotoxic insults to the CNS has been demonstrated. 19,41 Immortalized cells from rodent and human teratoma cell lines have also been shown to survive and differentiate when transplanted into the injured rodent brain; 28,35,36,40 however, the signals that mediate the differ- entiation and the mechanisms for the observed neurologi- cal improvement are not clear. Hypothetically, grafted cells may produce growth factors and cytokines that enhance the survival of injured neurons, but replacement of lost neurons may also be possible. The latter line of reasoning is corrob- orated by a recent study in which it was demonstrated that host cells in the hippocampus form functional synapses with transplanted embryonic stem cells. 1 These data are en- couraging for the possible transplantation of neural progen- itor cells in cases of TBI; however, additional knowledge of human progenitor cells in this context is required before cell therapy can be applied in humans. A supply of human cells that can be produced on a large scale is a prerequisite for J Neurosurg 100:88–96, 2004 88 Proliferation, migration, and differentiation of human neural stem/progenitor cells after transplantation into a rat model of traumatic brain injury ANDRÉ WENNERSTEN, M.SC., XIA MEIJER, M.D., PH.D., STAFFAN HOLMIN, M.D., PH.D., LARS W AHLBERG, M.D., PH.D., AND TIIT MATHIESEN, M.D., PH.D. Department of Clinical Neuroscience, Section of Neurosurgery, Karolinska Hospital, Stockholm, Sweden; and NS Gene A/S, Copenhagen, Denmark Object. Cultures containing human neural stem and progenitor cells (neurospheres) have the capacity to proliferate and differentiate into the major phenotypes of the adult brain. These properties make them candidates for therapeutic trans- plantation in cases of neurological diseases that involve cell loss. In this study, long-term cultured and cryopreserved cells were transplanted into the traumatically injured rat brain to evaluate the potential for human neural stem/progenitor cells to survive and differentiate following traumatic injury. Methods. Neural stem/progenitor cell cultures were established from 10-week-old human forebrain. Immunosuppressed adult rats received a unilateral parietal cortical contusion injury, which was delivered using the weight-drop method. Immediately following the injury, these animals received transplants of neural stem/progenitor cells, which were placed close to the site of injury. Two or 6 weeks after the procedure, these animals were killed and their brains were examined by immunohistochemical analysis. At both 2 and 6 weeks postoperatively, the transplanted human cells were found in the perilesional zone, hippocampus, corpus callosum, and ipsilateral subependymal zone of the rats. Compared with the 2-week time point, an increased num- ber of HuN-positive cells was observed at 6 weeks. In addition, at 6 weeks post–injury/transplantation, the cells were noted to cross the midline to the contralateral corpus callosum and into the contralateral cortex. Double labeling demonstrated neuronal and astrocytic, but not oligodendrocytic differentiation. Moreover, the cortex appeared to provide an environment that was less hospitable to neuronal differentiation than the hippocampus. Conclusions. This study shows that expandable human neural stem/progenitor cells survive transplantation, and migrate, differentiate, and proliferate in the injured brain. These cells could potentially be developed for transplantation therapy in cases of traumatic brain injury. KEY WORDS • brain injury • neurosphere • differentiation • migration • rat T J. Neurosurg. / Volume 100 / January, 2004 Abbreviations used in this paper: CNS = central nervous system; BrdU = bromodeoxyuridine; DMEM = Dulbecco modified Eagle medium; GFAP = glial fibrillary acidic protein; SVZ = subventricu- lar zone; TBI = traumatic brain injury.