In Vitro Derived "Neural Stem Cells" Function as Neural Progenitors Without the Capacity for Self-Renewal

¹ Program in Stem Cell Biology and Regenerative Medicine, University of Florida, Gainesville, Florida

^* To whom correspondence should be addressed. E-mail: gpm2{at}ufl.edu.

	Abstract

Hematopoietic stem cells have been defined by their ability to self-renew and successfully reconstitute hematopoiesis throughout the life of a transplant recipient. Neural stem cells (NSCs) are believed to exist in the regenerating regions of the brain in adult mice: the subependymal zone (SEZ) of the lateral ventricles (LV) and the hippocampal dentate gyrus. Cells from the SEZ can be cultured to generate neurospheres or multipotent astrocytic stem cells (MASC), both of which demonstrate the stem cell qualities of multipotency and self-renewal in vitro. Whether or not neurospheres and MASC possess the true stem cell quality of functional self-renewal in vivo is unknown. The definitive tests for this unique capability are long-term engraftment and serial transplantation. Both neurospheres and MASC transplanted into the LV of C57BL/6 mice resulted in short term engraftment into the recipient brain, with donor-derived migratory neuroblasts visible in the rostral migratory stream and olfactory bulb following transplantation. To test in vivo expansion/self-renewal of the transplanted cells, we attempted to re-isolate donor-derived neurospheres and MASC. Even when rigorous drug selection was used to select for rare events, no donor-derived neurospheres or MASC could be re-isolated. Furthermore, donor-derived migratory neuroblasts were not observed in the RMS for more than one month following transplantation, indicating a transient rather than long-term engraftment. Therefore, in vitro derived neurospheres and MASC do not function as NSCs with long term, self-renewal capabilities in vivo, but instead represent short-term neural progenitor cells as defined by an in vivo functional assay.