35 ES cells Embryonic stem (ES) cells, which were first, isolated from mouse blastocysts in 1981 ,36,37 have been shown to proliferate indefinitely in vitro in an undifferentiated state, and to differentiate into various lineages in response to different cell culture conditions. Current, extensive knowledge of cell biology, genetic manipulation, and in vitro culture methods make mouse ES cells an optimal system for potential development, of unlimited transplantable cell source with

reproducible genetic modification and cell biological methods.38 It has Inhibitors,research,lifescience,medical been known for several years that mouse blastocyst-dcrived cell lines could differentiate into teratomas containing cells of neuroectodermal Inhibitors,research,lifescience,medical lineage after transplantation of undifferentiated cells into syngeneic mice.39 Using retinoic acid (RA) treatment, Bain et al described the first, in vitro protocol for efficient generation of neurons from ES cells.40 However, the Bain protocol was not suitable to generate DA neurons, most probably due to the fact, that RA primes the neural cells towards more “dorsal” phenotypes. Recently, Inhibitors,research,lifescience,medical Barberi et al described several protocols for the generation of several kinds of neurons from mouse ES cells.41

Interestingly, some reports suggest that neural differentiation from ES cells may even be a “default” option occurring unless other cell fates are actively induced.42,43 This review will focus on the successful derivation of DA neurons from ES cells. In vivo differentiation of DA neurons from ES cells The first demonstration Inhibitors,research,lifescience,medical of ES cell-derived

DA cells after transplantation came from Deacon et al,44 when they showed that ES cells could spontaneously differentiate into DA neurons when see more grafted to either Inhibitors,research,lifescience,medical the brain or the kidney capsule. In this study, high numbers of cells (>50 000) were used and the grafts often became very large teratoma-like grafts that outgrew the target area, thus making any functional effects impossible to study. On the basis of the encouraging about findings of DA cells in these large grafts, the protocol used by Deacon et al was primarily modified by decreasing the number of cells grafted. This led to smaller primarily neural grafts with numerous DA neurons, which showed beneficial functional integration in a rat model of PD.45 Importantly, this study also highlighted the dangers of using dividing, undifferentiated ES cells for grafting, since about a quarter of the grafts still developed into teratomas, even when as few as 1000 ES cells were grafted. In vitro differentiation of DA neurons from ES eels Mouse ES cells The in vitro derivation of DA neurons from mouse ES cells was first, described by McKay and colleagues at the NIH.