Using two developmentally relevant morphogens, retinoic acid (RA)

and Sonic hedgehog (Shh), Wichterle Dinaciclib and colleagues showed that mouse ES cells could be directed to differentiate into functional spinal motor neurons (Wichterle et al., 2002). RA induces neuralization and caudalization of stem cells, while the ventralizing activity of Shh converts spinal progenitor cells to motor neurons (Peljto and Wichterle, 2011 and Wichterle et al., 2002). RA treatment and induction of SHH signaling have also been used to derive functional spinal motor neurons from human pluripotent stems cells (Boulting et al., 2011, Dimos et al., 2008, Hu and Zhang, 2009, Karumbayaram et al., 2009b, Lee et al., 2007b and Li et al., 2005). Lee and colleagues demonstrated the in vivo potential of hES cell-derived spinal motor neurons using transplantation assays into the spinal cord of developing chick embryos and of adult rats (Lee et al., 2007b). Both approaches yielded robust engraftment and maintenance of motor neuron phenotype and, when transplanted in developing chick spinal cord, were capable of forming long axonal projections to skeletal muscle (Lee et al., 2007b). Beyond its caudalizing role on neural progenitors, RA signaling also affects spinal motor neuron

subtype specification by imposing a rostral cervical identity (Peljto and Wichterle, 2011 and Wichterle et al., 2002). Using inhibition of Activin/Nodal signaling for neural induction, functional Veliparib in vitro science human spinal motor neurons could also be specified in a retinoid-independent pathway, which results in the production of more posterior motor neuron types (Patani et al., 2011). As disorders like ALS selectively target certain subtypes and pools of motor neurons (Kanning et al., 2010), the ability to direct the differentiation of stem cells into specific motor neuron subtypes could have important implications for disease modeling and studies aiming to understand mechanisms of selective vulnerability. Another clinically relevant neuronal subtype that has been generated in vitro from human pluripotent stem cells is midbrain dopaminergic (DA)

neurons, which are preferentially affected in PD. Several studies on the controlled differentiation of hES and hiPS cells into populations of neurons expressing tyrosine hydroxylase have been reported (Ben-Hur et al., 2004, Chambers et al., 2009, Cho et al., 2008, Cooper et al., 2010, Hargus et al., 2010, Perrier et al., 2004, Roy et al., 2006, Soldner et al., 2009 and Yan et al., 2005). Most of these differentiation strategies rely on the patterning of neural progenitors by the combined activity of SHH and FGF8, first shown to have a significant effect on dopaminergic differentiation by seminal work of Lee and colleagues using mouse ES cells (Lee et al., 2000). Methodological improvements to enhance human dopaminergic differentiation in vitro include coculture with immortalized human fetal astrocytes (Roy et al.