Brennand et al., 2012; Marchetto and Gage 2012). Moreover, elegant studies have described differentiation protocols that produce distinct types of neurons in vitro, although the number and properties of different types of human neurons in situ are largely unknown and are only now beginning to be defined. Overall, these studies suggest that derivation of neurons from human stem-cells may allow scientists to examine specific subtypes of neurons, to generate human neurons for regenerative medicine, and to investigate changes in human neurons in neuropsychiatric disorders (e.g., see Cho et al., 2008; Fasano et al., 2010; Kriks et al., 2011; Shi et al., 2012; Chambers et al., 2012; Ma et al., 2012). However, this approach of studying human neurons at present suffers from two major limitations.
