The second limitation is related to the cumbersome, variable, and slow procedures needed for deriving neurons with functional properties from ES or iPS cells. Generating neurons by differentiation of ES or iPS cells requires months of tissue culture procedures, and renders large-scale studies difficult (Johnson et al., 2007). Moreover, differentiation of ES and iPS cells into neurons depends on specific environmental factors such as pharmacological agents and bioactive proteins that may be difficult to obtain with a consistent composition, injecting a further element of variation (Soldner and Jaenisch, 2012).
