Unlike the glial disorders, which involve a limited number of phenotypes of relative homogeneity, the neuronal disorders are a diverse group, which includes both hereditary and acquired disorders of the CNS, all of which share a loss of central neurons. The neurodegenerative disorders in particular comprise a heterogeneous category, that include both multicentric and diffuse disorders such as Alzheimer’s, and those in which the loss of a single phenotype predominates, such as Huntington’s and Parkinson’s diseases. The latter category, those neuronal disorders in which a single region or phenotype is differentially affected, have proven to be the most amenable to cell type-specific neuronal replacement in animal models (Lindvall, 2012; Lindvall and Bjorklund, 2011). These include classical Parkinson’s disease, in which nigrostriatal neurons are lost before other neurons, and Huntington’s disease, in which striatal atrophy becomes apparent long before the onset of more widespread cortical neuronal loss. Clinical trials of cell transplantation have already been performed for each of these prototypic neurodegenerative conditions (reviewed in (Barker et al., 2015; Barker et al., 2013; Benraiss and Goldman, 2011; Lindvall and Bjorklund, 2011)).
