Neuronal differentiation of monkey ES cells into embryoid bodies (EBs, panel A), progenitor cells expressing nestin and musashi1 (panel B), neuronal cells expressing the neural filament TujIII (panel C) and neurotransmitter enzymes tryptophan hydroxylase (TPH, panel D), choline acetyltransferase (ChAT, panel E) and tyrosine hydroxylase (TH, panel F and G). Receptors of mature neurons such as α4-nicotinic receptors (panel H) and estrogen receptor β (panel I) were also observed. The image in panel A was taken directly from an inverted microscope. Images in panels B-H were taken after fluorescence-based immunocytochemistry. The image in panel I was taken after staining with the classic, non-fluorescent procedure using biotinylated antiserum and diaminobenzidine in the presence of nickel sulfate (shown in dark brown or black). Scale bars, when applied, are equivalent to 40 μm.