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refers to one of two components of an alternate segmentation of the dopaminergic cell groups found in the ventral midbrain tegmentum of the human and macaque. It is composed of columns of dopaminergic cells of the A9 cell group, which are distributed in the pars reticulata of the substantia nigra. The other component is the dorsal tier dopaminergic cell group. The cells of the ventral tier are more reduced in Parkinson\'s disease and more vulnerable to MPTP treatment than those of the dorsal tier ( Haber-2012 ).
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