Morphology and distribution of dopaminergic neurons intrinsic to the human striatum

The putative dopaminergic (DA) neurons intrinsic to the human striatum were studied by applying immunofluorescence and quantitative methods to postmortem tissue from seven normal individuals. Stringent morphological and chemical criteria were used to identify striatal DA neurons, including immunostaining for tyrosine hydroxylase, DA transporter and neuronal nuclear protein. The DA neurons were scattered throughout the striatum, but abounded particularly in its ventral portion. Frequency distribution of surface areas of DA cell bodies reveals that the most frequent DA neurons (x¯ = 58.0%, S.D. = 12.8%) had a medium-sized (≈200 ± 15 μm2) perikaryon with 3-5 varicose dendrites, whereas others (x¯ = 35.5%, S.D. = 14.0%) had a smaller (≈140 ± 15 μm2) perikaryon with 3-4 varicose dendrites. There was a small number (x¯ = 6.5%, S.D. = 8.5%) of larger DA neurons (209-584 μm2) with spiny dendrites and a few TH-immunoreactive cells displaying mixed neuron-glia morphology. Despite significant inter-individual variations in neuron density, the human striatum (mean volume of 8.76 cm3) harbored a mean of 331.9 DA neurons (S.D. = 199.2). A prolific zone, containing about 3000 cells, occurred in the ventral striatum in two brains. The addition of these cells would increase by about 10 times the total number of striatal DA neurons, which should not be confounded with segments of nigrostriatal DA fibers that displayed large (8-12 μm) varicosities and looked like small bipolar neurons. The function of striatal DA neurons is unknown but the fact that their number increases markedly following lesion of nigral DA input or administration of various growth factors, opens up new therapeutic avenues for treatment of Parkinson's disease.