Developing substantia nigra in human: a qualitative study.

Midbrains from 43 fresh human embryos and fetuses at 8-22 weeks of gestation were processed for routine histology, Golgi staining, tyrosine hydroxylase (TH) immunolabelling and retrograde tracing with the fluorescent dye DiI. Cells were immature and densely packed between 8 and 10 weeks. By 13 weeks cells could be identified as neurons and glia. Neurons matured gradually and achieved adult characteristics by 20-22 weeks. Neurons in the paramedian regions of the tegmentum, raphae region and substantia nigra were positive for TH from 13 weeks onwards, the earliest age group used for this technique. The presence of TH-positive neurons in the paramedian part of the tegmentum until 18 weeks and radial glial fibers extending from the aqueductal lining to the ventral part until 20 weeks were suggestive of migration of neurons to the ventral mesencephalon region. DiI labelling of the neurons and fibers of ipsilateral nigra from the caudate as early as 10 weeks demonstrated early nigrostriatal connections. The mature nature of the neurons appeared only by 13 weeks by this method. The present study shows that the nigral neurons in the human migrate and mature until mid-gestation. The nigrostriatal connection at 10 weeks suggests a trophic influence of nigra on the proliferating and maturing neurons of the striatum.