Cytoarchitectonics of substantia nigra grafts: a light and electron microscopic study of immunocytochemically identified dopaminergic neurons and fibrous astrocytes.

Maturation of dopaminergic (DA) neurons and astroglia was studied in transplants of the substantia nigra grown for up to 7 months in the brain of rats. The investigation had three specific aims. The first was to observe effects of different transplant positions on the longevity of DA neurons. Second, the grafts were examined for changes of synaptic interactions and associations between DA neurons and astroglia. Third, an answer was sought to the question whether transplanted DA neurons migrate into the adjacent host brain. The grafts were taken from the ventral mesencephalon of rat embryos of different ages (day 14 to 18 of gestation) and placed into the cerebral cortex, tectum, cerebellum, or ventricles of newborn host animals. Following different times of survival the immunocytochemical localization of tyrosine hydroxylase (TH) and of glia filament protein (GFA) in the transplants were observed. In all of the transplantation sites, except for one, neurons of different morphologies that contained TH were found in the grafts. The cerebellar white matter of the host brain failed to support the long-term survival of DA neurons. The overall structure of mature substantia nigra grafts had some resemblance to intact substantia nigra (SN). On the ultrastructural level, it was found that morphological expression of some immature features of DA neurons, such as glial sheaths, somatic spines, and lack of oligodendroglia, persisted in mature grafts. Specific associations of DA neurons and astroglia in the grafts suggested that the cytoarchitectonic appearance of a given brain region may be related to the existence of particular neuron glia relationships. In contrast to intact SN, transplants revealed deficiencies in unlabeled pleomorphic boutons and contained some TH-immunoreactive terminals. Migration of DA neurons and their processes into the adjacent host brain was rarely observed.