A re-evaluation of the ultrastructural localization of 5'-nucleotidase activity in the developing rat cerebellum, with a cerium-based method.

The membrane ectoenzyme 5'-nucleotidase converts 5'-AMP into adenosine which, in the nervous tissue, plays an important role as intercellular messenger. Moreover, during histogenesis, 5'-nucleotidase seems to be related to cell proliferation and migration. Conflicting data are reported in the literature about the localization (neuronal or glial) of 5'-nucleotides in the rat cerebellum. In the present report we have analyzed the distribution of 5'-nucleotidase activity with electron microscopy, using a cerium-based method, at different postnatal histogenetic stages (postnatal days (PND) 11, 17, 28). On PND 11 and 17, rims of reaction product outlined the plasma membranes of some neuroblasts in the external granular layer and of parallel fibers and some migrating cells in the developing molecular layer. Positivity was frequently observed on membranes of adjacent neuronal cells and glial processes. Moderate activity was also present on the membranes of granule cells and of mossy fiber rosettes and granule cell dendrites constituting the cerebellar glomeruli within the internal granule cell layer. At PND 28, the reaction product was slightly reduced in some localizations. Cytochemical patterns prove that the cerium-based method is suitable for demonstration of 5'-nucleotidase-specific activity. In fact, a continuous and fine reaction product appears strictly linked to the cell membranes, and no unevenly scattered precipitates can be observed. Data suggest that, during cerebellar histogenesis, 5'-nucleotidase may be involved in the mechanisms of cell migration and proliferation. However, in adulthood, prominent localization of the reaction product on neuronal elements suggests a major role in neuromodulation processes for the enzyme.