Acridine orange-RNA fluorescence of maturing neurons in the perinatal rat brain

Cytoplasmic RNA was demonstrated in neurons of the developing rat brain using acridine orange (AO) as a histochemical marker. Fetuses of 18 days and postnatal rats of 1, 7, 14, and 21 days as well as adults several months old were studied. Neuroblasts of the germinal matrix exhibited minimal or no orange-red AO-RNA fluorescence, but immature nerve cells in migration within the cerebral hemispheres of the rat showed a weak but definite orange colour. This finding contrasts with the absence of AO-RNA fluorescence in migrating human neuroblasts. Neurons of the neocortical plate showed uniformly strong fluorescence. In the hippocampus, the most pronounced increase in AO-demonstrated RNA was in pyramidal and granule cells during the first postnatal week. The cerebellum showed a paradoxically stronger fluorescence of granule cells in the 18-day fetus than at birth, and almost no AO-RNA fluorescence of granule cells at 21 days of age or in adults. Motor neurons showed the strongest fluorescence of all neurons. It is likely that the increase in cytoplasmic RNA in neurons corresponds to the onset of neurotransmitter biosynthesis, but transitory fetal neuropeptides may explain stronger fluorescence of some neurons in young individuals. The reliable and simple AO method provides a supplementary means of studying one aspect of neuronal maturation.