Glia-independent chains of neuroblasts through the subcortical parenchyma of the adult rabbit brain

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Abstract
In the brains of adult mammals long-distance cell migration of neuronal precursors is known to occur in the rostral migratory stream, involving chains of cells sliding into astrocytic glial tubes. By combining immunocytochemistry for polysialylated neural cell adhesion molecule (PSA-NCAM), neuronal and glial antigens, endogenous and exogenously administered cell-proliferation markers, and light and electron microscopy 3D reconstructions, we show that chains of newly generated neuroblasts exist both inside and outside the subventricular zone of adult rabbits. Two groups of chains were detectable within the mature brain parenchyma: anterior chains, into the anterior forceps of the corpus callosum, and posterior chains, close to the external capsule. Parenchymal chains were not associated with any special glial structures, thus coming widely in contact with the mature nervous tissue, including unmyelinated/myelinated fibers, astrocytes, neurons, and oligodendrocytes. These chains of cells, unlike those in the subventricular zone, do not display cell proliferation, but they contain BrdUrd administered several weeks before. Telencephalic areas, such as the putamen, amygdala, claustrum, and cortex, adjacent to the chains harbor numerous PSA-NCAM-positive cells. The counting of newly generated cells in these areas shows small differences in comparison with others, and a few cells double-labeled for BrdUrd/PSA-NCAM (after 1-month survival) and for BrdUrd/NeuN (after 2 months) were detectable. These results demonstrate the occurrence of glial-independent chains of migrating neuroblasts, which directly contact the mature brain parenchyma of adult mammals. These chains could provide a possible link between the adult germinative layers and a very low-rate/long-term process of cell addition in the telencephalon.