Abstract
Neuronal production persists during adulthood in the dentate gyrus and the olfactory bulb, where substantial numbers of immature neurons can be found. These cells can also be found in the paleocortex layer II of adult rodents, but in this case most of them have been generated during embryogenesis. Recent reports have described the presence of similar cells, with a wider distribution, in the cerebral cortex of adult cats and primates and have suggested that they may develop into interneurons. The objective of this study is to verify this hypothesis and to explore the origin of these immature neurons in adult cats. We have analyzed their distribution using immunohistochemical analysis of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) and their phenotype using markers of mature neurons and different interneuronal populations. Additionally, we have explored the origin of these cells administering 5′bromodeoxyuridine (5′BrdU) during adulthood. Immature neurons were widely dispersed in the cerebral cortex layers II and upper III, being specially abundant in the piriform and entorhinal cortices, in the ventral portions of the frontal and temporoparietal lobes, but relatively scarce in dorsal regions, such as the primary visual areas. Only a small fraction of PSA-NCAM expressing cells in layer II expressed the mature neuronal marker NeuN and virtually none of them expressed calcium binding proteins or neuropeptides. By contrast, most, if not all of these cells expressed the transcription factor Tbr-1, specifically expressed by pallium-derived principal neurons, but not CAMKII, a marker of mature excitatory neurons. Absence of PSA-NCAM/5′BrdU colocalization suggests that, as in rats, these cells were not generated during adulthood. Together, these results indicate that immature neurons in the adult cat cerebral cortex layer II are not recently generated and that they may differentiate into principal neurons.
Highlights
The production of new neurons in the adult mammalian CNS is mainly restricted to the subventricular zone (SVZ) surrounding the lateral ventricles and the subgranular zone (SGZ) of the hippocampal dentate gyrus
Subtypes and distribution of polysialic acid (PSA)-NCAM expressing cells in the adult cat cerebral cortex Polysialylated form of the neural cell adhesion molecule immunoreactivity was distributed throughout the cortical neuropil as well as on certain cell bodies
PSA-NCAM immunoreactive neuropil appeared more intense in layers III and V–VI, resembling the distribution observed in rodents (Varea et al, 2005) and humans (Varea et al, 2007)
Summary
The production of new neurons in the adult mammalian CNS is mainly restricted to the subventricular zone (SVZ) surrounding the lateral ventricles and the subgranular zone (SGZ) of the hippocampal dentate gyrus. The presence of immature neurons has been detected in the layer II of the paleocortex of rodents These cells are characterized by the expression of molecules related to neuronal development or plasticity, such as the microtubule associated protein doublecortin (DCX) or the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), ultrastructural features typical of immature neurons and a virtual absence of synaptic input (Gomez-Climent et al, 2008, 2010b). DCX expressing cells in layers II and upper III, can be found dispersed throughout the cerebral cortex, being specially abundant in the entorhinal cortex and in the ventral portions of the frontal and temporoparietal lobes, but relatively scarce in dorsal regions, such as the primary visual areas (Cai et al, 2009)
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