Interspecies variation of oligodendrocyte progenitor cell density and proliferation in the mammalian brain

The central nervous system dynamically changes its shape and structure in response to the environment. Such modifications are of paramount importance in the adult brain, mediating adaptation and brain repair. In mammals, brain plasticity can occur in different forms and with phylogenetic variations. Unlike rodents, large-brained, long-living mammals are endowed with reduced rate of stem cell-driven neurogenesis. Yet, their cerebral cortices contain large populations of non-newly generated, “immature” neurons (nng-Ins), which represent a novel type of plasticity providing undifferentiated, not-dividing cells in a region devoid of stem cells. Recently, similar cells have been also identified in brain subcortical regions (amygdala, claustrum). However, the real nature of these subcortical “immature neurons” is still unknown, as they were thought to be newly generated in the past. In this context, the widespread occurrence of proliferating cells in the brain parenchyma, mostly of oligodendroglial origin, can represent a confounding element, that might lead to wrong interpretations and conclusions. Here, the occurrence of oligodendroglia and proliferating oligodendrocyte progenitor cells (OPCs) was studied in three representative brain regions (amygdala, white matter and neocortex) of four mammalian species, characterized by different brain size and gyrencephaly. Sox10, Ki67 antigen, and doublecortin (DCX) detection showed that many dividing cells correspond to oligodendroglia progenitors rather than neurons. In addition, glial cell divisions do not substantially change among the species analyzed. These results support the idea that parenchymal cell proliferation and neurogenesis are independent processes.