Scientific Understanding of Consciousness
Neocortex Outer Region Subventricular Zone Development
Nature 464, 554-561 (25 March 2010)
Neurogenic radial glia in the outer subventricular zone of human neocortex
David V. Hansen Jan H. Lui, Philip R. L. Parker & Arnold R. Kriegstein
Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, Department of Neurology, Biomedical Sciences Graduate Program, Neuroscience Graduate Program, University of California San Francisco
Neurons in the developing rodent cortex are generated from radial glial cells that function as neural stem cells. These epithelial cells line the cerebral ventricles and generate intermediate progenitor cells that migrate into the subventricular zone (SVZ) and proliferate to increase neuronal number. The developing human SVZ has a massively expanded outer region (OSVZ) thought to contribute to cortical size and complexity. However, OSVZ progenitor cell types and their contribution to neurogenesis are not well understood. Here we show that large numbers of radial glia-like cells and intermediate progenitor cells populate the human OSVZ. We find that OSVZ radial glia-like cells have a long basal process but, surprisingly, are non-epithelial as they lack contact with the ventricular surface. Using real-time imaging and clonal analysis, we demonstrate that these cells can undergo proliferative divisions and self-renewing asymmetric divisions to generate neuronal progenitor cells that can proliferate further. We also show that inhibition of Notch signalling in OSVZ progenitor cells induces their neuronal differentiation. The establishment of non-ventricular radial glia-like cells may have been a critical evolutionary advance underlying increased cortical size and complexity in the human brain.
One of the most marked evolutionary changes underlying the unique cognitive abilities of humans is the greatly enlarged cerebral cortex. This change must be reflected in differences in progenitor cell number and/or proliferative output during development. There has been considerable progress in understanding progenitor cell behaviour in the developing rodent cortex, where neurogenic cell divisions are confined to a narrow region of proliferative cells near the cerebral ventricles. Cortical neurons arise from radial glial (RG) cells, the epithelial stem cells that line these ventricles. RG cells in the ventricular zone (VZ) generate intermediate progenitor cells that migrate into the SVZ and further proliferate to increase neuronal number.
A distinguishing feature of primate corticogenesis is the appearance of the OSVZ during mid-gestation. Cell-labelling studies in primates have shown that cell divisions in both the OSVZ and the VZ coincide with the major wave of cortical neurogenesis, suggesting that OSVZ cells produce neurons. However, the progenitor cell types in the OSVZ and the extent to which they participate in neurogenesis have not been characterized. Furthermore, unique features of human cortical development probably underlie neurodevelopmental disorders that affect the cerebral cortex, such as autism and schizophrenia. Here we describe classes of radial glia-like cells and transit-amplifying cells in the human OSVZ that contribute significantly to neurogenesis. OSVZ radial glia-like cells show unusual cell cycle behaviours that further distinguish them from traditional RG cells. These results indicate a new mechanism for cortical expansion outside the ventricular epithelium through the addition of radial columns arising from the OSVZ.
RG cells are characterized, in part, by their distinctive morphology, with an apical process extending to the ventricle and a basal process extending to the pia that can guide radial neuronal migration.
The radial processes of RG cells in the VZ support neuronal migration and underlie the columnar architecture of the cortex. The basal processes of OSVZ cells could serve a similar function, particularly if they extend to the pia.
These results show that the human OSVZ contains a new class of actively dividing, non-epithelial progenitors that can maintain contact with the pia but not the ventricle. Owing to their radial morphology and expression of nuclear and cytoplasmic markers characteristic of RG cells, we will refer to these cells as oRG (OSVZ radial glia-like) cells to set them apart from traditional RG cells in the ventricular epithelium (vRG cells).
An expansion of cortical progenitor cell number during evolution must have contributed to the increase in size of the human brain. However, it has been unclear whether expansion of cortical progenitor number is specific to stem versus transit-amplifying cell types—both of which could shape the eventual cellular composition of the cerebral cortex. Our results show that progenitor cells with markers and morphology of neural stem and transit-amplifying cells (RG cells and intermediate progenitor cells) are well represented in the human cortex in both the periventricular zone and the OSVZ. Proliferating cells in the OSVZ were observed decades ago, but the types of cells produced were unknown. Cells in the OSVZ resembling radial glia had also been observed, but their role in cortical development was not defined. We have shown that oRG cells, although distinct from the ventricular epithelium, can self-renew and produce neuronal precursors. They further resemble traditional RG cells in terms of marker expression and dependence on Notch signalling. The production of neurons by oRG cells and their daughters presents a more complex scheme of human neurogenesis. Although the origin of this previously unappreciated cell type remains unknown, we infer that oRG cells originate in the VZ and use mitotic somal translocation to migrate away. The stepwise translocation of oRG cells towards the cortical plate in coordination with cell divisions helps explain how the OSVZ expands while accumulating large numbers of oRG and intermediate progenitor cells.
Our finding that OSVZ progenitors undergo expansive proliferative divisions contrasts with observations of the rodent SVZ—in which intermediate progenitor cells usually divide only once—and provides a new cellular basis for understanding the evolutionary expansion of surface area in human cortex. It will be interesting to study whether the OSVZ and oRG cells are a general feature of gyrencephalic brain development, especially in non-primate species such as ferret and cat.
The radial unit hypothesis posits that the output of proliferative units (at the ventricle) is translated by glial guides to the expanded cortex in the form of ontogenetic columns. Our results add a layer of complexity to the radial unit model and show that the production of neurons in human cortex occurs simultaneously from primary and secondary progenitor cells in both periventricular and outer subventricular regions—therefore defining two distinct origins for neurons in the mature cortex. Our model suggests that ontogenetic columns can arise from both ventricular and non-ventricular sites during human cortical development. An intriguing future question is whether there are functional differences between neurons generated from these two regions, a feature that would further increase cortical complexity.
(end of paraphrase)