Figure 3.

Changes in the subventricular zone (SVZ; blue) in vertebrates (top row). Specific alterations in cell cycle kinetics, the overall thickness of the SVZ, the proportion of the SVZ corresponding to the inner and outer layers (ISVZ and OSVZ respectively), and the proportion of asymmetrical radial glial (RG) and outer radial glial (oRG) cell divisions producing intermediate progenitor cells account for expansion of the neocortex in some lineages, such as primates (right half of bottom panel). The SVZ and particularly the OSVZ is larger in primates than in many other species. While mammals with both a large and small neocortex share a number of aspects of neurogenesis (steps 1–5 bottom figure), several additional adaptations are observed in animals with a large neocortex such as primates. This includes increased thickness of particular layers (such as the OSVZ), additional rounds of division for RGs that re-enter the cell cycle in the ventricular zone (VZ) (6), division of oRGs into intermediate progenitor cells (IPC) in the OSVZ (7), which ultimately divide again to produce neurons (8). Whether oRGs also divide to produce IPCs in rodents is still contentious (dotted line on left side, 7). These figures have been modified from Molnár (2011) and Molnár and Molnár and Clowry (2012). Abbreviations in Table 1.