Diverse behaviors of outer radial glia in developing ferret and human cortex.

The dramatic increase in neocortical size and folding during mammalian brain evolution has been attributed to the elaboration of the subventricular zone (SVZ) and the associated increase in neural progenitors. However, recent studies have shown that SVZ size and the abundance of resident progenitors do not directly predict cortical topography, suggesting that complex behaviors of the progenitors themselves may contribute to the overall size and shape of the adult cortex. Using time-lapse imaging, we examined the dynamic behaviors of SVZ progenitors in the ferret, a gyrencephalic carnivore, focusing our analysis on outer radial glial cells (oRGs). We identified a substantial population of oRGs by marker expression and their unique mode of division, termed mitotic somal translocation (MST). Ferret oRGs exhibited diverse behaviors in terms of division location, cleavage angle, and MST distance, as well as fiber orientation and dynamics. We then examined the human fetal cortex and found that a subset of human oRGs displayed similar characteristics, suggesting that diversity in oRG behavior may be a general feature. Similar to the human, ferret oRGs underwent multiple rounds of self-renewing divisions but were more likely to undergo symmetric divisions that expanded the oRG population, as opposed to producing intermediate progenitor cells (IPCs). Differences in oRG behaviors, including proliferative potential and daughter cell fates, may contribute to variations in cortical structure between mammalian species.