Radial glia (RG) are progenitor cells that give rise to all of the neurons in the mammalian neocortex via asymmetric divisions. Ex vivo, RG can be cultured to form a self-renewing, expanding population, raising the question of which signals and cell division behaviours are modulated to achieve this switch. Now, David Míguez and colleagues tackle this question by analysing the role of FGF2 – previously shown to promote RG expansion in vitro – in division and differentiation dynamics. They first show that FGF2 increases growth rate in culture by shortening the cell cycle, and that it promotes the generation of progenitors as well as differentiated neurons. A theoretical framework termed a ‘branching process tool’ is then adopted to analyse cell cycle dynamics, revealing an increase in the number of pp (progenitor-to-progenitor) divisions, and uncovering the dynamics of cell cycle shortening in response to FGF2. Indeed, the branching process tool outperforms cumulative curve methods for monitoring cell cycle dynamics (which perform poorly in situations of variable proliferation/differentiation), and the values derived from the tool accurately reproduce experimental data. Thus, to promote RG culture propagation and expansion, FGF2 promotes symmetric divisions, increases growth fraction and shortens the average cell cycle length.
