Leaf starter

ow does a growing cell know that it’s big enough to divide? Challenging a long-held view, a new study argues that the decision depends mainly on external factors that stimulate growth and division rather than an internal mechanism for assessing size. The standard take on the relationship between growth and division envisions a “size checkpoint” during the cell cycle, where the cell somehow gauges its girth before advancing further. However, Ian Conlon (University College London, England) and colleagues found that they could separate growth from progression through the cell cycle by manipulating the concentrations of insulin-like growth factor-1 (IGF-1) and glial growth factor (GGF). Of the two, only IGF-1 incited growth in rat Schwann cells, while GGF drove cell cycle progression without speeding growth. Applying that discovery, the researchers used GGF to accelerate the cell cycle and gradually shrink cultured cells. The cells grown in high GGF concentrations were 20 percent smaller than cells grown in low GGF. According to Conlon, the findings demonstrate that the decision to divide doesn’t H