Rewriting the rules of cell division.

Animal cell division has long been thought to represent a strict division of labor. Transporting, organizing, and then separating the chromosomes is the job of microtubules and microtubule-associated proteins that constitute the mitotic spindle, whereas splitting the cell itself is the job of actin filaments (F-actin) and myosin-2 (an F-actin–based motor protein that drives contraction) in the cell cortex, the layer of cytoplasm at the cell periphery just beneath the plasma membrane. This view has persisted for more than five decades, with any exceptions considered as special cases. However, on page 835 of this issue, Hernandez et al. (1) report that F-actin has a role in chromosome transport, organization, and spindle assembly during mitosis in early mouse embryos. These findings demonstrate that successful mitosis may draw on more mechanisms than commonly assumed. [ABSTRACT FROM AUTHOR]