  Cells contain elaborate and interconnected networks of protein polymers which
make up the cytoskeleton. The cytoskeleton governs the internal positioning and
movement of vesicles and organelles, and controls dynamic changes in cell
polarity, shape and movement. Many of these processes require tight control of
the size and shape of cytoskeletal structures, which is achieved despite rapid
turnover of their molecular components. Here we review mechanisms by which
cells control the size of filamentous cytoskeletal structures from the point of
view of simple quantitative models that take into account stochastic dynamics
of their assembly and disassembly. Significantly, these models make
experimentally testable predictions that distinguish different mechanisms of
length-control. While the primary focus of this review is on cytoskeletal
structures, we believe that the broader principles and mechanisms discussed
herein will apply to a range of other subcellular structures whose sizes are
tightly controlled and are linked to their functions.
