When compressive forces push on a microtubule in a cell, such as when a growing polymer butts up against the cell edge, the fiber bends in multiple short wavelength curves like a snake. By contrast, when the end of an isolated microtubule is pushed with even small forces, the fiber compresses and bends in a single large arch. The minor forces necessary to bend isolated microtubules call into question the importance of the fibers in determining cell shape and strength.
Brangwynne et al. found that if they pressed on the end of a microtubule inside a cell with a microneedle, short wavelength bending occurred. Moreover, contraction of the actin–myosin cytoskeleton induced such buckling in rhythmically contracting cardiac myocytes, and neighboring microtubules bent in a coordinated pattern.
To find out if an elastic medium surrounding a fiber could cause a shift from long to short wavelength bending, the team compressed a thin plastic rod first in aqueous solution and then in a gelatin matrix. In water, the rod formed a long arc, but when constrained by gelatin, which the rod had to push out of the way, it bent in a shorter sinusoidal pattern. Mathematical modeling showed that the wavelength of bending in response to compression resulted from the combined strength of the fiber and the resistance of the medium.
When the team disrupted the actin–myosin matrix with cytochalasin and then compressed microtubules with a microneedle, they saw that the microtubule now bent in a longer arc than occurred when the actin fibers were intact.
The team concluded that the surrounding network adds substantial strength to the microtubules. Furthermore, by increasing the reinforcement in particular regions, the cell can hold one part of a microtubule straight while allowing small wavelength bends in other regions. Thus, microtubules can withstand and generate the forces necessary to support motility and tissue development.