Microtubules are tiny hollow cylinders made of protein.
Coating each microtubule are threadlike bristles that circulate particles
around the cell by dragging them from one bristle to the next.
Microtubules also guide the division of cells; first the microtubules dissolve,
then form again at just the right moment and place to pull the cell apart.
Microtubules also monitor what comes in and goes out of the cell
and organize different particles for specific purposes.