So we have this winding in a helical like fashion and we create a hollow center. So this structure here is the microtubule. So notice that the actin and the intermediate filament. So the microfilaments and intermediate filaments do not contain a hollow center but the micro tubules do contain a hollow center and that's exactly why they're called tubules because they create this hollow like tube. Now, just like our microfilaments, our micro tubules which are 23 nm in diameter so notice they are larger than the microfilaments or the intermediate filaments. The microtubules have a positive end as well as the negative end and the positive end is the end from where our microtubule grows.